EXTERNAL ANATOMY OF INSECTS.
THE HEAD, AND ITS PARTS.
Before I confine my observations to the head of insects, which I propose to consider separately in the present letter, I must premise a few words upon their body in general, or rather its crust, or external integument. In this we may notice its substance, general form, sculpture, pubescence, and composition.
i. I have already noticed the substance of this integument in the preparatory states of insects[1151]; I shall not, therefore, here repeat what I then said, but restrict myself chiefly to the consideration of it as it is found in their last state, in which it is usually firmer than in their previous stages of existence. In this respect, however, it varies much in the different Orders, and even in the different genera of the same Order. In some Coleopterous insects, for instance, it is very hard, and difficult to perforate; while in others it is soft, flexible, and a pin easily passes through it[1152]. And in general, from a substance in hardness resembling horn or shell, it passes through the intermediate degrees of that of leather and parchment, almost to a thin membrane. Yet in all cases there is enough of rigidity and hardness to answer the principal uses of a skeleton—to afford, namely, a sufficient point of attachment for the muscles, and to support and defend the interior organization; so that the play and action of the vital and secretory systems may not be interrupted or impeded.
With respect to the principles which enter into the composition of this integument, very little seems to be known at present; but few insects having been submitted to a chemical analysis. The blister-beetle (Cantharis vesicatoria), from its importance in medicine, has, however, been more than once analysed; and though the products have not been very precisely stated, yet we find amongst them phosphate of lime, albumen, and some other usual components of the substance of vertebrate animals[1153]. But which of these products belong to the integument, and which to its contents, cannot be ascertained, without a separate process for each; which would not, I conceive, be very feasible. The substance, however, of the integument of insects, though we know not its precise contents, which probably vary in different genera, &c., appears not to be exactly of the nature of any of those substances after which it has usually been denominated: it is not properly analogous either to real horn, shell, skin, or leather, &c. This seems to result from the following circumstance:—Most of the excretions of vertebrate animals, as horn, skin (at least when tanned), feathers, wool, hair, &c. when exposed to the action of fire liquify, more or less, before they incinerate; emitting at the same time a peculiar and disagreeable scent: but upon applying this test to the parts of insects of the different Orders, I found, in every instance, that incineration took place without liquefaction, and was unaccompanied by that peculiar scent which distinguishes the others. Even the claws, which to the eye appear, as to their substance, exactly like those of Mammalia, birds, &c. burn without melting, and retain their form after red heat. That the insect integument is not calcareous like that of the Crustacea, and the shells of Molluscæ, you may easily satisfy yourself, by immersing them in an acid test. I made this experiment upon portions of insects of several of the Orders, in an equal mixture of muriatic acid and water, and the result was, not only that all hexapods, but octopods, Arachnida, and even Scolopendridæ, upon immersion only emitted a few air-bubbles; while, when the other myriapods, Polydesmus, Iulus, Glomeris, &c. and the Oniscidæ, were immersed, a violent effervescence took place; proving the different nature of their substance. It is remarkable that the two great branches of the Myriapods, the Scolopendridæ and Iulidæ (Chilopoda and Chilognatha Latr.), should in this respect be so differently circumstanced—the latter having a calcareous integument, and the former not.—A further difference distinguishes these two tribes: old specimens of the Iulidæ usually lose their colour and turn white, like Oniscidæ; while those of the Scolopendridæ retain it.
ii. The form of insects is so variable, that it can be reduced to no other general rules—than that, for the most part, the length exceeds the breadth, and the breadth the depth, and that the upper surface is usually convex. But to these rules there are numerous exceptions. Thus many Tetyræ F. (Scutellera Latr.), a kind of bug, are as broad as they are long[1154]; in the genus Gonyleptes K.[1155] amongst the Aptera, and Epeira cancriformis, a crab-shaped spider, the breadth exceeds the length; in Cynips, and several other Hymenoptera, in Acrida K.[1156] (Locusta F.), and other Orthopterous insects, the depth exceeds the breadth; and in that singular beetle, Eurychora; the cockroach (Blatta), &c. the upper surface is flat.
iii. The sculpture of the integument of insects is often very remarkable; but as this will call for attention hereafter, I shall only here observe in general, that ornament and variety seem not to be the sole object of those elevations and depressions which form so prominent a feature of many of the animals in question; for by means of these, many important purposes, that at first sight do not strike the observer, may be served: such as giving firmness to the crust in those places where it is most wanted; diminishing its powers of resistance in others, so that it may yield somewhat to the action of the muscles; increasing or deducting from the weight of the body, so as to produce a proper equipoise during its motions, whether on the earth, in the air, or in the water. The depressions of the outer surface of the crust, in many instances, produce an elevation of it in the interior, and so afford a useful point of attachment to certain muscles. This observation seems more especially applicable to those excavations that are common to particular tribes or genera: thus the dorsal longitudinal channel to be met with on the prothorax of most of the Carabi of Linné on the inside of the crust have a corresponding ridge. In Locusta Dux, also, (a Brazil locust,) the same part has four transverse channels, corresponding with which on the inside are as many septa, or ridges, to which muscles are attached; and those larger impressed puncta denominated puncta ordinaria, which distinguish the same part in Geotrupes and many of the Scarabæidæ, within are elevated, so as to form a kind of ginglymous articulation with the base of the anterior coxæ. The other impressed puncta so often to be seen on the different parts of various insects, which sometimes so intirely cover the surface that scarcely any interval is discoverable between them, though in many cases they appear to be mere impressions that attenuate but do not perforate the crust—yet in others, perhaps equally or more numerous, they are real pores, which pass through the integument. If, for instance, you take the thoracic shield of the cockchafer (Melolontha vulgaris), and after removing the muscle &c., hold it against the light, with the inner side towards the eye, you will see the light through every puncture: or take the elytra of Geotrupes stercorarius, or any common beetle in which these organs have punctate striæ, and examine them under a lens on the inside, and you will see distinctly that the punctures pass through the elytrum, and the membrane that lines it[1157]. It is not improbable that in the case last mentioned these pores may be of use, as the spiracles are usually closely covered by the elytra, for the better transmission of the air to those respiratory organs. Whether the pores in the other parts of the body are for transpiration, is more than I shall venture to affirm; but as insects sometimes perspire, at least this has been ascertained with respect to the hive-bee[1158], this must be by the means of some pores.
iv. The integument of insects is often clothed, either partially or generally, with pubescence, or hairs of various kinds—a circumstance which seems to have more than one object. In Parnus, Heterocerus, Gerris, Argyroneta aquatica, and some other aquatic insects, the end in view seems to be to keep the water from wetting the crust; and in this case the covering of hairs is dense, silky, and decumbent. Another object is preventing friction from being injurious: thus humble-bees, that from their mode of nidification[1159], are usually more particularly exposed to it, are well clothed with hair; and in those articulations of insects where much friction takes place, we may often observe a dense fringe or coating of the same substance. This you may see in the common stag-beetle (Lucanus Cervus), where the thorax receives the head; and very remarkably at the same point in the Hercules-beetle (Dynastes Hercules MacLeay): but besides these uses, there is probably one more universal, which will apply as well to those thinly scattered bristles and hairs, here and there one, to be noticed in many insects: but concerning this I can only throw out a conjecture, as I do not recollect ever to have seen any experiments with regard to this use of animal hairs. But may they not act as conductors, either to introduce some invisible fluid into the body in a positive state, or to convey it out, when received by other means, in a negative state? Every one knows that the fur of a cat has electric properties, and there may be an important general use of this kind attached to the fur and hairs of animals[1160]. But, as I said, I give this as a mere conjecture; and only wish it may excite your attention to the subject, and put in exercise your natural tact for experiment.
M. Cuvier regards the hairs of insects as merely a continuation of the epidermis, with which they fall when the animal changes its skin[1161]; but this will apply only to the hairs of larvæ: for the hairs of perfect insects in many cases are implanted in a pore, and pass through epidermis or crust to the membrane that lines it, in which they terminate.
v. We are now to consider the composition of the integument; under which term I would include the different layers of which it consists, and its articulation.
1. With respect to the first of these circumstances, the layers of which the integument consists, it seems to exhibit some, although not an exact, analogy with the skin, rather than the skeleton, of the vertebrate animals[1162]. In these last, the skin is stated to consist of four layers. Of these the exterior one is the epidermis, or scarf-skin: under this is the rete mucosum, or mucous tissue, which gives its colour to the skin; next follows the papillary tissue formed by the extremities of the nerves, and in which the sense of touch principally resides; the last and innermost layer is the skin proper, or leather, called Dermis, Derma, or Corium[1163]. Two of these layers M. Cuvier assigns to insects. They have, he observes, in every state, a true epidermis[1164]; and in their state of larva he finds that the infinite variety of colours that so adorn many of them is produced by a mucous substance observable between the epidermis and the muscles[1165]: this seems analogous to the rete mucosum. To this, dried and mixed with their horny substance, he attributes also the colours of the perfect insect: "for," says he, "when the Lepidoptera are in the chrysalis, the little coloured scales which are to ornament their wings, are then in a state of mucosity similar to that which is found under the skin of the caterpillar. The colours of the Arachnida," he goes on, "are also due to this mucosity: it is discoverable under the skin, and has the appearance of minute glandular points of which the shades vary considerably. But in the Coleoptera, and many other Orders, the colours of the skin are mixed in its horny tissue, nearly as those of the Testacea are in their calcareous shells"[1166]. In the perfect insects, therefore, in most cases, we may consider the epidermis and rete mucosum as together forming the exterior and coloured integument of insects—that part which in the table, since it is not properly an epidermis, I have distinguished by the name of Exoderma.
The learned author just quoted has observed nothing under the skin of white-blooded animals that he regards as analogous to nervous papillæ[1167]. In some parts of insects, as in the lamellæ of the antennæ of the Petalocera, and the extremities of the joints, especially the last, of many palpi, there is however an appearance of them; and it seems reasonable to suppose that where the sense of touch resides, there must, even in insects, be something of a papillary tissue.
With regard to the innermost integument of the vertebrate animals,—the leather, or real skin,—this learned comparative anatomist finds nothing analogous to it in the integuments of insects[1168]; but as he does not notice it, he appears to have overlooked the substance that lines the outer crust, or exoderma, in the Coleoptera and most other orders. This is not always easily detected; but it may generally be discovered by breaking, or rather tearing (not cutting), after having cleared away the muscles, any part of the body of an insect. It is always very visible on the under side of elytra[1169], but is not discoverable in tegmina. It appears to consist, in many cases, of several layers of a whitish membrane, and generally breaks into fibres. In some elytra of the larger Dynastidæ, towards the sides the exterior layer is separated from the rest by a kind of cellular substance. The fibrous structure of this inner skin (which I call the Esoderma) seems to give it some affinity to the skin of vertebrate animals[1170]. In many parts of the body, however, it appears to be merely a thin pellicle. A medical friend, to whom I showed specimens of it, thinks it a kind of cellular membrane.
2. A few words are next necessary with regard to the articulation of the integument, or the mode by which the several pieces of which it and its members consist, are united to each other. In some, as in several of the parts of the head, the occiput, vertex, temples, cheeks, &c.—the line of distinction is merely imaginary; in others an impressed line separates a part from its neighbours, as is the case with the nose in Vespa, &c. the head in the Arachnida. But in the majority of instances the parts are separated by a suture, or form a real joint. The kinds of articulation observed by anatomists in vertebrate animals do not all occur in insects, and they seem to have some peculiar to themselves. Thus, for instance, they have no proper suture; for though they exhibit the appearance both of the harmonic and squamose (ecailleuse Cuv.) sutures[1171], yet these parts being all limited by the esoderma, or skin, above noticed as lining the integument, and all admitting a degree of motion more or less intense, rather afford examples, as the case may be, of other kinds of articulation[1172]. Again, they have no proper Enarthrosis, or ball and socket; though the anterior coxæ of the Capricorn-beetles (Cerambyx L.) approach very near to this kind of articulation, as will be shown more fully in another place. The inosculating segments or rings, which distinguish the abdomen, and sometimes other parts of insects, are an example of a kind of articulation not to be met with in the Vertebrata. The ginglymous articulation, in which the prominences of the ends of two joints are mutually received by their cavities, and which admits only of flexion and extension, often prevails in the limbs, &c. of insects; but in many cases the joints are merely suspended to each other by a ligament or membrane; and, in fact, the integument of insects, with regard to its articulation, even where the joints ginglymate, may be said in general to consist of pieces connected by the internal ligament, membrane, or skin that lines it; for even in the legs, where the ginglymous articulation is sometimes very remarkable and complex, as will be shown to you hereafter, the joints are also connected by this substance, as you may see if you examine the legs of any Coleopterous insect.
The number of articulations or pieces that form the integument and its members in these animals, varies greatly in different tribes, genera, &c. Thus, in the common louse (Pediculus humanus) they scarcely reach fifty, while in some cockroaches (Blatta) they amount to more than eight times that number.
Having premised these observations on the external anatomy of the body in general, in the remainder of the present letter I shall confine myself to the consideration of the head and its parts.
I. The Head of insects, as the principal seat of the organs of sensation, must be regarded in them, as well as in the vertebrate animals, as the governing part of the body. It may be considered with respect to its substance, figure, composition, superficies, proportion, direction, articulation with the trunk, motions—and more particularly as to its parts and appendages.
i. With regard to its substance—the head may be said in general to be the hardest part of the crust: and it is so for very good reasons. In the first place, as it has to make way for the rest of the body when the animal moves, it is thereby best fitted to overcome such resistance as may be opposed by the medium through which it has to pass; in the next, as it bears the organs of manducation, it was requisite that it should be sufficiently firm and solid to support their action, which is often upon very hard substances; and besides this, as no motion of its parts inter se, as in the case of the trunk, is requisite to facilitate the play of its organs, a thin integument was not wanted.
ii. The most general law relative to the figure or shape of the head seems to be, that it should approach to that of an equilateral triangle, with its angles rounded, and the vertex being the mouth; and that the vertical diameter should be less than the horizontal, whether longitudinal or transverse. But the infractions of this law are numerous and various. Thus, in some insects an isosceles triangle is represented by the head, the length being greater than the breadth; in others, instead of being flat it is compressed, so that the horizontal diameter is less than the vertical; in others, again, it is orbicular, or round and depressed; in others nearly spherical: occasionally it is rather cylindrical. In many instances it is very long; in others the width exceeds the length. Though often narrowest before, in some cases the reverse takes place. Its anterior end is often attenuated into a long or short snout or rostrum, and its posterior into a long or short neck. Its contour, though usually regular, is sometimes either cut into lobes, or scooped out into sinuosities. But to enumerate minutely all the variations of form which take place in the head of insects would be endless; I shall therefore proceed to the next particular.
iii. The composition of the head is very simple; for, exclusive of its organs, it consists only of a single piece or box, without suture or segment, with an aperture at the end below to receive the instruments of manducation, others for the eyes and stemmata when present, and also for the antennæ. In the Arachnida, &c., in which the head is not separated from the thorax, it is merely a plate, the under-side or cavity of which is occupied and filled by the above instruments.
iv. With regard to its superficies, the head of insects is generally more or less uneven, though in some cases it presents no inequalities. In many of the Lamellicorn tribes, and a few other individuals, in one sex at least, as has been before observed[1173], it is armed with long horns, or prominent tubercles; it is often covered with numerous puncta, or pores; and some of its parts, as the nose, after-nose, &c. in particular groups, marked out by an impressed line[1174]. In many Hymenoptera, Diptera, &c. its upper surface is convex, and the lower concave; in others both surfaces are convex.
v. It is the most general rule, as to its proportion, that it shall be smaller than either trunk or abdomen; but in some instances, as in the S. American ant, Atta megacephala, it is much larger than either.
vi. By the direction of the head, I mean its inclination with respect to the prothorax. The most natural direction, or that which obtains most generally, is for it to form an angle more or less obtuse with the part just mentioned. This seems to obtain particularly in Coleoptera; but in some, as Mylabris, it is inflexed, forming an acute angle with it. In the Heteropterous Hemiptera (Cimex L. &c.) it is generally in the same line with the body, or horizontal; and in many Diptera it is vertical.
vii. We now come to a circumstance which will detain us longer, namely, its articulation with the trunk, or rather with its anterior segment, the prothorax.—M. Cuvier makes two principal kinds of articulation of the head upon the prothorax, in one of which the points of contact are solid, and the movement subordinate to the configuration of the parts; in the other, the articulation is ligamentous, the head and the thorax being united and kept together by membranes.
1. The first of these kinds of articulation—that by the contact of solid parts—takes place, he says, in four different ways. "In the most common conformation, in the part that corresponds to the neck, the head bears one or two smooth tubercles, which receive corresponding cavities of the anterior part of the prothorax observable in the Lamellicorn and Capricorn beetles. In this case the head can move backwards, and the mouth forwards and downwards. The second mode of solid articulation takes place when the posterior part of the head is rounded, and turns upon its axis in a corresponding cavity of the anterior part of the prothorax; as may be seen in Curculio, Reduvius, &c. The axis of motion is then at the centre of articulation, and the mouth of the insect moves equally backwards and forwards, upwards and downwards, to right and left.—The third sort of articulation, by solid surfaces, takes place when the head, truncated posteriorly, and presenting a flat surface, is articulated, sometimes upon a tubercle of the thorax, and sometimes upon another flat and corresponding surface, as in almost all the Hymenoptera and the majority of the Diptera. The disposition of the fourth kind of articulation allows the head only the movement of the angular hinge (le seul mouvement de charnière angulaire). The only examples at present known are in some species of Attelabus F. The head of these insects terminates behind in a round tubercle, received in a corresponding cavity of the thorax: the lower margin of this cavity has a notch, and permits the movement of the head only in one direction[1175]."
2. The second kind of articulation, the ligamentous, he affirms takes place only in orthopterous and some neuropterous insects: "The head in this kind of articulation is only impeded in its movements towards the back, because it is stopped there by the advance of the prothorax; but below it is quite free. The membranes or ligaments extend from the circuit of the occipital cavity to that of the anterior part of the prothorax, which gives a great extent to the movement[1176]."
When I consider the well-deserved celebrity of the great author whose words I have here quoted, and the great and useful light that the genius and learning which conducted his patient labours and researches have thrown over every department of comparative anatomy,—a science he may be almost said to have founded,—I feel the most intire reluctance to differ in any thing from an authority so justly venerable to all lovers of that interesting study. But, however great my diffidence and hesitation to express an opinion at all opposed to his, the interests of truth and science require that I should state those particulars in which my own observations, made upon a careful examination of various insects of every Order, have led to results in some respects different from the above. "Aliquando bonus dormitat Homerus;" and if the Genius of Comparative Anatomy ever nodded, it sometimes happened when he was examining the structure of insects. An instance of this with regard to the mouth of the bee has been noticed by Mr. Savigny[1177]; and indeed it is not wonderful that in so extensive an undertaking, in which the number of examples to be examined upon every branch of his subject must be immense, that he did not always scrutinize minutely those that seemed less important. Every writer on every department of Natural History, especially where the objects of research, as in the insect world, are so infinite in number, will be liable to such mistakes; but these will meet with due allowance from every candid mind—
"Hanc veniam damus, petimusque vicissim:"
and I shall express my trust that you will overlook any errors of mine; and doubtless I shall not be free from them—
"————-Quas aut incuria fudit,
Aut humana parum cavit natura——"
The two kinds of articulation of the head which our learned author has stated as the principal ones, will, I think, be found upon examination not so widely distant as his expressions seem to indicate; for in fact in all insects, as well as the Orthoptera, this part is suspended by a membrane or ligament which unites the margins of the occipital cavity with those of the anterior one of the prothorax: thus forming all round some protection to the organs that are transmitted from the head through the latter part to the rest of the body. Though the head in most Orthoptera is not partly received into the cavity of the prothorax, as is the case in the Order Coleoptera, but is rather suspended to it, yet in some instances, for example in the mole-cricket (Gryllotalpa vulgaris), it is partially inserted.
Again: when, in his first mode of articulation by contact of solid parts, he speaks of one or two smooth tubercles of the neck, with their corresponding cavities in the prothorax, as forming the most common conformation, you would expect to find examples of this in very many insects; yet upon a close examination, unless in Oryctes nasicornis[1178], and perhaps in others of the Dynastidæ MacLeay, you would scarcely meet with any thing that could be called a tubercle and its corresponding cavity in the neck or prothorax of any Lamellicorn or Capricorn beetle that you might chance to examine. You would find, indeed, that the occiput was usually smooth and very slippery, as if lubricated; that in its margin were one or two notches (Myoglyphides), with muscles attached to them; that in the former of these tribes, the Lamellicorns, it projected on each side so as to form a more or less prominent angle; and that the throat (jugulum) was very convex, and lodged in a cavity of the lower margin of the prothorax: but further appearances of tubercles &c. you would in vain look for even in this tribe; and in the Capricorns you would find that the general conformation in this respect belonged to our learned author's second mode of solid articulation, resembling that of the majority of the weevils (Curculio L.), in which the head has no projecting angles or tubercles, or other elevation, but is received usually into the circular cavity of the prothorax.
His third mode of this articulation, that of the Hymenoptera and Diptera, is so peculiar, that it ought to be considered as a primary kind; since in this the head moves upon the prothorax as upon a pivot, and has a kind of versatile motion.
With regard to his fourth mode, which from his description appears to be that of Apoderus Oliv., he allows motion to the head only in one direction, observing that the lower margin of the prothoracic cavity has a notch. But M. Latreille calls the articulation of the head in this genus an Enarthrosis[1179], which admits motion in every direction; and if you examine the common species (A. Coryli), you will find that the prothorax has a sinus taken out of its upper margin, as well as out of its lower one—which at any rate will allow a motion upwards.
I merely mention these little inaccuracies, with due diffidence, as some apology for giving you a different and at least a more popular and general view of this part of my subject, which I shall now proceed to state to you. It seems to me most convenient for the Entomologist, and most consonant to nature, to divide insects, with respect to the articulation of the head with the trunk, into three primary sections, each admitting one or more subdivisions.
1. The first consists of those whose head inosculates more or less in the anterior cavity of the chest; and whose articulation, therefore, seems to partake in a greater or less degree of the ball and socket (Enarthrosis). The head, however, is often capable of being protruded from this cavity. If you take into your hand any common Harpalus that you may find under a stone, you will see, if pressed, that it can shoot forth its head, so as to be entirely disengaged from the prothorax: a neck of ligament intervening between them[1180]: of course this power of protruding the head enables the animal to disengage it at its will from the restriction imposed upon its motions by the surrounding margin of the prothoracic cavity. To this section belong all the Coleoptera, the Heteropterous Hemiptera (Cimex L., &c.), and some of the Neuroptera (Raphidia, Semblis, &c.).—It may be further divided into two subsections—those, namely, whose head inosculates in the prothorax by means of a neck: as for instance Latreille's Trachelides, Apoderus, and the Staphylinidæ, amongst the beetles; the Reduviadæ amongst the Heteropterous insects, and Raphidia in the Neuroptera; and those whose head inosculates in the prothorax without the intervention of a neck; as, the Petalocera, the aquatic beetles (Dytiscus, Hydrophilus, &c.), and most of the genus Curculio L. in the first of these orders, the great body of the Cimicidæ in the second, and Semblis, Corydalis, &c. in the third.
2. The second section consists of those insects whose head does not inosculate in the chest, but is merely suspended to it by ligament or membrane. To this belong most of the tribes of the Orthoptera Order, with the exception of the Mantidæ, the Dermaptera, the Homopterous Hemiptera, and such of the Aptera as have the head distinct from the prothorax.—This section admits of a triple subdivision: those, namely, whose head is wholly covered by the shield of the prothorax, as in Blatta L.; those whose head is partly covered by it, as Gryllotalpa, and other Gryllina; and those whose head is quite free, not being at all impeded in its motion by the prothorax, as the Dermaptera, Nirmus, Pediculus, &c.
3. The third section consists of those whose head is truncated posteriorly, and flat or concave, with a very small occipital aperture, and is attached to a neck of the prothorax upon which it turns, or is merely suspended to that part. This includes the Lepidoptera, Hymenoptera, Diptera, the Libellulina, &c. in the Neuroptera, and the Mantidæ in the Orthoptera. Three subsections at least, if not more, present themselves in this section: the first is, of those whose head is united to the prothorax, without the latter forming any neck. To this belong the Lepidoptera, Trichoptera? The second is of those the upper side of whose thoracic neck is ligamentous; and here you may range most of the Hymenoptera. The third is of those in whom it is a continuation of the ordinary integument. In this subsection the Diptera, Libellulina and Mantidæ will find their place. In this last section the head appears to turn upon the thorax as upon a pivot.
Before I finish what I have to say on the articulation of the head, I must direct your attention to the analogies that hold in this respect between the different tribes. Thus the Coleoptera are analogous to the Heteropterous Hemiptera; the Orthoptera, with the exception of the Mantidæ, to the Homopterous Hemiptera; the Mantidæ to the Libellulina; the Lepidoptera to the Trichoptera; the Hymenoptera to the Diptera, with a slight variation, and probably others might be traced.
viii. A word or two upon the motions of which the head of insects is capable. M. Cuvier, in the extracts lately laid before you, speaks of different powers of movement as the result of different configurations of the parts of the head. This probably is correct with regard to many cases; but a great deal will depend upon the power the insect has of protruding its head and disengaging its base from the restriction of the prothorax; for where, like the Harpali, Staphylini, &c. it is able to do this, it can probably move its head in every direction. It is only where the ligaments are less elastic, or allow of little tension, that its movements are confined; and few living insects have been sufficiently examined to ascertain how far this takes place. In those cases belonging to the third section of articulations, in which the head moves upon the thorax as upon a pivot, as is the case with Hymenoptera and Diptera, the movement is nearly versatile. I have seen a fly turn its head completely round, so that the mouth became supine and the vertex prone; and from the form and fixing of the head, it should seem that those of the Mantidæ were endued with the same faculty.
ix. The parts and appendages of the head are now in the last place to be considered. I shall begin with the Mouth, or rather the orifice in which the trophi or organs of manducation are inserted. In some insects, as was before observed, they occupy all the under-side of the head, as in the Arachnida, Myriapoda, &c; but in the great majority they fill an orifice in its anterior part, which in some instances, as in Lampyris, the Lepidoptera, Cimex L., Truxalis, appears to be nearly under the head; but in general it terminates that part, though it extends further below than above. In Chermes, a Homopterous genus, the promuscis is stated to be in the Antepectus, and consequently the mouth; but, as I shall endeavour to prove to you hereafter, this is a fallacy. In the males of the species of Coccus there is no mouth at all. In that of the elm (C. Ulmi) in lieu there are ten little shining points, arranged two before and two behind in a line, and three on each side in a triangle[1181]. It is to be observed that the orifice of which I am speaking is usually much smaller in those insects which take their food by suction, the Hemiptera, Lepidoptera, Diptera, &c., than in the masticating tribes. With regard to the real mouth, or that through which the food enters, I have nothing at present to observe, except that it lies between the upper-lip and tongue, is sometimes covered by a valve, as in Apis, Vespa[1182], &c., and is different in the masticators and suckers.
I shall next offer a few observations seriatim, as they stand in the Table, upon the organs of manducation; which, to avoid circumlocution, instead of Instrumenta cibaria, the name Fabricius gave them, I shall call trophi or feeders. It is upon these parts, you are aware, that the system of the celebrated Entomologist just mentioned is founded; and could they always, or even for the most part, be inspected with ease, they would no doubt afford characters as various and discriminative as those of the vertebrate animals. Differences in these parts indicate a difference in the mode in which the animal takes its food, and often in the kind of food, and sometimes in its general economy and habits,—circumstances which are powerful and weighty in supporting the claim of any set of animals to be considered as forming a natural genus or group. Trifling variations, however, of these parts, unless supported by other characters and qualities, ought not to have much stress laid upon them, since, if we insist upon these, in some tribes almost every species might be made a genus.
With respect to their trophi in general, insects of late have been divided into two great tribes[1183], masticators and suckers; the first including those that are furnished with instruments to separate and masticate their food; namely, an upper- and under-lip (labrum and labium), upper- and under-jaws (mandibulæ and maxillæ), labial and maxillary palpi, and a tongue (lingua): and the second those in which these parts are replaced by an articulate or exarticulate machine, consisting of several parts and pieces analogous to the above, which pierce the food of the animal, and form a tube by which it sucks its juices. If, however, the mode in which insects take their food be strictly considered, it will be found that in this view they ought rather to be regarded as forming three tribes; for the great majority of the Hymenoptera order, and perhaps some others, though furnished with mandibles and maxillæ, never use them for mastication, but really lap their food with their tongue: these, therefore, might be denominated lappers.
When a mouth is furnished with the seven ordinary organs used in taking the food and preparing it for deglutition—I mean the upper-lip and the two upper-jaws; the under-lip and the two under-jaws, including the labial and maxillary palpi; and the tongue—I denominate it a perfect mouth; but when it is deficient in any of these organs, or they exist merely as rudiments, or when their place is supplied by others, (which, though they may be analogous parts, have little or no connection with them in their use or structure,) I denominate it an imperfect mouth. This last I would further distinguish, according to the nature of its trophi, by other and more distinctive terms, as I shall presently explain to you.
1. Labrum[1184].—I shall first consider the organs present in a perfect mouth, beginning with the upper-lip (labrum). This part, which Fabricius sometimes confounded with the nose, miscalling it clypeus, is usually a moveable[1185] piece, attached by its base to the anterior margin of the part last named, and covering the mouth, and sometimes the mandibles, from above. In insects in their last state it is usually of a horny or shelly substance; yet in some cases, as in Copris and Cetonia, beetles that imbibe juices, it is membranous. In form and shape it varies greatly, being sometimes nearly square, at others almost round; in some insects representing a parallelogram, in others a triangle, and in many it is oblong. In some instances it is long and narrow, but more generally short and wide. It is often large, but occasionally very minute. In the majority it has an intire margin, but it is not seldom emarginate or bilobed, or even dentate. Its surface is commonly even, but it is sometimes uneven, sometimes flat, at others convex, and in a few species armed with a short horn or tubercle[1186]. As to its pubescence, it is often naked, but now and then fringed or clothed with down or hairs, or sprinkled with bristles. It consists in almost every instance of a single piece; but an exception to this occurs in Halictus, a little bee, in the females of which it is furnished with a slender appendage[1187].—The direction of the upper-lip is various. It is rarely horizontal, or in the same line with the nose, often vertical; it sometimes forms an obtuse angle with the anterior part of the head, and occasionally an acute one, when it is more or less inflexed. The use of this part is ordinarily to close the mouth from above, to assist in retaining the food while undergoing the process of mastication; but in many Hymenopterous insects its principal use seems to be, to keep the trophi properly folded; and in some cases where it is inflexed, as in the leaf-cutter bees (Megachile Latr.), to defend its base, while the mandibles are employed, from injury by their action[1188].
2. Labium[1189].—On the under-side of the head, and opposed to the upper-lip, the mouth is closed by another moveable organ, concerning the nomenclature and analogies of which Entomologists have differed considerably. At the first view of it, this organ seems a very complex machine, since the under-jaws or maxillæ are attached to it on each side, and the tongue is often seen to emerge from it above, so as to appear merely a part of it; but as the former answer to the upper-jaws, and the latter is the analogue of the part bearing the same name in the vertebrate animals, I shall consider these as distinct and primary organs, and treat of the under-lip (labium) of which I am now speaking, by itself. Linné takes no notice of this part, but his illustrious compatriot and cotemporary, De Geer, did not so overlook it: he appears to consider the whole apparatus, including the maxillæ, as the labium[1190]; but sometimes he distinguishes the middle piece by that name[1191]; and the tongue, in the case of the stag-beetle, he denominates a proboscis (trompe)[1192]. In the Hymenoptera he calls this part tongue, under-lip, and proboscis: but seems to prefer the last term[1193]. Fabricius originally regarded the whole middle piece as a labium[1194]; but afterwards (though his definition is not accurate, since he assigns the palpi to the ligula, which he affirms is covered by the labium—circumstances by no means universal in Coleoptera) he considers this as consisting of ligula and labium[1195]. Latreille at first regarded the ligula of Fabricius as the labium, and called the labium of that author the mentum[1196]; but afterwards he gave the name of labium to the whole middle piece of the lower apparatus of the mouth—calling the upper piece, with Fabricius, the ligula, and retaining the denomination of mentum for the lower[1197].
If the circumstances of the case are duly considered, I think you will be convinced that the term under-lip, or labium, should be confined to that part which the learned Dane so named. For I would ask, Which is the part on the under side of the head that is the antagonist, if I may so speak, of the upper-lip or labrum? Is it not that organ which, when the mouth is closed, meets that part, and in conjunction with it shuts all in? Now in numerous insects, particularly the Lamellicorn beetles (Scarabæus and Lucanus L.), this is precisely the case. In the Predaceous beetles, indeed, (Cicindela, Carabus, Dytiscus L. &c.) the under-lip does not meet the upper, to close the mouth and shut in the tongue; neither can the tongue be said so to do, but only, from some circumstances connected with its manner of taking its food, it is requisite that the last-mentioned organ should not be retractile or covered; but its miscalled mentum is still the analogue of that part which helps to close the mouth in the former tribe. Should not this, therefore, which so often performs the office, be distinguished by the name, of a lip? Again, is it not rather incongruous to consider that organ which confessedly more or less performs the office of a tongue, as a part of the lip? Though it often wears that appearance, yet I believe, if the matter is thoroughly and patiently investigated, it will be found that on their upper side its roots are attached to the interior of the upper side of the head, as well as on their lower side to the labium; so that it may be regarded as common to the two lips, and therefore not properly considered as an appendage of the under-lip alone.
Having assigned my reasons for preferring the name given to the part in question by Fabricius, rather than that of Latreille, I shall next make my observations on the part itself. In many cases the labium, or the middle piece of the lower oral apparatus, appears to consist of two joints: this you may see in the great water-beetle (Hydrophilus piceus), the burying-beetles (Necrophorus), the Orthopterous tribes[1198], the Hymenoptera[1199], and others. In this case the upper or terminal piece is to be regarded as the labium, and the lower or basal one (which Mr. MacLeay calls the stipes) as the mentum or chin—at other times, as in some Lamellicorn beetles, the only separation is a transverse elevated line, or an obtuse angle formed by the meeting of the two parts, and very frequently there is no separation at all, in which case the whole piece, the mentum merging in it, may be denominated the labium.
With respect to its substance, the labium in most Coleopterous insects is hard and horny, in Necrophorus it is membranous, and the mentum harder; in Prionus coriarius, our largest Capricorn-beetle, both are membranous; in the bee-tribes, Apis L., the labium rather resembles leather, while the mentum is hard. Its surface is often convex, sometimes plane, and sometimes even concave; as for instance in Melolontha Fullo, a rare chafer occasionally found on the coast of Kent. In some it is covered with excavated points; in others it is quite smooth. In numbers, as in the Predaceous beetles, both labium and mentum are perfectly naked; in others, as in the common cockchafer, they are hairy; in Geniates barbatus Kirby, another chafer in the male insect, the labium is naked, while the mentum, which forms a piece distinct from that part, is covered with a dense rigid beard[1200]. In shape the whole labium varies considerably, much more than the labrum; for in addition to most of the forms I enumerated when I described that organ, which I shall not here repeat, you may meet with examples of many others. Thus, to instance in the Petalocerous tribes (Scarabæus L.), in some, as in the Rutelidæ, the labium is urceolate, or representing in some degree the shape of a pitcher[1201]; in others it is deeply concave, and not a little resembles a basin or a bowl[1202]; this form is peculiar to the labium of Cremastocheilus Knoch, a scarce North American beetle; in another related to this, but of an African type (Genuchus Kirby MS. Cetonia cruenta F.), it is a trapezoid plate, which is elevated from the head, and hangs over the throat like a chin[1203]. In the Hymenoptera it is extremely narrow and long, and embraces the sides of the tongue, as well as covering it from below; so that it wears the appearance of a kind of tube[1204]. Generally speaking, the length of the labium exceeds its breadth; but in the Predaceous beetles the reverse of this takes place, it being very short and wide, and usually terminating towards the tongue in three lobes or teeth which form two sinuses varying in depth[1205].
The mentum taken by itself affords no very striking characters to which I need call your attention: I shall only observe, that in Hymenoptera it is generally of a triangular shape[1206]; but before I proceed to consider the labial palpi, it will be proper to notice the remarkable labium of Orthopterous insects, and of the Libellulina, between which there is no little analogy. At first you would imagine the terminal part of this organ in the former to be the analogue of the tongue, or ligula F.; as it is indeed generally regarded by modern Entomologists[1207]. It seems, like the tongue of the Carabi L., Dytisci, &c., to be a distinct piece, which has below it both labium and mentum; but when you look within the mouth, you will find a linguiform organ[1208], which evidently acts the part of a tongue, and therefore ought to have the name; and the piece just alluded to must either be regarded as the termination of the lip, or as an external accompaniment of the tongue, analogous, it may be, to the paraglossæ in bees[1209]. In a specimen of Acrida viridissima (Locusta F.) which I dissected, the tongue is as long as the appendage of the under-lip, and by its upper surface seems to apply closely to it. In the Libellulina a similar organ is discoverable[1210], which on its upper-side terminates in the pharynx, like that of one of the Harpalidæ before mentioned. In the Orthoptera, therefore, I regard the labium as consisting of three articulations, the upper one divided into two, three, or more lobes[1211]; the intermediate one more directly answering to the labium of other insects, and the basal one or mentum. This organ in the Libellulina is of a different structure: it has only two articulations representing labium and mentum; but the former consists of three parallel pieces, the two exterior ones rising higher than the intermediate one, and at their inner angle having an acute sinus from which the palpi emerge; and the intermediate piece, which is longitudinally channelled, lapping over the inner side of the lateral pieces. From the angle of the covered part of these pieces, a subulate short horizontal horn points inwards towards the tongue, which it must keep from closing with the labium[1212].
3. Palpi Labiales[1213].—The last-mentioned organs, the labial palpi, next claim our attention; but before I advert particularly to them, it will be proper to premise a few words upon palpi, or feelers, in general. These are usually jointed moveable organs, of a corneous or coriaceous substance, attached by ligaments to the labium and maxillæ, and in the Crustacea even to the mandibulæ. Their joints, which are usually more or less obconical, articulate also in each other by ligaments, with perhaps some little mixture of the ball and socket. Their ends, the last joint especially, seem furnished with nervous papillæ which indicate some peculiar sense, of which they are the instrument. What that sense is has not been clearly ascertained, and concerning which I shall enter more into detail in another place. Their motion seems restrained, at least in some, to two directions, towards and from the mouth. They were called palpi or feelers, because the insect has been supposed to use them in exploring substances. There seem to be no organs in the vertebrate animals directly analogous to the palpi of insects and Crustacea, unless, perhaps, the cirri that emerge from the lips of some fishes, as the cod, red mullet, &c. which Linné defines as used in exploring (prætentantes). Whether the vibrissæ, miscalled smellers, of some quadrupeds and birds have any reference to them, I will not venture to affirm; but the feline tribe evidently use their bristles as explorers, and they are planted chiefly in the vicinity of the mouth.
Having made these general remarks, I shall now confine myself to the labial palpi. I call them labial palpi, because that term is in general use, and because in many cases they really do emerge from what I consider as the labium, as in most of the chafers; but they might with equal propriety be denominated lingual palpi, since they sometimes appear to emerge from the tongue as in the stag-beetle (Lucanus Cervus). In some instances, as in the Predaceous beetles, they seem to be common to both labium and tongue, being attached at their base on the upper side to the former, and on the under side to the latter. As to their situation: they emerge from the base of the labium in the locusts (Locusta Leach)[1214]; from its middle in Hister maximus[1215]; from its summit in Amblyterus MacLeay[1216]; and from its lateral margin in Dynastes MacLeay, &c. They consist of from one to four joints; which, I believe, they never exceed. They vary in length; though generally shorter than the maxillary palpi, yet in the ferocious tiger-beetles (Cicindela L.) they equal them in length; and in the hive-bee and humble-bees, and many other bees, they are considerably longer[1217]. The two first joints of these palpi, however, in these bees are different in their structure from the two last, being compressed and flat, or concave; and the two last joints, instead of articulating with the apex of the second, emerge from it below the apex: so that these two first joints seem rather elevators of the palpi than really parts of them[1218]. With respect to the relative proportions of their joints to each other: in some cases the first joint is the longest and thickest, the rest growing gradually shorter and smaller[1219]; in others, the second is the longest[1220]; in others, again, the third[1221], and sometimes the last[1222]; and often all are nearly of the same size and length[1223]. They are more commonly naked, but sometimes either generally or partially hairy. Thus in Cicindela, the last joint but one is usually planted with long snow-white bristles in a double series, while the rest of the joints have none; and in Copris Latr. all of them are extremely hairy. In shape they do not vary so much as the maxillary palpi, being most frequently filiform or subclavate, and sometimes setaceous; the last joint varies more in shape than the rest, and is often remarkably large, triangular, and shaped like the head of a hatchet[1224]; and at others it resembles the moon in her first quarter[1225]. In the great dragon-fly, or demoiselle if you prefer the gentler French name (Æshna F.) the labial palpi, which are without any visible joints, are terminated by a minute mucro or point[1226]. With regard to their direction and flexure, they frequently, as in the instance just mentioned, turn towards each other, and lie horizontally upon the end of the labium. Sometimes, as in the Cicindelidæ, they appear to point towards the tail of the insect, the last joint rising, and forming an angle with the rest of the feeler. In other instances they diverge laterally from the labium, the last joint turning again towards it at a very obtuse angle.
4. Mandibulæ[1227].—Having considered the analogues of the lips in our little beings, I must next call your attention to the representatives of the jaws. The vertebrate animals, you know, are mostly furnished with a single pair of jaws, one above and the other below, in which the teeth are planted and which have a vertical motion. But insects are furnished with two pair of jaws, a pair of upper-jaws and a pair of under-jaws, which have no teeth planted in them, and the motion of which is horizontal.—I shall begin with an account of the upper-jaws. These by modern Entomologists, after Fabricius, are denominated mandibles (mandibulæ): a term appropriated by Linné to the beaks of birds. The upper-jaws of insects this great naturalist named maxillæ—and not improperly, since the office of mastication is more peculiarly their office than that of the under-jaws, which Fabricius has distinguished by that name: as the term mandible, however, is generally adopted, I shall not attempt to disturb it.
The mandibles close the mouth on each side under the labrum or upper-lip. They are generally powerful organs, of a hard substance like horn; but in the Lamellicorn beetles of Mr. MacLeay's families of Scarabæidæ and Cetoniadæ, they are soft, membranous, and unapt for mastication. In Coleopterous insects they commonly articulate with the head by means of certain apophyses or processes, of which in many cases there are three discoverable at the exterior base of the mandibles; one, namely, at each angle, and one in the middle. That on the lower side is usually the most prominent, and wears the appearance of the condyle of a bone: it is received by a corresponding deep socket (or cotyloid cavity) of the cheek, in which, being perfectly smooth and lubricous, it moves readily, but without synovia, like a rotula in its acetabulum. The upper one projects from the jaw, forms the segment of a circle, and is concave also on its inner face. A corresponding more shallow, or, as anatomists speak, glenoid cavity of the cheek, where it meets the upper-lip, receives it, and the concave part admits a lubricous ball projecting from the cheek, upon which it turns[1228]. This structure you will find in the stag-beetle, and some other timber-devourers. Other Coleoptera have only a process of a similar structure at each of the dorsal angles of the base of the mandible, the intermediate one being wanting; and the articulation does not materially differ, as far as I have examined them, in the Hymenoptera and Neuroptera. In the Orthoptera, the structure approaches more nearly to that of the stag-beetle, since there are three prominences: it is thus well described by M. Marcel de Serres: "This articulation," says he, "takes place in two ways. At first, in the upper surface of the mandible, and at its base, may be observed two small prominences and a glenoid cavity; these prominences are received in two glenoid cavities excavated in the shell of the front, as the cavity of the mandible receives a small prominence of the same part. Below the mandible, and at its base, there is a kind of condyle, which has its play in a cotyloid cavity excavated in the shell of the temple, far below the eye, and at the extremity of the coriaceous integument of the head[1229]." Within the head in this order, at least in Locusta Leach, is a vertical septum, which divides the head into two chambers, as it were, an occipital and a frontal, consisting of a concave triangular stem, terminating in two narrower concave triangular branches, so as to resemble the letter Y, and forming three openings, an upper triangular one, and two lateral subquadrangular ones, which last are the cavities that receive the base of the mandibles. This partition, which I would name Cephalophragma, doubtless affords a point of attachment to many of the muscles of the head. It does not appear to have been noticed, unless it be synonymous with the intermaxillary arcade of Marcel de Serres[1230]. Probably a corresponding support to the muscles, &c. may exist, as we have seen it does in Vespa L.[1231], in many other heads of the different Orders, which have not yet fallen under examination. Many mandibles, as those of the hornet &c., appear to be suspended to the cavity of the head on the inside by a marginal ligament sufficiently relaxed to admit of their play: those of the Orthoptera, M. Marcel de Serres informs us, are united to the head by means of two cartilages, the outermost being much the shortest, to which their moving muscles are attached. These he considers as prolongations of the substance of the mandible[1232]. The bottom of mandibles, when cleared of the muscles &c., inclines almost universally to a triangular form; but in some cases, as in the stag-beetle, it is nearly a trapezium. I cannot conclude this subject without noticing the motions of the mandibles. What the author lately quoted has said with regard to those of the Orthoptera, will, I believe, apply equally well to all the mandibulate orders. "The articulation of mandibles with the skull appears to take place by two points solely; and as these parts only execute movements limited to a certain direction, they may be referred to ginglymus[1233].—The movements of mandibles are limited to those from within outwards, and from without inwards[1234]." Whether they are restricted from any degree of vertical motion, has not yet been proved, as the jaws of vertebrate animals move horizontally as well as vertically—so those of insects may have some motion vertically as well as horizontally; and it seems necessary for some of their operations that they should. I am not anatomist enough to speak with confidence on the subject, but the ball and socket articulation at the lower part of the mandible, and the curving one at the upper, though a kind of ginglymus, seems to imply a degree of rotatory movement, however slight.
I must next say something upon the general shape of these organs. Almost universally they incline to a triquetrous or three-sided figure, with their external surface convex, sometimes partially so, and their internal concave. Most frequently they are arched, curving inwards; but sometimes, as in Prionus octangularis[1235], a Capricorn beetle, and others of that genus, they are nearly straight; and in Rhina barbirostris[1236], a most remarkable Brazilian weevil, their curvature is outwards. In Pholidotus lepidotus MacLeay, and Lucanus Elephas, two insects of the stag-beetle tribe, they are bent downwards; and in Lucanus nebulosus K. (Ryssonotus MacLeay) they turn upwards[1237]. They are usually widest at the base, and grow gradually more slender to the apex, but in the hornet (Vespa Crabro) the reverse takes place, and they increase in width from the base to the apex; and in the hive-bee, and others of that tribe, they are dilated both at base and apex, being narrowest in the middle; others are nearly of the same width every where. In those insects that use their mandibles principally for purposes connected with their economy, they are often more broad in proportion to their thickness, than they are in those which use them principally for mastication. In the locust tribes (Locusta Leach), they are extremely thick and powerful organs, and fitted for their work of devastation; but in the glow-worm (Lampyris), they are very slender and minute. In those brilliant beetles, the Buprestes, they are very short; but in the stag-beetles, and those giants in the Capricorn tribe, the Prioni, they are often very long[1238]. They either meet at the summit, lap over each other, cross each other, or are protended straight from the head; as you have doubtless observed in the stag-beetle, whose terrific horns are mandibles of this description. These organs are usually symmetrical, but in some instances they are not: thus in Hister lævus, a kind of dung-beetle, the left hand mandible is longer than the right; in Creophilus maxillosus K. (Staphylinus L.), a common rove-beetle, in the left hand mandible the tooth in the middle is bifid, and in the right hand one intire; and in Bolbocerus K. the mandible of one side, in some species the dexter, and in others the sinister, has two teeth, and the other none.
The next circumstance with respect to these organs which demands our attention, is the teeth with which they are armed. These are merely processes of the substance of the mandible, and not planted in it by gomphosis[1239], as anatomists speak, as they are in vertebrate animals. They have, however, in their interior, at the base at least, in the Orthoptera, a coriaceous lamina that separates them in some sort from the body of the mandible[1240]. Many insects, however, have mandibles without teeth; some merely tapering to a sharp point, others obtuse at the end, and others truncated[1241]. Of those that have teeth, some have them on the inside at the base, as Manticora, an African tiger-beetle[1242]; others in the middle, as Staphylinus olens, a rove-beetle, Lethrus cephalotes, &c.[1243]; others at the end, as many weevils (Curculio L.)[1244]; others again on the back, as the Rutelidæ, a tribe of chafers[1245], and Lethrus, a beetle just named; others once more on the lower side of the base, in the form of a tooth or spine, as in Melitta spinigera, a species of wild-bee, and some of its affinities[1246]; and lastly, others on the upper side of the base in the form of a long tortuous horn, as in that singular wasp Synagris cornuta F. before noticed as a sexual character[1247]. In the stag-beetle tribes (Lucanus L.) these teeth are often elongated into short lateral branches, or a terminal fork[1248]. They are sometimes truncated, sometimes obtuse, and sometimes acute.
But with regard to their kind, it will be best to adopt the ideas of M. Marcel de Serres; for though his remarks are confined to the Orthoptera, they may be applied with advantage to the teeth that arm the mandibles of insects in general. He perceives an analogy between those of this Order and the teeth of quadrupeds; and therefore divides them into incisive or cutting, laniary or canine, and molary or grinding teeth. He denominates those incisives that are broad, having in some degree the shape of a wedge, their external surface being convex, and their internal concave; whence they are evidently formed for cutting. The laniaries are those which have a conical shape, are often very acute, and in general the longest of any; and in some insects, as the carnivorous Orthoptera (and the Libellulina), they cross each other. The molaries are the largest of all, and their purpose is evidently to grind the food. There is never only a single one to each mandible, while the number of the incisives and laniaries is very variable. As the molaries act the principal part in mastication, they are nearer the inner base of the mandible or point of support: they serve to grind the food, which has been first divided by the incisives or torn by the laniaries. The carnivorous tribes are destitute of them; in the omnivorous ones they are very small, and in the herbivorous ones they are very large[1249]. So that in some measure you may conjecture the food of the animal from the teeth that arm its mandibles. Of incisive teeth you may find an example in those that arm the end of the mandibles of most grasshoppers (Locusta), and of the leaf-cutter-bees (Megachile Latr.)[1250]; of the laniary or canine teeth, you will find good examples in the mandibles of the dragon-flies (Libellulina); the two external teeth of the apex of those of the leaf-cutter bees may be regarded as between the incisives and laniaries; and the pointed mandibles without teeth may be deemed as terminating in a laniary one[1251]. The lower part of the inner or concave surface of the mandibles of grasshoppers will supply you with instances of the molary teeth, and the apex, also, of those of some weevils, as Curculio Hancocki K.[1252] But the most remarkable example of a molary organ is exhibited by many of the Lamellicorn beetles, especially those that feed upon vegetables, whether flower or leaf.—Knoch, who indeed was the first who proposed calling mandibles according to their teeth, incisive, laniary, or molary, but who does not explain his system clearly, observed that the mandibles of some Melolonthæ have a projection with transverse, deep furrows, resembling a file, for the purpose of bruising the leaves they feed upon[1253]: and M. Cuvier, long after, observed that the larvæ of the stag-beetle have towards their base a flat, striated, molary surface; though he does not appear to have noticed it in any perfect insect[1254]. This structure, with the exception of the Scarabæidæ and Cetoniadæ, seems to extend very generally through the above tribe; since it may be traced even in Geotrupes, the common dung-chafer, in which at the base of one mandible is a concave molary surface, and in the other a convex one, but without any furrows: a circumstance that often distinguishes those that have furrows.—In the Dynastidæ the affinity of structure with the Melolonthidæ &c. is more pronounced, the furrows to which ridges in the other mandible correspond being reduced to one or two wide and deep ones; whereas in some of the latter tribe they are very numerous. These mandibles, in many cases, at their apex are furnished with incisive teeth to cut off their food, and with miniature mill-stones to grind it[1255]. The part here alluded to I call the Mola.
Were I to ask you what your idea is with regard to the use of the organs we are considering, you would perhaps reply without hesitation, "Of what possible use can the jaws of insects be but to masticate their food?" But in this you would in many instances be much mistaken; as you will own directly if you only look at the mandibles of the stag-beetle—these protended and formidable weapons, as well as those of several other beetles, cannot be thus employed. "Of what other use, then, can they be?" you will say. In the particular instance here named, their use, independent of mastication, has not been satisfactorily ascertained; but in many other cases it has. Recollect, for instance, what I told you in a former letter, of those larvæ that use their unguiform mandibles as instruments of motion[1256]. Again: amongst the Hymenopterous tribes, whose industry and varied economy have so often amused and interested you, many have no other tools to aid them in their various labours and mechanical arts: to some they supply the place of trowels, spades, and pick-axes; to others that of saws, scissors, and knives—with many other uses that might be named. In fact, with the insects of this intire Order mastication seems merely a secondary, if it is at any time their use. Still comprehending in one view all the mandibulate Orders, though some use their mandibles especially for purposes connected with their economy, yet their most general and primary use is the division, laceration, and mastication of their food; and this more exclusively than can be affirmed of the under-jaws (maxillæ). This will appear evident to you, when you consider that insects in their larva state, in which universally their primary business is feeding, with very few exceptions use the organs in question for the purpose of mastication, even in tribes, as the Lepidoptera, that have only rudiments of them in their perfect state—while the maxillæ ordinarily are altogether unapt for such use. The exceptions I have just alluded to are chiefly confined to the instance of suctorious mandibles; or those which, being furnished at the end with an orifice, the animal inserting them into its prey, imbibes their juices through it. This is the case with the larvæ of some Dytisci, Hemerobius, and Myrmeleon[1257]; and spiders have a similar opening in the claw of their mandibles, which is supposed to instil venom into their prey[1258].
Under this head I must not pass without notice an appendage of the mandibles, to be found in some of the rove-beetles (Staphylinidæ), as in Ocypus, Staphylinus, and Creophilus Kirby. In the first of these it is a curved, narrow, white, subdiaphanous, submembranous, or rather cartilaginous piece, proceeding from the upper side of the base of the mandible[1259]; in the second it is broader, straighter, and fringed internally and at the end with hairs; and in this at first it wears the appearance of being attached laterally to the mandible under the tooth[1260], but if closely examined, you will find that it is separate: in Creophilus maxillosus it is broader. This is the part I have named prostheca. It is perhaps useful in preventing the food from working out upwards during mastication.
5. Maxillæ[1261]. The antagonist organs to the mandible in the lower side of the head, are the under-jaws, or maxillæ—so denominated by the illustrious Entomologist of Kiel. Linné appears to have overlooked them, except in the case of his genus Apis, in which he regards them, and properly, as the sheath of the tongue. De Geer looked upon them in general as part of the apparatus of the under-lip or labium; and such in fact they are, as will appear when we consider them more particularly. Fabricius has founded his system for the most part upon these organs, the principal diagnostic of ten out of his thirteen Classes (properly Orders) being taken from them; and in the modern, which may be termed the eclectic, system, although the Orders are not founded upon them, yet the characters of genera, and sometimes of large tribes, are derived from them: and as they appear less liable to variation than almost any other organ, as Mr. W. S. MacLeay has judiciously observed, there seems good reason for employing them—it is therefore of importance that you should be well acquainted with them.
Their situation is usually below each mandible, on each side of the labium; towards which they are often somewhat inclined, so that their tips meet when closed. In some cases, as in the Predaceous beetles (Carabus L. &c.), they exactly correspond with the mandibles; but in others their direction with respect to the head is more longitudinal, as in the Hymenoptera, &c. In substance they may be generally stated to be less hard than those organs; yet in some instances, as in the Libellulina, Anoplognathidæ, &c. they vie with them, and in the Scarabæidæ and Cetoniadæ exceed them, in hardness. In the bees, and many other Hymenoptera, they are soft and leathery. Their articulation is usually by means of the hinge on which they sit: it appears entirely ligamentous, and they are probably attached to the labium at the base, or mentum—at least this is evidently the case with the Hymenoptera, in which the opening of the maxillæ pushes forth the labium and its apparatus. In that remarkable genus related to the glow-worms, now called Phengodes (Lampyris plumosa F.), and in the case-worm flies (Trichoptera K.), the maxillæ appear to be connate with the labium, or at least at their base.—As to their composition, these organs consist of several pieces or portions. At their base they articulate with a piece more or less triangular, which I call the hinge (Cardo)[1262]. This on its inner side is often elongated towards the interior of the base of the labium, to which it is, as I have just observed, probably attached. This elongate process of the hinge in Apis, Bombus, &c. appears a separate articulation; and the two together form an angle upon which the mentum sits[1263], and by this the maxilla acts upon the labial apparatus.
The next piece is the stipes or stalk of the maxilla. This is the part that articulates with the hinge, and may be regarded in some cases, as in the Orthoptera &c., as the whole of the maxilla below the feeler; and in others, as in the Geotrupidæ, Staphylinidæ &c., as only the back of it, the inside forming the lower lobe. This piece is often harder and more corneous than the terminal part, is linear, often longitudinally angular, and in the bee-tribes (Apis L.) is remarkable on its inner side for a series of bristles parallel to each other like the teeth of a comb[1264]. In Pogonophorus Latr., a kind of dor or clock-beetle, it is armed on the back with four jointed spines, the intermediate one being forked[1265]. M. Latreille has thus described the stipes of the maxillæ of Coleoptera: "Next comes the stalk," says he, "which consists of three parts: one occupies the back and bears the feeler; the second forms the middle of the anterior face, and its figure is triangular; the third fills the posterior space comprised between the two preceding; and is that which is of most consequence in the use of the maxilla; the anterior feeler, where there are two, the galea, and the other appendages that are of service in deglutition, are part of that piece[1266]."
The third and terminal portion of the maxilla is formed by the lobe, or lobes (Lobi). This may be called the most important part of the organ, since it is that which often acts upon the food, when preparing for deglutition. When armed with teeth or spines at the end, its substance is as hard as that of the mandibles; but when not so circumstanced, it is usually softer, resembling leather, or even membrane[1267]; and sometimes the middle part is coriaceous, and the margin membranous. This part is either simple, consisting only of one lobe, as you will find to be the case with the Hymenoptera, Dynastidæ, Nemognatha, and several other beetles; or it is compound, consisting of two lobes. In the former case, the lobe is sometimes very long, as in the bee tribes, and the singular genus of beetles mentioned above[1268], Nemognatha; and at others very short, as in Hister, &c. The bilobed maxillæ present several different types of form. Nearest to those with one lobe are those whose lower lobe is attached longitudinally to the inner side of the stalk of the organ, above which it scarcely rises. Of this description is the maxilla in the common dung-beetle (Geotrupes stercorarius), and rove-beetle (Staphylinus olens).[1269] Another kind of formation is where the lower lobe is only a little shorter than the upper: this occurs in a kind of chafer (Macraspis tetradactyla MacLeay).[1270] A third is where the upper lobe covers the lower as a shield; as you will find in the Orthoptera order, and the Libellulina, and almost in Meloe[1271]. A fourth form is where the upper lobe somewhat resembles the galeate maxilla just named; but consists of two joints. This exists in Staphylinidæ, &c.[1272] The last kind I shall notice is when the upper lobe not only consists of two joints, but is cylindrical, and assumes the aspect of a feeler or palpus[1273]. This is the common character of almost all the Predaceous beetles (Entomophagi Latr.). This lobe, which has been regarded as an additional feeler, is strictly analogous to the upper lobe in other insects, and therefore should rather be denominated a palpiform lobe than a palpus. Where there are two lobes, the upper one is most commonly the longest; but in many species of the tribe last mentioned the lower one equals or exceeds it in length[1274].
The lobes vary in form, clothing, and appendages. The upper palpiform lobe in those beetles just mentioned, in general varies scarcely at all in form; but the genus Cychrus (which is remarkable for a retrocession from the general type of form of the Carabi L. making an approach towards that of those Heteromera which, from their black body and revolting aspect, Latreille has named Melosomes,) affords an exception, the upper joint being rather flat, linear-lanceolate, incurved, and covering the lower lobe[1275], which it somewhat resembles. The lower lobe also in this tribe varies as little as the upper, being shaped like the last joint of that lobe in Cychrus just described, except that in Cicindela it is narrowest in the middle[1276]. In other tribes the upper valve is sometimes linear and rounded at the apex, and the lower truncated, as in Staphylinus olens[1277]; sometimes the upper one is truncated or obtuse, and the lower acute, as in Trogosita and Parnus[1278]. In Ptinus, another tribe of beetles, before noticed as injurious to our museums[1279], the reverse of this takes place, the upper-lobe, which is the smallest and shortest, being acute, and the lower truncated[1280]. In Blaps both are acute[1281]. In Rhipiphorus and Scolytus the lobes are nearly obsolete. The lower lobe is bifid in Languria, a North American genus of beetles, so as to give the maxilla the appearance of three lobes[1282]; and in Erotylus, a South American one, the upper is triangular[1283]: it is often oblong, quadrangular, linear, &c. in others.—In those that have only one lobe the shape also varies. In Gyrinus, the beetle that whirls round and round on the surface of every pool, which, though it belongs to the Predaceous tribe, has only one lobe, the lobe represents a mandible in shape of the laniary kind, being trigonal and acute[1284]; and in the Anoplognathidæ, a New Holland tribe of chafers, in which it is, as it were, broken, the lobe forming an angle with the stalk, it is concavo-convex and obtuse, and somewhat figures a molary tooth[1285]. In the first tribe into which the bees (Apis L.) have been divided (Melitta Kirby), the lobe is often linear or strap-shaped, and bifid at the apex; and in the second (Apis K.) lanceolate and intire[1286]. In Cerocoma it is long and narrow[1287]. More variations in form might be named, but these are sufficient to give you a general idea of them in this respect. With regard to their clothing, I have not much to observe—in examining the Predaceous beetles you will observe, that the interior margin of the lower incurved lobe is fringed with stiff bristles or slender spines, and in many other beetles either one or both lobes have a thick coating or brush of stiffish hairs[1288]; but in several cases only the apex of the lobe is hairy. In the Orthoptera order, and many of the Melolonthidæ or chafers, the whole maxilla is without hairs, or nearly so.
The appendages of the maxillæ are next to be noticed. These are principally their claws, or laniary teeth; for they are seldom armed with incisive or molary teeth. The whole tribe of Predaceous beetles, with few exceptions, have the inner lobe of their maxilla armed with a terminal claw, which in the Cicindelidæ articulates with the lobe, and is moveable, but in the rest of the tribe is fixed[1289]. In Phoberus MacLeay the lower lobe has two spines[1290]. In Locusta this lobe has three or four spines or laniary teeth, and in Æshna there are six, which, like the claw of Cicindela, are moveable[1291]. In others both lobes terminate in a single spine or claw: this is the case with Paxillus MacLeay[1292]. In Passalus, nearly related to the last genus, the upper lobe is armed with a single spine, and the lower one with two[1293]. Those maxillæ that terminate in a single lobe are also often distinguished by the spines or teeth with which it is armed; thus in a nondescript chafer belonging to the Dynastidæ (Archon K. MS.) it terminates in two short teeth; in that remarkable Petalocerous genus Hexodon Oliv. in three truncated incisive ones[1294]; in Dynastes Hercules in three acute spines[1295]. Four similar ones arm the apex of the maxilla in that tribe of Rutelidæ which have striated elytra; and five that are stout and triquetrous those of Melolontha Stigma F. Many others have six spines, sometimes arranged in a triple series[1296]. Besides teeth or spines, in some cases the lobes of maxillæ terminate in several long and slender laciniæ or lappets fringed with hairs. At least those of a Leptura (L. quadrifasciata L.) described by De Geer, appear to be thus circumstanced. He conjectures that this beetle uses its maxillæ to collect the honey from the flowers[1297].
As the principal use of the mandibles is cutting and masticating, so that of the organs we are considering seems to be primarily that of holding the food and preventing it from falling while the former are employed upon it. I say this is their primary use; for I would by no means deny that they assist occasionally in comminuting or lacerating it. In fact, were there no organs appropriated to this use, and if both mandibles and maxillæ were employed at the same time in comminuting the food, it seems to me that it must fall from the mouth. In a large proportion of insects the lobes of the maxillæ are not at all calculated for laceration or comminution; and in those tribes—as the Melolonthidæ, Rutelidæ, Dynastidæ—in which they seem most fitted for that purpose, the mandibles have incisive teeth at their apex, and at their base a powerful mola or grinder: circumstances which prove, that even in this case the business of mastication principally devolves upon them.
6. Palpi Maxillares[1298]. There is one circumstance that particularly distinguishes the maxillæ from the mandibles—they are palpigerous, as well as the under-lip. The feelers, or palpi, emerge usually from a sinus observable on the back of the maxillæ where the upper lobe and stalk meet. Their articulation does not materially differ from that of the labial palpi. Each maxilla has properly only one feeler; but, as was lately observed[1299], in certain tribes the upper lobe is jointed and palpiform, which has occasioned it to be considered as a feeler, and these tribes have been regarded as having six feelers. The most general rule with regard to the length of the palpi is, that the maxillary shall be longer than the labial; but the reverse often takes place. In many bees the maxillary consist only of a single joint, and are very short; while the labial consist of four, and are very long[1300]: and in some insects (as in Pogonophorus Latr.) the four palpi are of equal length[1301]. The antennæ are most commonly longer than the palpi; but in several aquatic beetles, as Elophorus, Hydrophilus, &c., whose antennæ in the water are not in use, the organs we are considering are the longest.—As to the number of their articulations, it varies from one to six; which number they are not known to exceed. In each of the Orders a kind of law seems to have been observed as to the number of joints both in the maxillary and labial palpi, but which admits of several exceptions. Thus in the Coleoptera, the natural number may be set at four joints for the maxillary, and three for the labial palpi: yet sometimes, as in Stenus, Notoxus, &c., the former have only three joints, and the latter, as in Stenus and Tillus, only two. In the Orthoptera the law enjoins five for the maxillary, and three for the labial; and to this I have hitherto observed no exception. In the Hymenoptera, the rule is six and four, but with considerable exceptions, especially as to the maxillary palpi, which vary from six joints to a single one: thus in the hive-bee and the humble-bee, the labials, including the two flat joints or elevators, have four joints, while the maxillaries are not jointed at all[1302]. In Chrysis, in which the latter consist of five, the former are reduced to three. The Libellulina may almost be regarded as having no maxillary palpi, since they exhibit no organ that is distinctly palpiform. It seems to me that the upper lobe of their maxilla, which articulates with the stalk in the same manner as a feeler, may be regarded as an instance in which that lobe and the feeler coalesce into one; and the mucro that proceeds from the lobe has the aspect of an emerging feeler, and corresponds somewhat with the labial one above noticed[1303]. In the remainder of the Neuroptera and the Trichoptera, the prevailing number is five and three. In the latter there are exceptions, which will furnish good characters for genera. In the Lepidoptera we find two, and sometimes three, the maxillary being very minute[1304]. The Diptera Order presents two tribes in this respect quite distinct from each other. The most natural number of joints in the maxillary palpi of the Tipulidæ, Culicidæ, &c. is four or five: the last joint, however, in Tipula, Ctenocera, &c. like that of the antennæ in Tabanus L., appears to consist of a number of very minute joints[1305]; but in the Asilidæ and Muscidæ, &c., the number two seems to be most prevalent[1306]. The labial palpi in this order are obsolete.—As to shape, the maxillary palpi, as well as the labial, are usually filiform; but in the weevil tribes (Curculio L.) they are most commonly very short and conical[1307]; in the chafers (Scarabæus L.) they usually are thickest at the apex[1308]; in Megachile and Euglossa, wild bees, they are setaceous, growing gradually more slender from the base to the summit[1309]: a tribe of small water-beetles (Haliplus), the saw-flies (Tenthredo L.), and several other Hymenoptera, have them thickest in the middle[1310]. Their most important part, however, and that which varies most in form, is the terminal joint:—of this I have already related some singular instances[1311], and shall now describe a few more. This joint is sometimes acute, at others blunt, at others truncated: in figure it is ovate, oblong, obtriangular, hatchet-shaped, lunate, transverse, conical, mammillate, subulate, branched, chelate, laciniate, lamellate, &c. &c.[1312]: terms which I shall more fully explain to you hereafter, and which I only mention here to show the numerous variations as to figure, of which this joint exhibits examples. The palpi in general at their vertex are often rather concave; and this concavity is formed by a thin papillose membrane, which it is supposed the animal has the power of pushing out a little, so as to apply it to surfaces. The primary use of the palpi of insects will be considered when I treat of their senses; but they probably answer more purposes than one. For instance, when I was once examining, under a lens, the proceedings of a species of Mordella, which was busily employed in the blossom of some umbelliferous plant, it appeared to me to open the anthers with its maxillary palpi, and they often held the anther between them: when not so employed, they were kept in intense vibration, more than even its antennæ; and at the same time, as far as I could judge, an Elater made the same use of them.
7. Lingua[1313].—This name was applied by Linné to the part in insects representing the tongue in vertebrate animals; and as it performs most of the common offices of a tongue, and the pharynx is situated with respect to it, as we shall presently see, nearly as it is in those animals, there seems no more reason for giving it a new name, than there is for giving a new name to the head or legs of insects, because in some respects they differ from those of the higher animals. I shall not therefore call it Ligula, with Fabricius and Latreille, nor Labium, with Cuvier and others, but adhere to the original term, which every one understands.
The tongue lies between the two lips—the labrum and labium. On its upper side, at the base, it meets the palate or roof of the mouth, below which it is attached, it may be presumed, by its roots to the crust of the head, on each side the pharynx or swallow; and on its lower side, in many cases, it is attached to the labium, and that very closely, so as to appear to be merely a part of it, and to form its extremity: but in the Orthoptera and Libellulina, it is more free, and in form somewhat resembling the tongue of the quadrupeds[1314].—In substance the tongue varies. In general it seems something between membrane and cartilage; but in the Predaceous beetles, in which it is not covered by the labium, it approaches nearer to the substance of the general integument, and in Anthia F. it is quite hard and horny:—that just mentioned of the Orthoptera and Libellulina is more fleshy[1315]. With regard to its station, in many cases, as in the instance just named, in the Lamellicorn tribe (Scarabæus L.) and others, it is, when unemployed, concealed within the mouth; the lips, mandibles, and maxillæ all closing over it. The tongue of some Hymenoptera also is retractile within the mouth. "When ants are disposed to drink," says M. P. Huber, "there comes out from between their lower jaws, which are much shorter than the upper, a minute, conical, fleshy, yellowish process, which performs the office of a tongue, being pushed out and drawn in alternately: it appears to proceed from the lower-lip.—This lip has the power of moving itself forwards in conjunction with the lower jaws: and when the insect wishes to lap, all this apparatus moves forward; so that the tongue, which is very short, does not require to lengthen itself much to reach the liquid[1316]." M. Lamarck thinks that the labium of insects has a vertical motion (de haut en bas ou de bas en haut)[1317]. This it certainly has in some degree; but it has also, as in the above case, a more powerful horizontal one, which is produced, in Hymenoptera at least, by the opening of the maxillæ—as I have already observed[1318].
I have little to say with respect to the structure of the tongue: it generally seems to be without articulations; but in many bees it articulates with the labium where it enters it, so as when unemployed to form a fold with it. In the hive-bee it terminates in a kind of knob or button, which has been falsely supposed to be perforated for imbibing the honey by suction. The upper part of this tongue is cartilaginous, and remarkable for a number of transverse rings: below the middle, it consists of a membrane, longitudinally folded in inaction, but capable of being inflated to a considerable size: this membranous bag receives the honey which the tongue, as it were, laps from the flowers, and conveys it to the pharynx[1319]. In Stenus this organ is retractile, and consists of two joints[1320].
The shape of the tongue of insects probably varies as much as any other part; but as it is apt to shrink when dried[1321], and is not easy to come at, we know but little of its various configurations:—in the bees it is very long, in most other insects very short. Though frequently simple and undivided, in many cases it presents a different conformation. Thus in the saw-flies (Tenthredo L.) it terminates in three equal lobes[1322]; in Stomis and Geotrupes in three unequal ones, the intermediate being very short[1323]; in Carabus, in three short teeth[1324]; in Pogonophorus it represents a trident[1325]; in the wasp it is bifid, each lobe being tipped with a callosity[1326]; in Melolontha Stigma it is bipartite[1327]; in Elaphrus, the analogue of the tiger-beetles, it terminates in a single tooth or point; in the aquatic beetles, Dytiscus L., it is quadrangular and without teeth[1328]; in some Ichneumonidæ it is concavo-convex, and forms a demitube; and in others it is nearly cylindrical[1329].
In many insects it has no hairs, but in the Predaceous beetles it generally terminates in a couple of bristles[1330]. In the hive- humble- and other bees, it is extremely hairy[1331]; a circumstance which probably enables it more effectually to despoil the flowers of their nectar. In Geotrupes stercorarius, the common dungchafer, and Melolontha Stigma lately mentioned, the lobes of the tongue are fringed with incurved hairs[1332]; and in Æshna it is hairy on the upper side, each hair or bristle crowning a minute tubercle. In many cases the tongue is attended, and sometimes sheathed at the base, by two usually membranous appendages:—these the learned Illiger has denominated paraglossæ; and I shall adopt his term. You will find them frequently attached to the tongue of the Predaceous beetles[1333], and to that of many Hymenoptera. In the hive-bee and humble-bee they are short, and take their origin within the labial feelers[1334]: in Euglossa, another bee, they are long, involute at the tips, and, what is not usual with them, very hairy[1335]: in the wasp, like the lobes of the tongue, they are tipped with a callosity.
Under this head I may observe to you, that the insects whose oral organs we are considering besides a tongue appear likewise to be furnished with a palate (Palatum). This, though a part of the roof of the mouth, is not precisely in the situation of the palate of vertebrate animals, since it seems rather the internal lining of the labrum. If you take the common dragon-fly (Æshna viatica), you will find that the under side of this part and of the rhinarium is lined with a quadrangular fleshy cushion, beset, like the upper surface of the tongue, with minute black tubercles, crowned with a bristle. This cushion is divided transversely into two parts by a depression; the anterior or outer piece being attached to the labrum, and the other piece to the rhinarium. The former has a central longitudinal cavity, black at the bottom, on the sides of which the tubercles are flat and without a bristle. From its base on each side a spiniform process emerges, forming a right angle with it. These processes seem the antagonists of those mentioned above[1336], that emerge from the labium. The posterior or inner piece has on each side a roundish space, attached to the under surface of the two sides of the rhinarium, beset also with bristle-bearing tubercles. You will find something similar lining the labrum and nasus of some Coleoptera,—say Geotrupes, Necrophorus, and Dytiscus. The first piece I regard as the analogue of the palate, and the second as connected with the sense of smelling. In Necrophorus the circular pieces are covered with a finely striated membrane, and in Dytiscus each has a little nipple.
8. Pharynx[1337].—On the upper side of the tongue, usually at its base or root, is the pharynx, or aperture by which the food passes from the mouth to the œsophagus. This orifice, which is situated with respect to the tongue of the Orthoptera and Libellulina nearly as in those insects (at least as far as I have been able to examine them), whose tongue is called a ligula or labium,—of course exists in all the mandibulate Orders whose mouth we are now considering. In the Hymenoptera it is covered by a valve, the Epipharynx of Savigny; and it appeared to me to be so likewise in one of the Harpalidæ that I examined. The formation seems different in Geotrupes, as far as I can get an idea of it; but it is so difficult to examine the interior of the mouth without laceration of some of the parts, that I can only tell you what the appearances were in one instance, upon removing the labrum from the mandibles; and in another, separating the whole apparatus of the labium, including the maxillæ, from the mandibles and labrum. In the former case, the mandibles coincided at the base, the two molary plates (molæ), which in this genus are narrow, transverse and not furrowed, are so applied as evidently to have an action upon each other, as the mandible opens and shuts, proper for trituration. Within these is the base of the tongue, under the form of a ventricose sack. The upper part of this last organ, which forms the internal covering of the labium, appears to consist of three (in the recent insect fleshy) lobes, the middle one being bent downwards internally, so as to form a kind of sloping cover to an orifice in the part I call the base. After two or three days, the tongue shrinks and dries to a hard substance;—between the mandibles and the base of the tongue I could not discover the pharynx. The above apparent opening covered by the tongue was the only one I could perceive. In the latter case, the form and structure of the base of the tongue is more visible: it is an oblong ventricose tubular sack, projecting above anteriorly into an acute angle formed by a fine white membrane, most beautifully and delicately striated with oblique striæ, to be seen only under a powerful lens: on the anterior side of this sack are two parallel cartilaginous ridges close to each other, fringed with short hairs, which take their origin from the angle. I could not be certain whether the orifice covered by the intermediate lobe was only apparent, or real; but I did not succeed in my endeavour to find any other pharynx, though from the molary structure of the base of the mandibles one may conjecture that there must be one situated at the base of this sack to receive the food they render after trituration. The excrement of this animal is not fluid. In the Libellulina the pharynx seems closed by two valves meeting. This part in Hymenoptera, and probably in other Orders, has the aspect of being cartilaginous and fitted to sustain the action of the substances that have to pass through it[1338].
The Epipharynx is a valve, called by M. Latreille sublabrum (sous labre[1339]), attached by its base to the upper margin of the pharynx, or that next the labrum. In the bees it is said by Reaumur to be of a fleshy substance, and capable of changing its figure. He seems to think it the real tongue of the bee[1340]; but as it does not appear to have any of the uses of a tongue, and merely closes the orifice of the mouth, it surely does not merit that name. M. Savigny calls it a membranous appendage which exactly closes the pharynx[1341]. De Geer has examined the epipharynx of the wasp, which he describes as of a scaly substance, and regards merely as the cover of the part just named[1342].
With regard to the Hypopharynx, which Latreille considers as a support and appendage of the epipharynx, I have little to add to the definition I have given of it above. In the Libellulina the base of the tongue terminates towards the pharynx in a fleshy cushion, armed at each angle next to that part with a short hard horn or tooth of a black colour. This cushion, I suppose, may be analogous to the hypopharynx of M. Savigny[1343]. On the opposite side the pharynx is closed by another fleshy cushion (epipharynx?), which appears to line the nose, behind those two mammillæ before described[1344], which form the internal covering of the rhinarium.
Before I call your attention to what I would denominate an imperfect mouth, in which some one or more of the seven organs above enumerated exist under another form, or only as rudiments,—I must say something upon the mouth of the Myriapods and Arachnida, in which there seem to be redundant organs of manducation.—M. Latreille, in the Essay lately quoted, in which, though some of his notions seem fanciful, he has shown a vast depth and range of thought and research, has asserted,—from the admirable and curious observations of M. Savigny, and those which since their publication he has made himself,—that the masticating organs of annulose animals (called by him condylopes) are a kind of legs[1345]. And M. Savigny, whose indefatigable labours and unparalleled acuteness have opened the door to a new and vast field in what may be denominated analogical anatomy,—has observed, that with certain Apiropods[1346] the organs that serve for manducation do not differ essentially from those which, with the other Apiropods and the Hexapods, serve for locomotion[1347]: and the unguiform mandibles of the larvæ of certain Diptera, you have before been told, are used not only in manducation, but also as legs[1348]. These remarks will satisfactorily prove to you, that organs which at first sight possess no visible affinity or analogy—as for instance, jaws and legs—may, if traced through a long series of beings, exhibit a very great one;—and will lessen your surprise when you find, that in certain tribes such commutations of organs and their use take place.
The following is the structure, as to its organs, of the mouth of the myriapods, as exhibited by the centipedes (Scolopendridæ). The part which appears to perform the office of the upper lip (but which M. Savigny regards as the nose, calling it the chaperon,) is a transverse piece with a deep anterior sinus, in the centre of which is a minute tooth[1349]. This piece is separated from the forepart of the head by a suture; but it probably is not moveable: however, it covers the mouth, and may be regarded rather as analogous to the labrum. Below this are two mandibles, armed at their end with five sharp triangular teeth[1350], under which are the maxillæ, terminating in a moveable concavo-convex lobe, resembling the valve of a bivalve shell[1351]; and between them is the labium, of a rhomboidal shape, divisible into two lobes, attached laterally to the maxillæ: these lobes M. Savigny terms the second maxillæ, forming with the others, according to him, the labium[1352]. Affixed to the base of this labium, or covering it on the outside, are a pair of pediform palpi, which he considers as the first auxiliary labium, and representative of the first pair of legs of hexapods and Iuli[1353]. I imagine them to be also the analogues, in some degree, of the labial palpi of a perfect mouth. The last of the organs in question is a large rhomboidal plate affixed to the first apparent segment of the trunk, crowned at its vertex with two truncated denticulated teeth, and from the upper sides of which emerge a pair of moveable organs terminating in a powerful incurved claw, and which entirely covers all the other parts of the mouth[1354]. This, M. Savigny deems as a second auxiliary labium, and the lateral organs of prehension,—which may be regarded each as a kind of maxillary hand, and as the only representatives in this tribe of the maxillary palpi, though widely different,—he looks upon as really analogous to the second pair of legs in Iulus and the hexapods[1355]. These two pairs of pedipalpes (to use an expressive French term) show their relation to legs by their general structure, and their analogy with palpi by their use as oral organs, though belonging to the trunk: so that here we see the legs and their appendages assume a material function in manducation, forming a singular contrast to what we had observed before with regard to mandibles becoming instruments of locomotion. The mouth of the Iulidæ, with little variation, is upon the same plan[1356] with those here described.
The next type of form with regard to the oral organs is that of the Arachnida. In these, as you know, the head is confounded with the trunk; so that they are a kind of Blemmyes in the insect world. Their organs of manducation, amongst which there is no labrum or upper lip, are, in the first place, a pair of mandibles planted close and parallel to each other in the anterior part of the head, which they terminate. In the spiders they consist of two tubular joints, of which the first is much the largest, more or less conical or cylindrical, and armed underneath with a double row of stout teeth; and the terminal one is more solid and harder, in the form of a very sharp crooked claw, which in inaction is folded on the first joint between the teeth. Under its extremity on the outside is a minute orifice, destined to transmit a venomous fluid, which is conducted there by an internal canal from the base of the first joint, where is the poison-bag[1357]. In the scorpion and harvest-man (Phalangium) the mandible consists of two joints terminated by a chela or double claw, the exterior one being moveable[1358].—M. Latreille, as has been before observed, regards these not as representatives of the mandibles of hexapods, but as replacing the interior pair of antennæ, in the situation of which they are precisely placed, of the Crustacea[1359]: and M. Savigny is of opinion that the Arachnida may in some sort be defined as Crustacea without a head, and with twelve legs, of which the two first pair are converted into mandibles and maxillæ[1360]. From the situation of the organs in question, the first of these opinions seems preferable; but the conversion of the legs in other cases, at least the coxæ, into organs of manducation, gives some weight to the last. With regard to their use, it is said to be to retain the insect which the animal has seized, and to facilitate the compression which the maxillæ exercise upon it for the extraction of the nutritive matter[1361]. If this be correct, in this respect the mandibles may be said to represent the maxillæ of the mandibulate hexapods; and, vice versa, the sciatic maxillæ, as they have been denominated[1362], of the Arachnida, their mandibles. The palpi are pediform, and the first joint of the coxa, or hip, acts the part of a maxilla:—this is composed of a single piece or plate, more or less oval or triangular, sometimes straight and sometimes inclined to the labium, with the interior extremity very hairy. The labium consists also of a single piece, and is only an appendage of the anterior extremity of the breast. The interior of the mouth, or palate, presents a fleshy, hairy, linguiform piece, which is usually applied to the internal face of the labium. An opening is supposed to exist in its sides, for the transmission of the alimentary juices[1363]. If you examine the under side of the body of a scorpion, you will find that not only the palpi, but the two anterior pair of legs, by means of their coxæ, are concerned in manducation: so that these insects have in fact three pairs of maxillæ—a circumstance that M. Savigny has observed to take place also in the harvest-men (Phalangium L.)[1364]. The palpi of the scorpion, which may be called its hands, like the anterior legs of the lobster and crab, terminate in a tremendous chela or forceps, consisting of a large triangular joint, armed at the end with a double claw internally toothed; the exterior one of which, contrary to what takes place in the animals just named, is moveable, and not the interior[1365].
Having given you this full account of the trophi of those animals that have all the organs of manducation developed, I must next advert to those in which one part receives an increment at the expense of others, and the whole oral machine is fitted for suction; or where some parts appear to be deficient, so that this may be called an imperfect mouth. At first sight one would regard the trophi of a bee as of this description; but this is not the case, since it has all the ordinary organs, though the tongue is unusually long, and looks as if it was made for suction; which, however, as you have been informed, is not the case.
There are five kinds of imperfect mouth to be met with in insects that take their food by suction, each of which I shall distinguish by a separate denomination. The first is that of the Hemiptera Order:—this I term the Promuscis; the second is that of the Diptera, which with Linné I call Proboscis; the third, peculiar to the Lepidoptera, is with me an Antlia; the fourth, which I name Rostrulum, is confined to the Aphaniptera order, or genus Pulex L.; and the last is Rostellum, which I employ to denote the suctory organs of the louse tribe (Pediculidæ).
i. Promuscis[1366].—The organ we are first to consider has usually been denominated Rostrum: but since that term is likewise in general use for the snout of insects of the weevil tribes (Curculio L.), I think you will concur with me in adopting the one here proposed, for the very different oral instruments of the Hemiptera. Illiger has employed promuscis to denote those of bees[1367]: but since, as I have just observed, they consist of all the ordinary organs, they seem to require no separate denomination: the term, therefore, may be applied to represent a different set of trophi, without any risk of producing confusion. This part consists of five pieces: viz. a minute, long, conical piece, commonly very slender, which covers the base of the promuscis, and represents the labrum[1368]; a jointed sheath (vagina), consisting of either three or four joints, the analogue of the labium, and four slender rigid lancets (scalpella), the two exterior ones, according to M. Savigny, representing the mandibles, and the intermediate pair the maxillæ[1369]. By the union of these four pieces a suctorious tube is formed, which the animal inserts into the substance, whether animal or vegetable, the juices of which form its nutriment. These pieces are dilated at their base, and serrated at their apex; and the two central ones, though at their origin they are asunder, form one tube, which has often been mistaken for a single piece. A pharynx and tongue have been discovered by M. Savigny in this apparatus; who thinks that in Nepa there are also rudiments, but very indistinct, of labial palpi: so that the maxillary palpi seem to be the only part absolutely wanting[1370].
The Promuscis when at rest is usually laid between the legs; but when employed, in most cases its direction is outward. In the genus Chermes L. (Psylla Latr.) the origin of the promuscis has been supposed to be in the breast; but if closely examined, this anomaly in nature will be found not to exist. If you take one of these insects, the first thing that strikes you upon inspecting the head, is a pair of remarkable conical processes into which the front appears to be divided. Look below these, and you will there discover the upper-lip: and from this you may follow the promuscis till it gets beyond the forelegs, when it takes a direction perpendicular to the body[1371]; a circumstance which has given rise to the above false notion. Though in Coccus, Chermes, &c. this instrument is short, in some Aphides it is longer in proportion than in any other insect. In A. Quercus it is three times the length of the body; so that when folded, it stretches out beyond it, and looks like a long tail[1372]; and in A. Abietis it even exceeds that length[1373].
ii. Proboscis[1374].—Linné long since, and after him Fabricius, has employed this term to designate the oral instruments, or rather their sheath, in the Muscidæ and some others, calling the same organ, when without fleshy lips, rostrum and haustellum: but as the parts of the mouth in all true Diptera (for Hippobosca and its affinities can scarcely be deemed as co-ordinate with the rest), are analogous to each other; although in some they are stiff and rigid, in others flexile and soft, and in Œstrus (except the palpi) mere rudiments,—the same appellation ought to designate them all. I am happy to find that M. Latreille agrees with me in this opinion; and to his sensible observations on this head, if you wish for further information, I refer you[1375]. The mouth of Dipterous insects appears to vary in the number of pieces that it presents; but in all, the theca or sheath is present, which represents the labium (including the mentum) of the mandibulate Orders[1376]. It consists of three joints, the last of which is formed by the liplets (Labella). Those in the Muscidæ are large, turgid, vesiculose, and capable of dilatation; in the Bombylidæ and other tribes they are small, slender, long and leathery, and sometimes recurved. The second joint or stalk, which may be said to represent the mentum, the liplets being properly in a restricted sense the analogue of the labium, its sides being turned up, forms a longitudinal cavity, which contains the haustellum. The upper piece of this, the valvula, is long, rigid, and very sharp, representing the labrum[1377]. Beneath this cover, in the above cavity, are the lancets; which, as far as they are at present known, vary in number and form: sometimes there are five of them, sometimes four, sometimes two, and sometimes, it should seem, only one[1378]. In the gnat (Culex) they are finer than a hair, very sharp, and barbed occasionally on one side[1379]; in the horse-fly (Tabanus L.) they are flat and sharp like the blade of a knife or lancet[1380]. In this tribe the upper pair, or the knives (Cultelli), represent the mandibles; the lower pair, or the lancets (Scalpella), usually palpigerous, the maxillæ; and the central one the tongue. In the horse-fly Reaumur has figured only four, exclusive of the labrum and labium; but in a specimen I have preserved there appear to be five, one of which, as slender as a hair, I regard as the analogue of the tongue[1381].—When the lancets are reduced to two, they probably represent the maxillæ, the mandibles being absorbed in the labrum; and where there is only one, the maxillæ also are absorbed by the labium, which then bears the palpi, the lancet representing the tongue[1382]. The lancets are so constructed in many cases, as to be able by their union to form a tube proper for suction, or rather for forcing the fluid by the pressure of the lower parts to the pharynx[1383]. Labial palpi appear not usually present in the proboscis; but M. Savigny thinks he has discovered vestiges of them in Tabanus[1384]. In this genus the maxillary ones are large, and consist of two joints[1385]. The proboscis is often so folded, as to form two elbows; the base forming an angle with the stalk, and the latter with the lips, so as in shape to represent the letter Z, only that the upper angle points to the breast, and the lower one to the mouth: this is the case with the flesh-fly and many others. In other flies, as Conops and Stomoxys, whose punctures on our legs so torment us[1386], there is only a single fold, with its angle to the breast. The proboscis is received in a large oblong cavity of the underside of the anterior part of the head.
It may here be observed, that in the promuscis the elongation of the organs seems to be made chiefly at the expense of all the palpi, but in the proboscis at that of the labial only; and in some cases at that also of the mandibles or maxillæ;—the former merging in the labrum and the latter in the labium.
iii. Antlia[1387].—The third kind of imperfect mouth is that of the Lepidoptera, which I have called Antlia. Fabricius denominates it lingua: but as this organ has no analogy with the real tongue of insects, this is confessedly improper, and it appeared necessary therefore to exchange it for another denomination: I have endeavoured to apply a term to it that indicates its use—to pump up, namely, the nectar of the flowers into the mouth of the insect. On a former occasion I described to you the structure of this instrument[1388]; but further discoveries with regard to it having since been made by MM. Savigny and Latreille, I shall here give you the result of their observations. The former of these able physiologists has detected in the mouth of the Lepidoptera rudiments of almost all the parts of a perfect mouth. Of the correctness of this assertion you may satisfy yourself, if you consult his admirable elucidatory plates, and compare them with the insects. Just above the origin of the spiral tongue or pump, the head is a little prominent and rounded; and immediately below the middle of this prominence there is a very minute, membranous, triangular or semicircular piece; which from its position, as covering the base of the antlia, may be regarded as the rudiment of the upper-lip (labrum)[1389]. On each side of the outer base of the antlia is another small immoveable piece, resembling a flattened tubercle, the end of which is internally hairy or scaly: these pieces appear to represent the mandibles[1390]. Near the base of each half of the antlia, just below a sinus, may be distinctly seen the minute, usually biarticulate rudiment of a maxillary palpus[1391]; demonstrating to a certainty that these spiral organs, at least their lateral tubes or Solenaria, are real maxillæ[1392]. The rudiment of the under-lip (Labium) is the almost horny triangular piece united by membrane to the two stalks of the maxillæ, and supporting at its base the recurved labial palpi; which are so well known that I need not enlarge upon them[1393]. Amongst these parts there seems at first sight no representative of the tongue; but M. Latreille has advanced some very ingenious, and I think satisfactory arguments[1394], which go to prove that this part, at least the tongue of Hymenoptera, has its analogue in the intermediate tube or Fistula formed by the union of the two maxillæ, and which conveys the fluid aliment of this Order to the pharynx. As in Diptera the maxillæ sometimes merge in the labium, so here the tongue (as it were divided longitudinally) merges in the maxillæ. He further observes, that in a transverse section of the maxilla of the death's-head hawk-moth (Sphinx Atropos), the lateral tube appeared to be divided into two by a membranous partition, and to contain in the upper cavity a small cylindrical tube, which seemed to be a trachea[1395]. To animals that are without lungs, and breathe by tracheæ, suction must be performed in a very different way from what it is by those that breathe by the mouth: and as in the very extended organs in question the fluid has a long space to pass before it reaches the pharynx, in some way or other these lateral tubes may have the power of producing a vacuum in the middle tube, and so facilitate its passage thither. We see, in the antlia, that the maxillæ receive their vast elongation at the expense of all the other organs, except the labial palpi.
iv. Rostrulum[1396].—An animal very annoying to us affords the type of the next kind of imperfect mouth—I mean the flea. Its oral apparatus, which I would name rostrulum, appears to consist of seven pieces. First are a pair of triangular organs, the laminæ, which together somewhat resemble the beak of a bird, and are affixed, one on each side of the mouth, under the antennæ: these represent the mandibles of a perfect mouth[1397]. Next, a pair of long sharp lancets (Scalpella), which emerge from the head below the laminæ: these are analogous to maxillæ[1398]: a pair of palpi, consisting of four joints, are attached to these near their base[1399], which of course are maxillary palpi. And lastly, in the midst of all is a slender setiform organ (ligula), which is the counterpart of the tongue[1400]. Rösel, and after him Latreille, seem to have overlooked this last piece, since they reckon only six pieces in the flea's mouth[1401]: but the hand and eye of our friend Curtis have detected a seventh, as you see in his figure. From this account it appears, that the elongation of the organs of the Aphaniptera Order is at the expense of the labium and its palpi.
v. Rostellum.—So little is known of the composition of the next kind of imperfect mouth, that I need not enlarge upon it. It is peculiar to the louse tribe (Pediculidæ), and it consists of the tubulet (Tubulus), and siphuncle (Siphunculus). The former is slenderer in the middle than at the base and apex, the latter being turgid, rather spherical, and armed with claws which probably lay hold of the skin while the animal is engaged in suction. When not used, the whole machine is withdrawn within the head; the siphuncle, which is the suctorious part, being first retracted within the tubulet, in the same way as a snail retracts its tentacula[1402]. This apparatus seems formed at the expense of all the other organs.
There are some other kinds of imperfect mouth, which, though they seem not to merit each a distinct denomination, should not be passed altogether without notice. The first I shall mention is that of the family of Pupipara Latr. (Hippobosca L.). It consists of a pair of hairy coriaceous valves, which include a very slender rigid tube or siphuncle, the instrument of suction, which Latreille describes as formed by the union of two setiform pieces[1403]. In Melophagus, the sheep-louse, the union of the valves of the sheath is so short, that they appear like a tube; but if cut off they will separate, and show the siphuncle, as fine as a hair, between them. This organ is of a type so dissimilar, as was before observed, to that of the Diptera in general, and approaches so near to that of the dog-tick (Ixodes), that they may be deemed rather apterous insects with two wings, than to belong to that Order; and the circumstance that some of the family are apterous confirms this idea. In fact they are a transition family that connects the two Orders, but are nearest to the Aptera. In Nycteribia the oral organs differ from those of the other Pupipara in having palpi. This also is the case with those of the genus Ixodes, the palpi of which are placed upon the same base with the instrument of suction, than which they are longer: they appear to consist of two joints, the last very long and flat. The instrument of suction itself is formed by three hard rigid laminæ; two shorter parallel ones above, that cover the third, which is longer and broader, and armed on each side with several teeth like a saw, having their points towards the base[1404]. Many of the other Acari L. have mandibles, and several have not: but their oral organs have not yet been sufficiently examined; and from the extreme minuteness of most of them, this is no easy task; nor to ascertain in what points they differ or agree.
If you consider the general plan of the organs of manducation in the vertebrate animals, how few are the variations that it admits! An upper and a lower jaw planted with teeth, or a beak consisting of an upper or a lower mandible with a central tongue, form its principal features. But in the little world of insects, how wonderful and infinite is the diversity which, as you see, in this respect they exhibit! Consider the number of the organs, the varying forms of each in the different tribes, adjusted for nice variations in their uses:—how gradual, too, the transition from one to another! how one set of instruments is adapted to prepare the food for deglutition by mastication; another merely to lacerate it, so that its juices can be expressed; a third to lap a fluid aliment; a fourth to imbibe it by suction—and you will see and acknowledge in all the hand of an almighty and all-bountiful Creator, and glorify his wisdom, power, and goodness, so conspicuously manifested in the structure of the meanest of his creatures. You will see also, that all things are created after a pre-conceived plan; in which there is a regular and measured transition from one form to another, not only with respect to beings themselves, but also to their organs—no new organ being produced without a gradual approach to it; so that scarcely any change takes place that is violent and unexpected, and for which the way is not prepared by intermediate gradations. And when you further consider, that every being, with its every organ, is exactly fitted for its functions; and that every being has an office assigned, upon the due execution of which the welfare, in certain respects, of this whole system depends, you will clearly perceive that this whole plan, intire in all its parts, must have been coeval with the Creation; and that all the species,—subject to those variations only that climate and different food produce,—have remained essentially the same, or they would not have answered the end for which they were made, from that time to this.
Having given you this particular account of the trophi or organs of the mouth of insects, I must now make some observations upon the other parts of the head. I have divided it, as you see in the Table, into face and subface; the former including its upper and the latter its lower surface. Strictly speaking, some parts of the face, as the temples and cheeks, are common to both surfaces; but I do not therefore reckon them as belonging to the subface, which, exclusive of the mouth and its organs, consists only of the throat, and where there is a neck, the gula.
i. Nasus[1405].—I shall consider the parts of the face in the order in which they stand in the Table, beginning with the nasus or nose. Fabricius has denominated this part the clypeus, in which he has been followed by most modern Entomologists. You may therefore think, perhaps, that I have here unnecessarily altered a term so generally adopted, and expect that I assign some sufficient reasons for such a change. I have before hinted that there is good ground for thinking that the sense of smell in insects resides somewhere in the vicinity of this part; and when I come to treat of their senses, I shall produce at large those arguments that have induced me to adopt this opinion: and if I can make out this satisfactorily, you will readily allow the propriety of the denomination. I shall here only state those secondary reasons for the term, which, in my idea, prove that it is much more to the purpose than clypeus. This last word was originally applied by Linné in a metaphorical sense to the ample covering of the head of the Scarabæidæ, and the thoracic shield of Silpha, Cassida, Lampyris, and Blatta: in all which cases there was a propriety in the figurative use of it, because of the resemblance of the parts so illustrated to a shield. But when Fabricius (though he sometimes employs the term, as Linné did, merely for illustration,) admitted it into his orismological table, as a term to represent universally the anterior part of the face of insects to which the labrum is attached (though in some cases he designates the labrum itself by this name), it became extremely inappropriate; since in every case, except that of the Scarabæidæ, the part has no pretension to be called a shield;—so that the term is rather calculated to mislead than illustrate. This impropriety seems at length to have struck M. Latreille, since in a late essay[1406] he has changed the name of this part to Epistomis, a term signifying the part above the mouth. But there are reasons, exclusive of those hereafter to be produced concerning the sense of smell, which seem to me to prove that nasus is a preferable term; not to mention its claim of priority, as having been used to signify this part a century ago[1407]. When we come to consider the terms for the other parts of the head, as lips, jaws, tongue, eyes, temples, cheeks, forehead, &c. the concinnity, if I may so speak, and harmony of our technical language, seem to require that the part analogous in point of situation to the nose of vertebrate animals should bear the same name. And any person who had never examined an insect before, if asked to point out the nose of the animal, would immediately cast his eye upon this part: so that one of the principal uses of imposing names upon parts—that they might be more readily known—would be attained. If it is objected, that calling a part a nose that has not the sense of smell, supposing it to be so, might lead to mistakes—I would answer, that this objection is not regarded as valid in other cases: for instance, the maxillæ are not generally used as jaws, and yet no one objects to the term; because, from their situation, they evidently have an analogy to the organs whose name they bear. But enough on this subject—we will now consider the part itself.
To enable you to distinguish the nose of insects when it is not separated from the rest of the face by an impressed line, you must observe that it is the terminal middle part that sometimes overhangs the upper-lip, and at others is nearly in the same line with it; that on each side of it are the cheeks, which run from the anterior half of the eyes to the base of the mandibles. Just below the antennæ is sometimes another part distinct from the nose, which I shall soon have to mention; so that the nose must not be regarded as reaching always nearly to the base or insertion of the antennæ, since it sometimes occupies only half the space between them and the upper-lip, which space is marked out by an impressed line. But you will not always be left at such uncertainty when you want to ascertain the limits of the nose; for it is in many cases a distinct piece, separated by an elevated or impressed line from the rest of the face. This separation is either partial or universal. Take any species of the genera Copris, Onitis, or Ateuchus, and you will see the nose marked out in the centre of the anterior part of the face by two elevated lines, forming nearly a triangle and bounded by the horn[1408]. Or take a common wasp or hornet, and you will find a similar space, though approaching to a quadrangular figure, marked out by impressed lines[1409]. In Rhagio and Sciara, two Dipterous genera, this impression is so deep as to look like a suture. Between these lines, in those cases, is included what I call the nose. As to substance, in general it does not differ from the rest of the head; but in the Cleridæ it is almost membranous. You must observe, that in all these, what at first sight appears to be the termination of the front, is not the nose, but the narrow depressed piece that intervenes between it and the lip. With regard to its clothing, it is most commonly naked, but in some genera it is covered with hair; in Crabro F. often with golden or silver pile, which imparts a singular brilliance to the mouth of the insects of that genus: M. Latreille supposes that the brilliant colours of the golden-wasp (Chrysis L.) may dazzle their enemies, and so promote their escape[1410]; the brilliance of the mouth of the Crabro may on the contrary at first dazzle their prey for a moment, so as to prevent their escape. The form of the nose, where distinct from the rest of the face, admits of several variations: thus in the Staphylinidæ and Cleridæ it is transverse and linear; in Copris it is triangular, with the vertex of the triangle truncated; in Vespa Crabro it is subquadrate and sinuated. In many Heteromerous beetles[1411] it is rounded posteriorly: in Pelecotoma, a new genus in this tribe, related to Asida, there is a deep anterior sinus; in Blaps the anterior margin is concave; in Cetonia[1412] Brownii, and atropunctata (forming a distinct subgenus), it is bifid: it varies in the Scarabæidæ, in some being bidentate, in others quadridentate, and in others again sexdentate, including the cheeks: in Mylabris, a kind of blister-beetle, it is transverse and nearly oval; in Lamia, a capricorn-beetle, it represents a parallelogram; and in most Orthoptera it is subtriangular: in Tettigonia F. it is prominent, transversely furrowed, and divided by a longitudinal channel: in Otiocerus K. it presents the longitudinal section of a cone[1413]: in the Diptera Order, with the exception of the Tipulidæ and some others, in which it unites with the cheeks, &c. to form a rostrum, the nose in general, as to form, answers to its name, resembling that of many of the Mammalia: in some of the Asilidæ it is very tumid at the end, and terminates in a sinus, to permit the passage of the proboscis to and fro: in many of the Syrphidæ, &c. it is first flat and depressed, and then is suddenly elevated, so as to give the animal's head the air of that of a monkey: in some tribes, as Rhingia, Nemotelus, Eristalis, &c., in conjunction with the cheeks it forms a conical rostrum: in Tabanus bovinus, and other horse-flies, it terminates in three angles or teeth. Many more forms might be mentioned, but these will suffice to give you a general idea of them. In size and proportions the nose also varies. It is frequently, as in Tettigonia, the most conspicuous part of the face, both for size and characters; but in the Staphylinidæ it is very small, and often scarcely discernible, being overshadowed by its ample front: and it may be observed in general, that when the antennæ approximate the mouth, as in this genus and many others, the front becomes ample, and the nose is reduced to its minimum: but when they are distant from the mouth, the reverse takes place; and the nose is at its maximum and the front at its minimum. Mutilla, Myrmecodes, Scolia, &c. in the Hymenoptera, are an example of the former; and the Pompilidæ, Sphecidæ, Vespidæ, &c. of the latter. In Myopa buccata, &c. its length exceeds its width; but more commonly the reverse takes place. The circumscription of the nose also deserves attention. It is usually terminated behind by the front (frons), or, where it exists, by the postnasus, in the sides by the cheeks, and anteriorly by the labrum. But this is not invariably the case; for in the Cimicidæ, in which the cheeks form the bed of the Promuscis, the front embraces it on each side by means of two lateral processes, that sometimes meet or lap over each other anteriorly, which gives the nose the appearance of being insulated; but it really dips below these lobes to join the labrum. This structure you may see in Edessa F., and many other bugs. This part sometimes has its arms. Thus in Copris, and many Dynastidæ, the horns of the head seem, in part at least, to belong to this portion of it; in Tipula oleracea (the crane-fly), &c. it terminates before in a horizontal mucro. In Osmia cornuta, a kind of wild-bee, each side of the nose is armed with a vertical horn. The margin of the nose in most Lamellicorn insects, though mostly level, curves upwards.
I am next to mention a part of the nose which merits a distinct name and notice, which I conceive in some sort to be analogous to the nostrils of quadrupeds, and which I have therefore named the Rhinarium or nostril-piece. I had originally distinguished it by the plural term nares, nostrils; but as it is usually a single piece, I thought it best to denote it by one in the singular. When I treat of the senses of insects, I shall give you my reasons, as I have before said, for considering this part as the organ of scent, or connected with it, which you will then be able to appreciate. I shall only here observe, that the piece in question is in the usual situation of the nostrils—between the nose and the lip. In a large number of insects this part may be regarded as nearly obsolete; or at least it is merely represented by the very narrow membranous line that intervenes between the nose and the lip and connects them; which, as in the case of the head of Harpali before noticed, may be capable of tension and relaxation, and so present a greater surface to the action of the atmosphere. But I offer this as mere conjecture. In the lady-bird (Coccinella) this line is a little wider, and becomes a distinct Rhinarium; as it does also in Geotrupes. With respect to its insertion, the rhinarium is a piece that either entirely separates the nose from the lip, or only partially: the former is the most common structure. It is particularly remarkable in a New Holland genus of chafers (Anoplognathus Leach). In A. viridiæncus it is very ample, and forms the under side of the recurved nose, so that a large space intervenes between the margin of the latter and the base of the labrum. In Macropus Thunb., of the Capricorn tribe (Cerambyx L.), the nostril-piece, which forms a distinct segment, is narrower than the nose, and the upper-lip than the nostril-piece, forming as it were a triple gradation from the front to the mouth. Again, in others the part in question is received into a sinus of the nose. This is the case with the dragon-flies (Libellulina), in which this sinus is very wide; in the burying-beetle (Necrophorus)[1414], in some species of which it is deep but narrow; and in a species of Tenebrio from New Holland, which perhaps would make a subgenus. If you examine with a common glass any of the larger rove-beetles (Staphylinidæ), you will find that the nose itself seems lost in the nostril-piece, both together forming a very narrow line across the head above the labrum, without any apparent distinction between them; but if you have recourse to a higher magnifier, you will find this divided into an upper and lower part, the former of the hard substance of the rest of the head, and the latter membranous. I once was of opinion that the prominent transversely furrowed part, so conspicuous in the face of Tettigonia F.[1415], was the front: but upon considering the situation of this, chiefly below the eyes and antennæ, and comparing it with the analogous piece in Fulgora laternaria and other insects of the Homopterous section of the Hemiptera, I incline to think that it represents the nose, and that the longitudinal ridge below it is the nostril-piece[1416]. In the Heteropterous section it is merely the vertical termination of their narrow nose. In other insects again, this part approaches in some measure to the common idea of nostrils; there being two, either one on each side the nose, or two approximated ones. If you catch the first humble-bee that you see busy upon a flower, you will discover a minute membranous protuberance under each angle of the nose. Something similar may be observed in some species of Asilus L. In the Orthoptera, especially in Blatta, Phasma, and some Locustæ, two roundish or square pieces, close to each other on the lower part of the nose, represent the nostrils[1417].—With regard to substance, in the chafer-tribes, at least those that feed on leaves or living vegetable matter, as the Melolonthidæ, Anoplognathidæ, and in many other insects, the rhinarium is of the same substance with the rest of the head; but in Macropus Thunb., Staphylinus, Necrophorus, &c., it consists of membrane.
ii. Postnasus[1418].—This is a part that appears to have been confounded by Entomologists with the front of insects; in general, indeed, it may be regarded as included in the nose, and does not require separate notice: but there are many cases in which it is distinctly marked out and set by itself, and in which it forms a useful diagnostic of genera or subgenera. There is a very splendid and beautiful Chinese beetle, to be seen in most collections of foreign insects (Sagra purpurea), in which this part forms a striking feature, and helps to distinguish the genus from its near neighbour Donacia. If you examine its face, you will discover a triangular piece, below the antennæ and above the nasus, separated from the latter and from the front by a deeply-impressed line: this is the postnasus or after-nose. Again: if you examine any specimens of a Hymenopterous genus called by Fabricius Prosopis (Hylæus Latr.), remarkable for its scent of baum, you will find a similar triangle marked out in a similar situation[1419]. In many Coleopterous insects, besides Sagra, you will discover traces of the part we are considering: as in Anthia, Dytiscus, and several others of the Predaceous beetles. In Cistela it is larger than the nose itself; but it is more conspicuous in the Orthoptera, particularly in Locusta (Gryllus F.), in which it is the space below the antennæ, distinguished by two or four rather diverging ridges[1420]. In the Libellulina, Myrmeleonina, &c. it is a distinct transverse piece. In Dasyga Latr., a kind of bee, it is armed with a transverse ridge or horn—But enough has been said to render you acquainted with it; I shall therefore proceed to the next piece.
iii. Frons[1421].—The Front of insects may be denominated the middle part of the face between the eyes, bounded anteriorly by the nose, or after-nose, where it exists, and the cheeks; laterally by the eyes; and posteriorly by the vertex. Speaking properly, it is the region of the antennæ; though when these organs are placed before the eyes, under the margin of the nose, as in many Lamellicorn and Heteromerous beetles, they seem to be rather nasal than frontal. This part is often elevated, as in the elastic beetles (Elater), whose faculty of jumping, by means of a pectoral spring, has been related to you[1422]. In Anthia, a Predaceous beetle, it has often three longitudinal ridges. In many of the Capricorn beetles (Cerambyx L.), it is nearly in the shape of a Calvary cross, with the arms forming an obtuse angle, and then terminating at the sinus of the eyes in an elevation for the site of the antennæ. In the ants also (Formicidæ), the front is often elevated between those organs. In Ponera, one tribe of them, this elevation is bilobed, and receives between its lobes the vertex of the postnasus. In the hornet (Vespa Crabro) the elevation is a triangle, with its vertex towards the mouth. In Sagra it is marked out into three triangles, the postnasus making a fourth, with the vertexes meeting in the centre. In the Dynastidæ and Scarabæidæ the horns are often frontal appendages, as is that of Empusa Latr., a leaf-insect, and probably those of Sphinx Iatrophæ F., which affords a singular instance of a horned Lepidopterous one. Sometimes it is an ample space, reducing the nose to a very narrow line, as in the Staphylinidæ, or sending forth a lobe on each side, as before mentioned, which embraces the nose. In a species of bug from Brazil, related to Aradus F., these lobes are dilated, foliaceous, and meet before the nose, so as to form a remarkable extended frontlet to the head. In others this part is extremely minute: thus in many male flies and other insects, as the Libellulina, where the eyes touch each other, the front is cut off from the vertex and reduced to a small angle. In the female flies the communication with the vertex is kept open, and the front consequently longer. In the horse-flies (Tabanidæ), in Hæmatopota, and Heptatoma, the frontal space is wider than in the rest of that tribe. Many of these are distinguished by a levigated area behind the antennæ in the part we are treating of. In the Libellulina, and in the drone-bee, whose eyes are confluent, the stemmata are in the front. In many Orthoptera also, as Locusta Leach, one of them is below the antennæ; and in the lanthorn-fly tribe (Fulgoridæ), both these organs, which are situate between them and the eyes, as they do also in Truxalis, appear to be in it[1423]. In this tribe the rostrum is an elongation of the part in question; and perhaps you would think at first that what I have considered as the nose in Tettigonia F. was also a tendency to this kind of rostrum; but if you examine the great lanthorn-fly (Fulgora laternaria), you will find besides, at the lower base of the lanthorn, a triangular piece analogous to the nose of Tettigonia, and below it another representing its nostril-piece:—the horizontal part of the nose in that genus may perhaps be regarded as part of the front. In Truxalis F. the face consists of a supine and prone surface, and the latter is composed of the front, after-nose, nose, and organs of the mouth. I may notice here a most remarkable and singular tribe of bugs, of which two species have been figured by Stoll[1424]: in these the head, or rather those parts of it that we have now been describing, the nose, namely, the after-nose, and front, are absolutely divided longitudinally in two, each half having an eye and antenna planted in it; or perhaps, as it is stated to be divided in one instance to the commencement of the promuscis, the nose is left intire, and dips down, as in cases before alluded to: so that in this the nose appears to leave the lobes of the front, which in others embrace its sides.
iv. Vertex[1425].—We now come to the vertex, or crown of the head; which is situated behind the front, and, except where the communication is intercepted by confluent eyes, adjoins it. It is laterally bounded by the hind part of the eyes and the temples; and posteriorly, where that part exists, by the occiput. The vertex may be denominated the ordinary region of the stemmata: for though in several cases, as we have just seen, one or more of them are planted in the front; yet this in the great majority, especially in the Hymenoptera, is their natural station. In Blatta and some other Orthoptera the posterior angle of the head is the vertex. In many dung-chafers of Latreille's genus Onthophagus, which are said to have occipital horns, as O. nutans, nuchicornis, Xiphias, &c., the horn really arms the part I regard as the vertex. In Locusta Leach, this part is very ample, and in Truxalis very long; but more generally it is small, and not requiring particular notice.
v. Occiput[1426].—The occiput, or hind-head, is that part of the face that either forms an angle with the vertex posteriorly, or slopes downwards from it. It has for its lateral boundaries the temples, and behind it is either terminated by the orifice of the head, or in many cases by the neck. In those beetles that have no neck, as the Lamellicorn and Capricorn, the hind-head is merely a declivity from the vertex, usually concealed by the shield of the thorax, very lubricous, to facilitate its motion in the cavity of that part, and at its posterior margin distinguished by one or two notches, which I shall notice hereafter, for the attachment of the levator muscles: but in those beetles or other insects that have a neck, or a versatile head, the occiput forms an angle with the vertex, often rounded, and sometimes acute. This structure may be seen in Latreille's Trachelides, and several other beetles. In the Hymenoptera, Diptera, and others with a versatile head, the part now under consideration curves inwards from the vertical line, so as with the temples and under parts of the head to form a concavity adapted to its movement upon the trunk.
vi. Genæ[1427].—The cheeks of insects (Genæ) usually surround the anterior part of the eyes, and lie between them and the mandibles or their representatives. Where they approach the latter, as in the Predaceous beetles (Cicindela, Carabus L. &c.), they are very short, and of course longer where the eyes are further removed from the mouth; as in the Rhyncophorous beetles (Curculio L.), where they form the sides of the rostrum, and often contain a channel which receives the first joint of the antennæ, when they are unemployed. In the Scarabæidæ and many other Lamellicorn beetles, their separation on each side from the nose is marked by a ridge[1428]; and in the wasps (Vespa) by an impressed line or channel. In an African tribe at present arranged with Cetonia F., to which C. bicornis Latr.[1429] and another, which he has named, I believe, C. vitticollis, belong, the cheeks are porrected on each side of the mouth into a horizontal horn. These horns have at first the aspect of a pair of open mandibles. In the magnificent Goliathi Lam., the horns of the male are rather a process of the cheek than of the nose. In Alurnus, Hispa, and other beetles, these parts, by their elevation and conjunction with the lower side of the head, form a kind of fence which surrounds and protects the oral organs; in many Cimicidæ, by a similar elevation of the cheeks, the bed of the promuscis is formed. In the Homopterous Hemiptera they run parallel nearly with the rhinarium or nostril-piece. In the Hymenoptera they are almost always ample, but they are confined to the lower side of the eye. In Sirex grandis, and others of that genus, the cheek at the base of the mandible is dilated so as to form a rounded tooth below it. In the Capricorn-beetles it is considerable, and sometimes terminates, at the base of the mandible, in two or three notches. In Scaurus and Eurychora, darkling-beetles, the cheek below projects into a lobe that covers the base of the maxilla. But the animal distinguished by the most remarkable cheeks is a species of Phryganea L. (Phryganea personata Spence); for from this part projects a spoon-shaped process, which curves upwards, and uniting with that of the other cheek, forms an ample mask before the face, the anterior and upper margin of which, in the insect's natural state, are closely united; and the posterior part being applied to the anterior part of the eye, causes the face to appear much swoln. It looks as if it was a single piece; but upon pressing the thorax it opens, both above and in front, into two parts, each convex without and hollow within, and each having attached to its inside a yellow tuft of hair resembling a feather. The use of this machinery at present remains a mystery[1430].
vii. Tempora[1431].—The temples (Tempora) are merely a continuation of the cheeks to the posterior limit of the head, forming its sides and posterior angles, and including the hinder part of the eyes, the vertex, and the occiput. They seldom exhibit any tangible character, except in certain ants (Atta Latr.), in which their angle terminates in one or two strong spines, giving the animal a most ferocious aspect; and in that remarkable genus Corydalis they are armed below with a tooth or point, which was not overlooked by De Geer[1432].
viii. Oculi[1433].—I must now call your attention to organs of more importance and interest, and which indeed include a world of wonders: I mean the eyes (Oculi) of insects. These differ widely from those of vertebrate animals, being incapable of motion. They may be regarded as of three descriptions—simple, conglomerate, and compound.
1. Simple Eyes[1434]. We will consider them as to their number, structure, shape, colour, magnitude, situation, and arrangement.
As to their number, they vary from two to sixteen. In the flea, the louse, the harvest-man (Phalangium), there are only a pair; in the bird-louse of the goose (Nirmus Anseris), and probably in others of the same genus, there are four[1435]; in some spiders (Scytodes, Dysdera, and Segestria Latr.[1436]), and some scorpions[1437], there are six. In the majority of spiders and Scolopendra morsitans, Scorpio maurus, &c. there are eight; and in Podura and Sminthurus Latr. there are sixteen[1438].
As to their structure, nothing seems to have been ascertained; probably their organization does not materially differ from that of one of the lenses of a compound eye; which I shall soon explain to you.
Their colour in the many is black and shining, but in the bird-louse of the goose they are quite white and transparent. In spiders they are often of a sapphirine colour, and clear as crystal. In Scolopendra morsitans and many spiders, scorpions, and phalangia[1439], they appear to consist of iris and pupil, which gives them a fierce glare, the centre of the eye being dark and the circumference paler. In the celebrated Tarantula (Lycosa Tarantula), the pupil is transparent, and red as a ruby; and the iris more opaque, paler, and nearly the colour of amber.
Where there are more than two, they vary in magnitude. In the enormous bird-spider (Mygale avicularia) the four external eyes are larger than the four internal[1440]; but in the Tarantula and Sphasus, the two or four internal are the largest. In Clubiona and Drassus they are all nearly of the same size[1441]; and in the Micrommata family they are very small[1442].
They vary also in shape. In Scolopendra morsitans the three anterior ones are round, and the posterior one transverse, and somewhat triangular. In Mygale calpeiana, a spider, the two smallest are round and the rest oval[1443]. In the trapdoor or mason spider (Mygale cæmentaria), the four small internal ones are round, and the large external ones oval[1444]; and those that are circumscribed posteriorly with an impressed semicircle, are shaped like the moon when gibbous[1445].
The situation and arrangement of simple eyes are also various. In many they are imbedded, as usual, in the head; but in the little scarlet mite, formerly noticed[1446], (Trombidium holosericeum), they stand upon a small foot-stalk[1447]: the hairiness of this animal might otherwise have impeded its sight. In spiders they are planted on the back of the part that represents the head, sometimes four on a central elevation or tubercle, and the remaining four below it—as in Lycosa; sometimes the whole eight are on a tubercle, as in Mygale; and sometimes, as in the common garden-spider (Epeira Diadema), upon three tubercles, four on the central one and two on each of the lateral ones. Other variations in this respect might be named in this tribe. In the scorpions a pair are placed one on each side, on a dorsal tubercle, and the other four or six on two lateral ones of the anterior part of the head[1448]. In the Phalangidæ the frontal eyes of the scorpion cease, and only a pair of dorsal ones are inserted vertically in the sides of a horn or tubercle, either bifid or simple, often itself standing upon an elevation which emerges from the back of the animal[1449]. If their eyes were not in a vertical and elevated position, the sight of these insects would be very limited; but by means of the structure just stated, they get a considerable range of surrounding objects, as well as of those above them. With regard to the arrangement of the eyes we are considering, it varies much. Sometimes they are placed nearly in the segment of a circle, as in those spiders that have six eyes only, before noticed[1450]; sometimes in two straight lines[1451]; at others in two segments of a circle[1452]; at others, in three lines[1453], and at others in four[1454]. Again, in some instances they form a cross, or two triangles[1455]; in others, two squares[1456]; in others, a smaller square included in a large one[1457]; in others, a posterior square and two anterior triangles[1458]; sometimes a square and two lines. Though generally separate from each other, in several cases two of the eyes touch[1459]; and in one instance three coalesce into a triangle[1460]. But it would be endless to mention all the variations, as to arrangement, in the eyes of spiders.
2. Conglomerate Eyes[1461] differ in nothing from simple eyes, except that instead of being dispersed they are collected into a body, so as at first sight to exhibit the appearance of a compound eye:—they are, however, not hexagonal, and are generally convex. They occur in Lepisma, the Iulidæ, and several of the Scolopendridæ. In Scolopendra forficata the eye consists of about twenty contiguous, circular, pellucid lenses, arranged in five lines, with another larger behind them, as a sentinel or scout, placed at some little distance from the main body. In the common millepede (Iulus terrestris) there are twenty-eight of these eyes, placed in seven rows, and forming a triangle, thus
—the posterior row containing seven lenses, the next six, and so on, gradually losing one, till the last terminates in unity. Each of these lenses is umbilicated, or marked with a central depression. In Craspedosoma Leach, you will find a similar formation. In Glomeris zonata, a kind of wood-louse that rolls itself into a ball, the lenses are arranged in a line curved at the lower end, with a single one by itself at the posterior end on the outside; they are oblong and set transversely, and their white hue and transparency give them the appearance of so many minute gems, especially as contrasted with the black colour of the animal[1462]. Between these eyes and the antennæ is another transverse linear white body, but opaque, seemingly set in a socket, and surrounded by a white elevated line, like the bezel of a ring. Whether it is an eye, or what organ, I cannot conjecture[1463]. Its aspect is that of a spiracle.
3. Compound Eyes[1464].—These are the most common kind of eye in hexapod insects, when arrived at their perfect state; in their larva state, as we have seen, their eyes being usually simple[1465]; except, indeed, those whose metamorphosis is semicomplete, which have compound eyes in every state.—In considering compound eyes, I shall advert to their structure, number, situation, figure, clothing, colour, and size.
As to their structure,—when seen under the microscope they appear to consist usually of an infinite number of convex hexagonal pieces. If you examine with a good glass the eye of any fly, you will find it traversed by numberless parallel lines, with others equally numerous cutting them at right angles, so as apparently to form myriads of little squares, with each a lens of the above figure set in it. The same structure, though often not so easily seen, obtains in the eyes of Coleoptera and other insects. When the eye is separated and made clean, these hexagons are as clear as crystal. Reaumur fitted one eye to a lens, and could see through it well, but objects were greatly multiplied[1466]. In Coleopterous insects they are of a hard and horny substance; but in Diptera, &c. more soft and membranous. The number of lenses in an eye varies in different insects. Hooke computed those in the eye of a horse-fly to amount to nearly 7,000[1467]; Leeuwenhoeck found more than 12,000 in that of a dragon-fly[1468]; and 17,325 have been counted in that of a butterfly[1469]. But of all insects they seem to be most numerous in the beetles of Mr. W. S. MacLeay's genus Dynastes. In the eyes of these the lenses are so small as not to be easily discoverable even under a pocket microscope, except the eye has turned white[1470]: it is not, therefore, wonderful, that Fabricius should call these eyes simple[1471]. In some insects, however, as in the Strepsiptera Kirby, the lenses are not numerous: in Xenos they do not exceed fifty, and are distinctly visible to the naked eye[1472]. These lenses vary in magnitude, not only in different, but sometimes in the same eyes. This is the case in those of male horse-flies and flies, those of the upper part of the eye being much larger than those of the lower[1473]. The partitions that separate the lenses, or rather bezels, in which they are set, are very visible in the eyes just mentioned, and those of Xenos; but in many insects they are only discernible at the intersecting lines of separation between the lenses. In hairy eyes, such as those of the hive-bee, the hairs emerge from these septa. Every single lens of a compound eye may be considered as a cornea, or a crystalline humour, it being convex without and concave within, but thicker in the middle than at the margin: it is the only transparent part to be found in these most remarkable eyes. Immediately under the cornea is an opaque varnish, varying according to the species, which produces sometimes in one and the same eye spots or bands of different colours. These spots and bands form a distinguishing ornament of many of the Tabani and other flies. And to this varnish the lace-winged flies (Hemerobius, &c.) are indebted for the beautiful metallic hues that often adorn them. When insects are dead, this varnish frequently loses its colour, and the eye turns white: hence many species are described as having white eyes which when alive had black ones. The consistence of this covering is the same with that of the varnish of the choroid in the eyes of vertebrate animals; but it entirely covers the underside of the lens, without leaving any passage for the light. Below this varnish there are numbers of short white hexagonal prisms[1474], every one of which enters the concavity of one of the lenses of the cornea, and is only separated from it by the varnish just described: this may be considered as the retina of the lens to which it is attached; but at present it has not been clearly explained how the light can act upon a retina of this description through an opaque varnish. Below this multitude of threads (for such the bodies appear), perpendicular to the cornea, is a membrane which serves them all for a base, and which consequently is nearly parallel with that part. It is very thin, of a black colour, not produced by a varnish; and in it may be seen very fine white tracheæ, which send forth branches still finer, that penetrate between the prisms of the cornea: this membrane may be called the choroid. Behind this is a thin expansion of the optic nerve, which is a true nervous membrane, precisely similar to the retina of red-blooded animals. It appears that the white pyramidal threads which form the retina of each lens are sent forth by this general retina, and pierce the choroid by a number of almost imperceptible holes[1475]. From this description it appears that the eyes of insects have nothing corresponding with the uvea or humours of those of vertebrate animals, but are of a type peculiar to themselves.
Having explained to you the wonderful and complex structure with which it has pleased the Creator to distinguish the organs of vision of these minute beings, proving, what I have so often asserted, that when animals seem approaching to nonentity, where one would expect them to be most simple, we find them in many cases most complex, I shall now call your attention to the next thing I am to consider—the number of the eyes in question. Most insects have only two; but there are several exceptions to this rule. Those that have occasion to see both above and below the head, the eyes of all being immovable, must have them so placed as to enable them to do this. This end is accomplished in many beetles, for instance Scarabæus L., Helæus Latr., &c., by having these organs fixed in the side of the head, so that part looks upward and part downward; but in others four are given for this purpose. If you examine the common whirlwig (Gyrinus Natator) that I have so often mentioned[1476], which has occasion, at the same time, to observe objects in the air and in the water, you will find it is gifted with this number of eyes. Lamia Tornator (Cerambyx tetrophthalmus Forst.) and some others, of which I make a genus, under the appellation of Tetrops, are also so distinguished. In these insects, one eye is above and the other below the base of the antennæ; in fact, in these the canthus, instead of dividing the eye partially, as in the other Capricorn-beetles, runs quite through it at considerable width[1477]. In Ryssonotus MacLeay (Lucanus nebulosus K.) the eye appears also to be divided in two by the canthus. In the Neuroptera Order there is more than one instance of the same kind. In Ascalaphus there are two considerable eyes on each side of the head, which, though clearly distinct, meet like those of many male flies and the drone. The male, likewise, of more than one species of Ephemera, besides the common lateral eyes and the stemmata on the back of the head, have a pair of compound eyes on the top of a short columnar process[1478]. In the Hemiptera Order, also, an instance occurs of four eyes in the genus Aleyrodes[1479]. Amongst the vertebrate animals, there is an example of eyes with two pupils in Anableps, a genus of fishes[1480], but no vertebrate animal has four of these organs. That many insects should have more than two eyes, will not seem to you so extraordinary as that any should be found that, like the Cyclops of old, have only one. There is, however, an insect, before celebrated for its agility[1481] (Machilis polypoda Latr.), which has a single eye in its forehead; or we may say, its eyes are confluent, without any line of distinction between them except a small notch behind. Now that I am treating of the number of eyes, I must not forget to observe to you, that in some insects no eyes at all have been discovered. In Polydesmus complanatus, on each side of the head there is an eye-shaped portion separated by a suture, in which under a powerful lens I cannot satisfy myself that I can discern any thing like the facets that usually distinguish compound eyes. In Geophilus electricus, another myriapod, they certainly do not exist[1482]. Whence we may conclude, as was before observed[1483], that the faculty of emitting light is rather given it as a means of defence than to guide it in its path.
The situation of compound eyes differs in different tribes. In some, as in the Staphylinidæ, they are planted laterally in the anterior part of the head; in others, the Carabi &c., in the middle; in others again, Locusta Leach &c., in the posterior part. In some, their station is more in the upper surface, either before or behind; so that a very narrow space separates them, or perhaps none at all. Instances of this position of the eyes occur in a minute weevil (Ramphus Clairv.[1484]), and many Diptera, &c. Of those that form an union on the top of the head, some are placed obliquely, so as to leave a diverging space below them, as in many Libellulina[1485], the drone[1486], &c. Others, as Atractocerus, in which the eyes occupy nearly the whole head, and unite anteriorly, have this diverging space above their conflux. In Rhina barbirostris Latr., another kind of weevil, they are confluent below the head, at the base of the rostrum, and a very narrow interval separates them above. In a large number of the Heteromerous beetles, they are set transversely, in the Capricorn ones longitudinally. Their surface, when they are lateral, has usually two aspects, one prone to see below, the other supine to see above. In general the eyes are situated behind the antennæ, so that their position, whether it shall be anterior or posterior, depends upon that of those organs. Often, indeed, as in the last-named beetles, part of the eye is behind and part before the antennæ; but except where there are four eyes, as in Tetrops, they are never placed before or below them.
Though the eyes of insects are generally sessile, yet to give them a wider range they are sometimes, but it rarely occurs, placed, like those of many Crustacea, on a footstalk, but not a moveable one. An instance of this in certain male Ephemeræ has already been mentioned. In the Hemiptera De Geer has figured two species of bugs (Cimicidæ) that are so circumstanced[1487]; as are also all the known Strepsiptera K., though in these the footstalk is very short[1488]: but the most remarkable example of columnar eyes is afforded by that curious Dipterous genus Diopsis, in which both eyes and antennæ stand upon a pair of branches, vastly longer than the head, which diverge at a very obtuse angle from its posterior part[1489].
In their figure eyes vary much. Sometimes they are so prominent as to be nearly spherical: this is the case with some aquatic bugs, as Ranatra, Hydrometra, and several male Ephemeræ[1490]. Very often they are hemispherical, as in the tiger-beetles (Cicindela L.), and the clocks or dors (Carabus L.); but in a large number of insects they are flat, and do not rise above the surface of the head.—With regard to their outline, they are often perfectly round, as in many weevils; oval, as in various bees; ovate, as in other bees (Andrena F.); triangular, as in the water-boatman (Notonecta). They are also often oblong, and occasionally narrow and linear; as in that singular beetle Helæus. In many of the Muscidæ they form nearly a semicircle, or rather, perhaps, the quadrant of a sphere. The eyes of the Capricorn-beetles (Cerambyx L.) have a sinus on their inner side, as it were, taken out of them; so that they more than half surround the antennæ, before which is the longest portion of them. An approach to this shape is more or less observed in the darkling-beetles (Tenebrio L.); but in these the sinus is not so deep. I may under this head observe, that in those Mantidæ that represent dry leaves, and some others, these organs usually terminate in a spine[1491].
Though not distinguished by the beauty and animation that give such interest to the eye of vertebrate animals, and exhibiting no trace of iris or pupil, yet from the variety of their colours the compound eyes of insects, though most commonly black or brown, are often very striking. Look at those of one of the lace-winged flies that commit such havoc amongst the Aphides[1492], and it will dazzle you with the splendour of the purest gold, sometimes softened with a lovely green. The lenses of those of Xenos blaze like diamonds set in jet[1493]. You have often noticed the fiery eyes of many horse-flies (Tabanus L.) with vivid bands of purple and green[1494]. Others are spotted[1495]; and Schellenberg has figured one (Thereva hemiptera)[1496], that exhibits the figure of a flower painted in red on a black ground. These colours and markings are all most vivid and brilliant in the living insect, and often impart that fire and animation to the eyes for which those of the higher animals are remarkable. Take one of the large dragon-flies that you see hawking about the hedges in search of prey, examine its eyes under a lens, and you will be astonished at the brilliance and crystalline transparency which its large eyes exhibit, and by the remarkable vision of larger hexagons which appear in motion under the cornea, being reflected by the retina—all which give it the appearance of a living eye. This moving reflexion of the hexagonal lenses in living insects was noticed long since in some bees (Nomada F., Cœlioxys Latr.)[1497]
Compound eyes differ greatly in their size. In some insects, as Atractocerus, the drone-bee, many male Muscidæ, &c., they occupy nearly the whole of the head; while in others, as numerous Staphylinidæ, Locusta Leach, &c., they are so small as to be scarcely larger than some simple eyes of spiders: and they exhibit every intermediate difference of magnitude in different tribes, genera, and species.
Under this head I must say something of the Canthus of the eye; by which I mean an elevated process of the cheek, which in almost all the genera of the Lamellicorn beetles enters the eye more or less, dividing the upper portion from the lower. Though usually only a process of the cheek, yet in the Scarabæidæ the whole of that part forms the canthus[1498]. It only enters the eye in the Rutelidæ, Cetonidæ, &c.; it extends through half of it in Copris; it goes beyond the half in Ateuchus; and in Ryssonotus MacLeay (Lucanus nebulosus K.) it quite divides the eye into two[1499], as I before observed. In Lucanus, Passalus &c. it projects before the eye into an angle; in Lucanus femoralis nearly into a spine; but in Lamprima and Œsalus it does not exist. The part, also, that enters the eye in the Capricorn-beetles may be regarded as a kind of canthus, though it is merely a dilatation of the front.
4. Stemmata[1500].—Having given so full an account of the kinds and structure of the ordinary eyes of insects, you may perhaps expect that I should now dismiss the subject: you would, however, have great cause to blame me, did I not make you acquainted with a kind of auxiliary eyes with which a large portion of them are gifted; I mean those pellucid spots often to be found on the posterior part of the front of these animals, or upon the vertex, frequently arranged in a triangle. These, Linné, from his regarding them as a kind of coronet, called Stemmata. They have been of late denominated Ocelli; but as this latter term is also in general use for the eyelets on the wings of Lepidoptera, I have adhered to that of the illustrious Swede. Neither he nor Fabricius has expressed any opinion as to the use of these organs; but Swammerdam and Reaumur were aware that they were real eyes. The former found that there are nerves that diverge to them though not easily traced, and that they have a cornea, and what he takes for the uvea[1501]; and the latter has supposed that the compound eyes and these simple ones have, the one the power of magnifying objects much, and the other but little, so that the former are for surveying those that are distant, and the latter those that are near[1502]. The same author relates some experiments that he tried with the common hive bee, by which he ascertained that the stemmata, as well as the compound eyes, were organs of vision. He first smeared the latter over with paint, and the animals, instead of making for their hive, rose in the air till he lost sight of them. He next did the same with the former, and placing the bees whose stemmata he had painted within a few paces of their hive, they flew about on all sides among the neighbouring plants, but never far: he did not observe that these ever rose in the air like the others[1503]. From this experiment it seems as if the compound eyes were for horizontal sight, and the stemmata for vertical.
The definition of them by Linné and Fabricius as smooth, shining, elevated or hemispheric puncta, conveys a very inadequate idea of them; for, except in a very few instances, they are perfectly clear and transparent, and their appearance is precisely the same as that of the simple eyes of Arachnida &c., under which head they might very well have been arranged; but as the last are primary eyes, and the stemmata secondary, it seemed to me best that they should stand by themselves. The structure of both is probably the same, and their internal organization that of one of the lenses of a compound eye, and both are set in a socket of the head.
Though a large number of insects have them, they are by no means universal, since some Orders, as the Strepsiptera, Dermaptera, and Aptera, are altogether without them. The Coleoptera, also, have been supposed to afford no instance of species furnished with them; but in the last number of Germar and Zincken Sommer's Magasin, it is affirmed that they are discoverable in Gravenhorst's genus Omalium, but not in the kindred genera Micropeplus and Anthophagus[1504]. Upon examining the former genus, I find, that although Omalium planum and affinities, O. striatulum, and some others, appear not to have them, yet with the aid of a good magnifier they may be discovered in most species of that genus; as likewise in Evœsthetus Grav. I find them also very conspicuous in A. Caraboides and other Anthophagi, but some species appear to want them. In these insects they are two in number, situated in the vertex a little behind the eyes but within them, and either at each end of a transverse furrow, or at the posterior termination of two longitudinal ones. Nor are they found in all the genera of the other Orders. In the Orthoptera, the Blattidæ, unless a white smooth spot on the inner and upper side of the eyes may be regarded as representing them, have them not; but in all the other genera of that Order they are to be found[1505]. In the Hemiptera all the Cicadiadæ are gifted with them; as are likewise Tetyra, Pentatoma, with many other Cimicidæ, and the Reduviadæ very remarkably; but many others in both sections of this order, as Thrips, Coccus, Aphis, Capsus, Miris, Naucoris, Nepa, and Notonecta, &c. are deprived of them[1506]. Of the Neuroptera the Libellulina add stemmata to their large eyes, in the anterior angle of which they are stationed[1507]; but many other genera of that Order are without them; as Myrmeleon, Ascalaphus, Hemerobius, &c. The Trichoptera and Lepidoptera universally have them; though in the latter, except in Castnia and the Sphingidæ, they are not easily seen. In the Hymenoptera they are usually very conspicuous, but in Larra and Lyrops, two genera of this order, the posterior pair are scarcely discernible; and in the neuter ants they are quite obsolete. In the Diptera, though many genera are furnished with them, yet many also want them; amongst the rest Latreille's Tipulariæ, and all the horse-flies (Tabanus L.). The Pupiparæ (Hippobosca L.) usually have none; but in Ornithomyia avicularia, one of that tribe, though extremely minute they are visible, arranged in a triangle, in the polished space of their vertex.
As to the Number of the stemmata, three appears to be most universal. Reaumur mentions an instance in which he counted four in a fly with two threads at its tail; but great doubt rests upon this statement[1508]. Some Orthopterous genera, as Gryllotalpa, and many Hemipterous, as Tetyra, Pentatoma, Reduvius[1509], Cercopis, Fulgora[1510], &c., have no more than two; and in Larra and its affinities, as just observed, the posterior ones are obsolete, so as to leave only one discernible.
Where there are three of these organs, they are usually arranged in an obverse triangle in the space behind the antennæ, at a greater or less distance from them. In those male flies (Muscidæ) whose eyes are confluent, the stemmata are in a little area behind their conflux; but, as before observed, in the drone-bee and the Libellulina they are before it. This triangle is in some cases nearly equilateral, as in Perla related to the may-flies, and many Hymenoptera; in others it is acutangular, as in Locusta &c., in which the stemma forming the vertex of the triangle is before the antenna[1511]: in others, again, it is obtusangular, as you will see in Pepsis and various Hymenoptera. In the humble-bees (Bombus), a line drawn through them would form a slight curve. Their situation also varies. In insects that have only two, they are sometimes placed a little behind the eyes, or in the back part of the space between them: this is the case with most of the bugs (Cimex L.) that have them.—They are often distant, as in Tetyra F., Edessa F.; and sometimes approximated, as in Reduvius F.[1512] In many of the Homopterous Hemiptera, as Cercopis, Ledra, &c. they are planted in the upper part of the head[1513], but in Iassus their situation is on the under part; and in a North American subgenus, as yet without a name, they are exactly between the two, being placed in the frontal angle. In Fulgora their station is between the eyes and antennæ[1514]. They are most commonly sessile, and as it were set in the head; but in some, as Fulgora candelaria, they stand on a footstalk. The stemmata are set in the side of a frontal tubercle in that four-winged fly of threatening aspect, Corydalis, which in its perfect state has mandibles, but longer and more tremendous, like those that distinguish the larva only of the kindred genus Hemerobius[1515]. These organs differ little in shape, being usually perfectly round and somewhat convex; but occasionally they vary in this respect. In Fulgora serrata they are oblong, with a longitudinal depression; in F. Diadema they are also umbilicated, but the umbilicus is circular; in Corydalis they are oval; in other insects they are ovate; in some semicircular, and in a few triangular. They vary much in size: in some of these animals being so minute as to be scarcely visible, while in others, as Corydalis, Dorylus, Vespa pallida F., Reduvius, &c.[1515], they are as large as some compound eyes. They differ also in colour, though often black: in Fulgora laternaria they are of a beautiful yellow; in F. candelaria they are white; in many Hymenoptera they are crystalline, in others red: the fierce look of Reduvius personatus is rendered more hateful by its stemmata having a pale iris round a dark pupil[1516].
Let us here stop and adore the goodness of a beneficent Creator, who, though he has deprived these little beings of the moveable eyes with which he has gifted the higher animals, has made it up to them by the variety and complex structure of their organs of vision, where we have only two points of sight, giving them more than as many myriads.
5. Antennæ.—But of all the organs of insects, none appear to be of more importance to them than their Antennæ, and none certainly are more wonderful and more various in their structure, and probably uses. Upon this last particular I shall enlarge hereafter. Their structure, as far as it differs in the sexes, I fully discussed in a former letter[1517]; and the most remarkable kinds of them will be included in a set of definitions which I shall draw up for you before our correspondence on this part of my subject closes: I shall therefore now confine myself to the following particulars—namely, their number, insertion, substance, situation, proportion, general form and structure, clothing, expansion, motions, and station of repose.
As to their Number, in the majority of crustaceous animals the antennæ amount to four, but no insect has more than two. A genus recently established (Otiocerus Kirby[1518]) seems to afford an exception to this rule, since the species composing it at first sight appear to have four, and in some instances even six antennæ; but as only two of them terminate in a bristle, the other, though proceeding from the same bed of membrane, may perhaps be regarded as merely appendages. Germar, who has described a species of this genus[1519] under the name of Cobax Wintheri, considers these appendages as analogous to palpi: but as they do not proceed from the oral organs, but from the bed of the antennæ at the base of the nose[1520], they ought certainly to be regarded rather as accessories to the latter, than as representing the former. In the Aptera order the mites (Acacus L.) appear to be without these organs. In the pupiparous tribe Hippobosca they seem about to disappear; and in the Arachnida &c., as has been more than once observed[1521], the mandibulæ have been thought to represent, not indeed the antennæ of insects, but the inner pair of those of the Crustacea.
In considering the insertion of antennæ, by which I mean their articulation with the head, we must advert first to the orifice (Torulus) that receives them[1522]. This is a perforation of the crust of the head; commonly, though not invariably, circular: in Coleopterous insects often with concave lubricous sides, forming an acetabulum, with processes usual in ginglymous articulations, larger than the bulb or root of the antennæ; and which is commonly covered, except the central space occupied by the bulb, with a tense membrane. Though not in general remarkable, in some cases it merits attention. In the genus Rhipicera Latr., the elegant antennæ of whose males I have described in a former letter[1523], particularly the Brazilian species, it is a long process on each side of the nose, and might be mistaken for the first joint: in another Coleopterous genus, Priocera K.[1524], it has somewhat of the shape of a trumpet: in Cupes a tubercle rises just above the base of the antenna: a circular process forms the torulus in Fulgora and others. It is also often placed in a cavity of the front, as in several wild-bees, Melitta K., and in Locusta Leach on the sides of an elevation of that part[1525]. In a large majority of insects the bulb (Bulbus) or ball which is received by the bed, wears the appearance, especially in the Hymenoptera, of a distinct joint; but if you carefully examine it, you will clearly see that it is merely the base of the scape swelled out into a spherical or other kindred form[1526]; and often marked, as in the Cicindelidæ, with impressed points: as it is the piece by which the antenna moves in its socket, this form of a rotula was doubtless given for its more ready motion in all directions. This structure is principally conspicuous in the Coleoptera and Hymenoptera Orders: in the others the base is not so distinguished from the rest of the scape. If you carefully extract the antennæ of a beetle, say a Copris or Lamia, and examine its base or bottom, you will find that it is open for the transmission of muscles and nerves; that in its upper margin it has a deep notch or sinus, on each side of which is a smaller notch; and that all round the margin, which is very lubricous, a membranous ligament is attached, by which it was affixed in the torulus. Its articulation, therefore, seems of a mixed kind, like that of most other organs and parts of insects, partaking of the ligamentous, ginglymous, and ball and socket. In the Orthoptera, Hemiptera, &c. the articulation seems more purely ligamentous.
With regard to their substance—these organs are regulated, in some degree, by the nature of the integument of the animal of which they are appendages; in the softer insects being of a softer substance than they are in hard ones. The vertex of the joints, where they receive the succeeding one, appears in many cases to be softer than the rest of it, and especially towards the apex, often papillose. The antennæ are generally opaque; but in Nebria complanata, a beetle common on the sea-coast in Wales and Lincolnshire, they are semitransparent.
The situation of antennæ must next be considered. In this respect it seems necessary that they should be so situated as to be under the direction of the eyes: for if you examine ten thousand insects (except, as was before observed[1527], where there are four eyes), you will not find one in which these organs are situated either above or immediately behind them; their station being always either somewhere in the space between the eyes or that below them. In Ptinus F. they are placed near the vertex; but in Gibbium, which is so nearly related to that destructive genus[1528], they are beneath them. In many Melittæ K. they are in the middle of the space between the eyes; and in many other Hymenoptera and Coleoptera (Staphylinus &c.), in the anterior part of it. In many Lamellicorn genera (except in some Acridæ, as A. viridissima) as Melolontha, Cetonia, Lucanus, &c. they may be regarded as planted in the lower surface of the cheek before the eyes; but in Copris &c., in which they are inserted further under the shield of the head, they are properly in the prone surface of the front. In the Capricorn-beetles (Cerambyx L.) and Cnodalon F. they may be termed inocular, or placed in a sinus of the eye; in the former tribe in its interior, and in the latter its anterior side. In the Rhynchophorous or rostrum-bearing beetles (Curculio L.) they vary in their situation. Thus in Macrocephalus Oliv. they are inserted at its apex; in Anthribus in its middle, and in Calandra at its base[1529]. In the water-scorpions (Nepa, Belostoma, &c.) they may be called extraocular, being placed under the head in its prone part, outside the eyes[1530]. In Nirmus Fringillæ, a kind of bird-louse, they appear to be oral, being situated, according to De Geer, under the head near the mouth, at a great distance from the eyes[1531].
In their proportions, both as to length and thickness, antennæ vary extremely. Thus sometimes they are very short—much shorter than the head; as in the aquatic beetles Gyrinus, Parnus, and the water-scorpion; and some land-beetles, as Anthrenus, &c. At other times they far exceed the length of the insect: the males of many Capricorn-beetles are so distinguished. In that of Lamia ædilis they are more than four times as long as the body; and every intermediate length between these two may be found amongst them. They vary also greatly in thickness: in Paussus, whose antennæ emit light in the night[1532], and Cerapterus, they are nearly as thick,—at least their knob, which forms the chief part of them,—as the body of the insect[1533]; while in Mantis, Acrida K. and Psocus, they are as slender as a hair. The antennæ in many of the Prioni, especially in P. imbricornis, are thick from base to tip; while in other Capricorn-beetles they are quite the reverse.
It will not be necessary to enlarge here upon the general form of these organs: I shall therefore only notice the two principal divisions of them in this respect.—Antennæ, regard being had to one of their uses, may be divided into two sections, distinguished by forms extremely different: those, namely, that are employed by insects as tactors to explore their way, and those that cannot be so employed. The great majority are of the former kind; but those that may be denominated setigerous,—as the antennæ of the Libellulina, Ephemerina, of the Homopterous Hemiptera, and of many Diptera, the last joint of which terminates in a bristle, or is furnished with a lateral one, and of some gnats that have short feathered antennæ,—appear not fitted to be used as tactors to explore by touch, and form the latter description. This difference in these organs, as I shall have occasion to prove more at large hereafter, furnishes a strong presumption that their primary function is not touch. Were this the case, it would be common to them all.
As to their structure, antennæ consist in general of a number of tubular joints; each of which having separate motion, the animal is thereby enabled to give them every flexure necessary for its purposes. The scape, or first joint, by means of the bulb inosculates in the torulus, or is suspended to it; and the others, sometimes by a similar, though less pronounced knob at their base, inosculate in the preceding one; but in some cases the inosculation seems not so perfect, the joints being simply suspended by ligament. In pectinated or lamellated antennæ, the branch is usually a lateral process of the joint from which it issues; but in Phengodes (Lampyris plumosa L.) its involute plumose branches appear to articulate with the apex of each joint[1534]. I have a specimen of one of the Cleridæ, of a genus undescribed, in which each branch is forked. In some tribes of the Capricorn-beetles (Stenocorus, &c.) the antennæ are often armed at their apex with spines, sometimes on the upper side and sometimes below. In some aquatic beetles (Gyrinus, Parnus) they are furnished with an auricle at their base, which, like the lid of a box, shuts them in when unemployed, and protects them from the water[1535].
The portions into which antennæ may in general be considered as divided, have been sufficiently explained to you above; but it may not be amiss to add here a few words on the principal variations in their structure that I have had an opportunity of observing. The scapus[1536] or first joint, which includes the bulbus, is usually the most conspicuous joint in the antenna (exclusive, I mean, of the capitulum, in those in which that organ terminates in a knob), it being thicker and often longer than the succeeding ones. In the Capricorn and Darkling beetles, indeed (Cerambyx and Tenebrio L.), the third joint is the longest, but the scape is still the thickest; and in the stag-beetles (Lucanus L.), many of the weevil tribes (Curculio L.), and those of the bees (Apis L.), except in the males, it is as long nearly as the remainder of the antennæ, which forms an angle with it. In shape it is generally somewhat curved and subclavate, or increasing in size from the base to the summit; but it is sometimes straight and filiform, at others oblong or square, at others again triangular, in several instances three-sided: in one (Cetonia cruenta F. Genuchus K.) it is, as it were, broken, the upper part forming nearly a right angle with the lower; in Cerocoma Schæfferi it is foliaceous; and it is occasionally suborbicular: and probably many other forms might be enumerated.
The Pedicellus[1537] is the second, and may be deemed the least conspicuous joint of the antennæ. Though more slender than the scape, it is generally thicker than that which immediately follows it. In broken antennæ it is the hinge or pivot on which the clavola or upper member turns: it is usually very short, campanulate or bell-shaped, or obconical; but in a species of bug (Tetyra, from New Holland—T. pedicellata Kirb. MS.) it is nearly as long as all the rest of the joints taken together. In those species of Lycus, a genus of beetles related to the glow-worm, that have flattened antennæ (as L. reticulatus, fasciatus, &c.), this joint is almost received into the socket of the scape, so that their antennæ appear at first to have only ten joints, but in those which have those organs filiform (as L. minutus, Aurora, &c.) it is more conspicuous.
The Clavola[1538], or remaining joints of the antennæ taken together, constitutes the principal part of the organ, which, especially at its extremity, exercises its functions of touch, or any other sense. The principal variations, as to form and structure, that occur in this part will be mentioned in another place. I shall only here observe, that in many instances the first joint of this part is longer than the rest; but in Tetyra pedicellata just mentioned, it is by far the shortest, and shaped like the pedicel of most insects. In the Libellulina, the Homopterous Hemiptera, and those flies whose antennæ terminate in a bristle, the clavolet is represented by the bristle. But in the flies which have a lateral bristle, on the last joint, and those with triarticulate antennæ that have no bristle, the terminal joint represents it. The clavolet often terminates in a knob, or in several joints thicker than that which precedes them. This varies greatly, not only in its form, but also in the number of joints of which it is composed. Thus in Paussus, Platypus, and many Calandræ, it consists of only a single joint[1539]; in Anthrenus, Ditoma, &c. of two; in Nitidula, Geotrupes, &c. of three[1540]; in Tetratoma, the Silphidæ, of four[1541]; of five in Scaphidium[1542]; of six in one species of Languria, of seven in the common cockchafer (Melolontha vulgaris[1543]); of eight in Diaperis Boleti, in which the whole clavolet forms the club[1544]; of nine in Oenas; and ten in Cerapterus[1545]. All the above, you will observe, are beetles. In the other orders there are eleven joints in the knob of some butterflies; twelve in that of Ascalaphus[1546] and Myrmeleon; and lastly, fourteen in Trachelus[1547].
Under structure also, the number of joints of which antennæ in general consist, should be considered. If you examine the insects belonging to the different orders, you will find remarkable variations in this respect. Let us run through them:—In the Coleoptera the natural number of joints is eleven; but this rule is not without many exceptions. Thus, many have fewer than the prescribed number: Paussus has only two[1548], Claviger and Platypus five, Dorcatoma and Calandra eight[1549], Geniates K. and Phanæus MacLeay nine[1550], and lastly Melolontha ten[1551]. Others, again, have more than eleven joints: Cebrio grandis, Chrysomela stolida, some Saperdæ, and several others, have twelve. In Prionus imbricornis the female has nineteen, and the male twenty[1552]. Rhipicera marginata has thirty-two; and in a New Holland species of this genus I counted thirty-eight. In the Orthoptera I can trace no general law in this respect. In Locusta Leach in some species you may count fourteen joints, in others sixteen, and in others twenty-five. In one, which appears to be a pupa, I found only thirteen. In Mantis they exceed thirty; but in Blatta, from between thirty and forty, they reach nearly one hundred and fifty; often varying in number in different individuals of the same species. The order Hemiptera exhibits two peculiar types of antennæ, which, with some exceptions, distinguish the two natural sections into which M. Latreille has judiciously divided it. In the Heteropterous section they are without a bristle at their end; and in the Homopterous one, with the exception of Aphis, Thrips, &c. they have one. In the genera of both these tribes, the number of joints varies in these organs. Thus, exclusive of the seta, in Flata and Cixius there are only two joints; in Galgulus, Fulgora, and Cercopis, there are three; in Lygæus, Coreus, &c. there are four; in Tetyra, Pentatoma, Tettigonia, there are five[1554]; in Aleyrodes there are six; in Aphis seven; in Thrips eight; in Psylla ten, the last of which is terminated by two bristles[1555]; and in Coccus eleven. The Neuroptera order, as it stands at present, is regulated by no general rule with regard to the number of joints in the antennæ of the insects that compose it. Several types of form in these organs distinguish its discordant tribes. The first is that of the Ephemeræ, in which the antennæ consist of two short joints, crowned by a short, tapering, unjointed bristle. The second is that of the Libellulina, similar to the above, but with a jointed bristle. The third is that of Psocus, in which the antenna has two short thick joints at the base, terminated by a long filiform bristle, consisting of seven or eight joints, and finer than a hair. Perhaps these three may be regarded as belonging to a common type. The fourth type is presented by the short filiform antennæ of Termes; the fifth by the setaceous ones of Corydalis, Hemerobius, &c.; and the sixth and last by the clavate and capitate ones of Myrmeleon and Ascalaphus. In the Lepidoptera and Trichoptera orders the antennæ, though varying in their general form in the three tribes of which Linné formed his genera Papilio, Sphinx, and Phalæna, with the exception of Hepialus, in which the joints are few, are always multiarticulate:—we will therefore, without further delay, proceed to the Hymenoptera. In Latreille's tribe Aculeata the general rule is, that the females shall have twelve joints and the males thirteen. In his Ichneumonides the law seems to be, that the antennæ shall be multiarticulate and setaceous; but in most of the other tribes of the order, even those that in other respects are most nearly related,—as in his Tenthredinetæ,—the number of joints of these organs varies without end. Thus in Hylotoma there are only three joints[1556]; in Cimbex læta[1557] five; in C. axillaris and Perga Leach[1558], six: and so on to twenty-five or more[1559]. The same fluctuation in this respect runs throughout the rest of the order. In the Diptera there are two general types of antennæ:—those of the Tipulariæ Latr., consisting usually of from fourteen to sixteen joints, in the males often resembling beautiful plumes; and those of the remainder of the order, in which they do not exceed three joints[1560]: though the last, or patella, is often further divided into obsolete or indistinct ones[1561]. These antennæ may be further subdivided into filatæ and aristatæ, or those without and those with a bristle, either lateral or terminal.
The clothing of antennæ also merits attention, since it is often not a little remarkable. By clothing I understand the down or hairs of every kind with which they are either generally or partially covered. A great number of filiform and setaceous antennæ of Predaceous beetles (Cicindela L., Carabus L.) have the first two, three, or four joints naked, and the rest covered with a fine down. In insects that have a knob at the end of these organs, whether lamellated or perfoliate, this down is often confined to it, or to its intermediate joints, and seems intermixed with nervous papillæ. These are particularly visible in the flabellate antennæ of Rhipicera, Lampyris Latreillii[1562], Elater flabellicornis[1563], &c. covering both surfaces of the processes of the joints. In some male bees these papillæ are inclosed in hexagonal spaces into which the antennæ are marked out[1564]. It is to be observed, that in many antennæ the joints of the clavolet have one or two bristles or more at their apex, one above perhaps, and one below; the lower angle in those of the serrated antennæ of Elater is usually so furnished, and sometimes the upper. In many Capricorn-beetles and various insects the antennæ are clothed, instead of down, with stiffish hairs or short bristles. Other insects have these organs, at least the clavolet, beset with longer hairs standing out from them on all sides: of this kind are those of a singular beetle (Sarrotrium muticum) sometimes found in this country[1565]. Again, there are some that have only their underside bearded with longer hairs; as Lamia curculionoides, speculifera K., and other Capricorns[1566]. In another of this tribe, Saperda hirsuticornis, the three intermediate joints are ornamented with branches of long black hairs, which give them an elegant and feathery appearance[1567]. In Callichroma alpina the apex of the slate-coloured joints of its antennæ is bearded with black hairs. In Lamia reticulata, and Saperda fasciculata and plumigera, all also Capricorns, a single bunch of hairs, resembling the brush of a bottle-cleaner, signalizes the middle of the antenna[1568]: in Saperda scopulicornis K. this is star-shaped[1569]. Sometimes the scape is externally bearded, as in Trox, a beetle found in horns and bones; and in many other Lamellicorns[1570]. In this last tribe the two exterior leaves of the knob of the antennæ are often set with short bristles[1571]; and in a minute beetle called by De Geer Dermestes atomarius, the hairs of this part are said to form a brush[1572].
When insects, I mean more particularly Coleoptera, are about to move from any station where they have been at rest, the first thing they usually do, before they set a step, is to bring forward and expand their antennæ, which have either been carefully laid up in a cavity fitted to receive them, or back upon the body: if they terminate in a lamellated knob, they separate the lamellæ as far as possible from each other; or if it is perfoliate, the joints of it mutually recede. The object of this is evidently to collect notices from the atmosphere, since the papillose part of these joints cannot be applied to surfaces. When the animal begins to move, in many cases the antennæ do the same, and continue their motion till it stops and returns to a state of repose. In the parasitic tribes of the Hymenoptera (Ichneumon L.) they are kept in an almost constant vibration. Many other insects move them in all directions without any order or regularity; and others, when they elevate one depress the other, and so proceed as if balancing themselves by means of these organs like a rope-dancer. I have before stated to you how by motions of their antennæ, ants and bees communicate their wants or discoveries to each other, or make inquiry concerning any thing they wish to know[1573]. But as I shall have occasion to make some further remarks upon this subject, when the senses of insects are under discussion, I shall for the present take my leave of it.
I shall conclude what I have to communicate to you relative to the organs of which we are treating, with a few observations with respect to their station when the insect reposes. In the Capricorn-beetles, Eucera and other insects with long antennæ, they are merely turned back or on one side with no particular cavity for their reception when unemployed, but probably the apex passes under the body. In the Predaceous and Darkling beetles (Carabus L., Tenebrio L.) their station is usually under the sides of the prothorax, and in the Tortoise beetles (Cassida), under its anterior margin. In the Elastic beetles (Elater) they are received into a groove between the under margin of that part and the fore-breast (antepectus). In Anthrenus, when the animal reposes or counterfeits death, the antennæ are concealed in a cavity of the underside of the prothorax, at right angles with the throat[1574]. In the kindred genus Byrrhus, another simulator of death, a large cavity is excavated under the same part, to receive both the forelegs and antennæ, a narrow space being left between the angle of the prothorax and fore-breast exactly admitting the base of the latter, which are quite concealed under the former. In Cryptocephalus and Chlamys, kindred beetles, when at rest they are withdrawn, except their scape and pedicel, with the head within the cavity of the prothorax. In others they are turned under the head, without any particular cavity for their reception; as in many moths, Apion, &c. In most of the Lamellicorn beetles their station is in the cavity formed by the eye and the throat, the knob forming an angle with the rest of the antenna. In Heterocerus they follow the contour of the eye[1575]. In Brentus, a genus of weevils remarkably long and slender, they are turned back and received by a slight longitudinal cavity of the rostrum; but in those of this tribe (Curculio L.) in which the clavolet forms an angle with the long scape, this latter part, bending back, is laid up in an oblique channel of that part; and the former, pointing in the contrary direction, is folded upon it. In many flies (Muscidæ) a vertical frontal cavity receives the antennæ, which point downwards during repose[1576]. Cryptocerus, a very remarkable ant, has on its head a singular square plate, the sides of which form a deep longitudinal cavity: in this cavity the antennæ, quite concealed, repose in safety. A cavity equally remarkable is exhibited by the water-scorpions, particularly Belostoma, in which is a very deep kidney-shaped box, between the eye and throat, to receive and defend its singular antennæ[1577]; which, when they are reposing, is closed by the exterior harder joints, and from which it seems as if they turned out, like a sentinel out of his box. In some aquatic genera of beetles, as Gyrinus, Parnus, &c. they are withdrawn within a lateral cavity of the same part, and are defended from the water externally by the auricle at their base[1578]. The flabellated and lamellated antennæ, previous to their being folded for repose, close all their plates; which in action are as widely expanded as possible, so as to form a knob; and in some the middle piece is entirely concealed, as if in a box. In broken antennæ, or those in which the clavolet forms an angle with the scape, the former is folded upon the latter, with its point downwards.
II. Subfacies.—Having dispatched the Facies, or upper side of the head, I am next to consider the Subfacies, or under side: but as the principal parts that occupy this side have been already considered, I shall have no occasion to detain you long.
i. Jugulum[1579].—This part, which may be regarded as analogous to the throat in vertebrate animals, lies between the cheeks; from which it may usually be distinguished by being more lubricous and tumid, and often separated by an impressed line. It is particularly conspicuous and elevated in the Lamellicorn beetles, and calculated by its lubricity for easy motion in the lower side of the cavity of the chest. Its apex is the base in which the mentum sits. It is not necessary to enlarge further upon it, as it seldom exhibits striking characters.
III. Collum[1580].—In a large proportion of insects the head inosculates in the trunk without the intervention of a neck, or a constriction of the head behind. In the Orders Orthoptera, Trichoptera, Lepidoptera, Hymenoptera, and Diptera, no instance of it that I recollect occurs: in the Coleoptera there are many. In the Predaceous beetles, though several have no distinct neck, yet others, as Anthia, &c. have a short and thick one; and some few, as Colliuris, Agra, &c. one more pronounced. Latreille has named a tribe in this Order Trachelides, from the circumstance of their having a neck: in this tribe you will find the blister-beetles (Cantharis and Mylabris) both of the moderns and the ancients. In the Hemiptera order the water-scorpions Nepa, &c. have a thick short neck; and Zelus, (a kind of bug,) one longer and more slender; and, like Raphidia, the snake's-head fly, which is similarly circumstanced, has the air of a serpent. Other Neuroptera, likewise, have a neck; as Hemerobius, Corydalis, &c. This part presents no other features that merit notice.
IV. Myoglyphides[1581].—The Myoglyphides, or muscle-notches, are sinuses, some shallow and some deeper, in the posterior margin of the upper side of the head, to which the levator muscles are affixed. They seem principally confined to the Coleoptera; though, in some cases at least, they may be traced in the Heteropterous Hemiptera. These notches vary in number and depth in different insects. Thus in Buprestis there is only one deep one[1582]: in Copris there are two shallow ones, in a deep sinus separated by a small prominence[1583]: in Elater and Lamia there are also two not in a sinus; and in Calandra Palmarum there are four, two on each side, with a prominent lobe between them[1584]. To each of these notches, at its under margin, below the ligament that unites the occiput to the trunk, a muscle to raise the head is usually attached.
[LETTER XXXV.]
EXTERNAL ANATOMY OF INSECTS, CONTINUED.
THE TRUNK, AND ITS PARTS AND ORGANS.
As the head of insects is the principal seat of the organs of sensation, so is the trunk of those of motion; and in it are contained the muscles by which they are moved: it may therefore be regarded as the great centre of motion, and as the main support and prop of the two other primary sections of the body—the head and abdomen, between which it is situated—it may be deemed the most important part of the insect, the key-stone of the whole structure. In treating upon it, for the greater clearness, I shall consider its substance, general form, proportions, composition, internal processes, and members. It will first, however, be necessary to assign my reasons for the nomenclature of its parts that I have adopted.
Had the entomological world been universally agreed upon this subject, and there was an established system of Orismology[1585], I should have proposed no alteration without great reluctance, and the fullest conviction of the absolute necessity of some change; but as the standard of language in our science is still unsettled, and different terms are used by different writers, there seems full liberty left to me to select those that appear upon the whole most appropriate; and where proper and significant terms seem wanting, to invent new ones. M. Latreille, in a late Essay[1586], has proposed many changes of this kind, and seems to hesitate concerning the adoption of some of those recently coined in France for the parts of the trunk[1587]; it may therefore, I think, be permitted me to labour a little in this hitherto imperfectly cultured field, and to suggest such improvements as the subject may seem to require or admit.
Linné called the part we are now considering the trunk, its upper-side he usually denominated the thorax, and its under-side the breast: he notices also the scutellum and sternum[1588]. As the prothorax and scutellum are the only apparent parts of the back of the trunk in his first Orders (Coleoptera, Hemiptera L.), the rest being covered, in noticing these he puts the part for the whole, calling the prothorax the thorax, but which strictly was not synonymous with what he called by the same name in the other Orders. Linné's phraseology with regard to the trunk of insects was adopted by Fabricius and other Entomologists, till Illiger employed the term thorax to designate the whole of the trunk[1589], calling the upper part thorax superior and the lower thorax inferior. M. De Blainville, M. Latreille, and other French writers, improved upon this, naming the upper part the back (dorsum), and the lower the breast (pectus); and dividing the trunk, or according to them thorax, into three sections, each bearing a pair of legs. But I see no sufficient reason for this alteration—the terms trunk, thorax, and breast, in the common acceptation are well understood, and lead to no confusion or glaring impropriety; I shall therefore adhere to the old phraseology, especially as French Entomologists in popular language still do the same.
As to the division of the trunk into segments by M. Latreille and others, it has been regarded as consisting of three primary ones, which have been called in the order of their occurrence, reckoning from the head—prothorax, mesothorax, metathorax. The first of these segments, however—and the learned Entomologist just named seems to hint as much[1590]—is usually more distinct from the other two, than they are from each other. If this idea be correct, the trunk is properly resolvable into two primary segments, the first bearing the arms or fore-legs, and the other the proper legs and the organs of flight. M. Chabrier calls the latter tronc alifère, or wing-trunk;—a happy term, which I have adopted and latinized, calling it the alitrunk (alitruncus): the first segment, because it bears the fore-legs, I have named manitrunk (manitruncus). I adopt likewise the terms above mentioned, prothorax, mesothorax, metathorax, to signify the three segments into which the thorax of Linné, or the upper side of the trunk, is resolvable; and those of the breast I denominate antepectus, medipectus, and postpectus. If terms be thought necessary to designate the two intire segments into which the alitrunk is resolvable, the first may be the meditrunk (meditruncus), and the other the potrunk (potruncus).
I. Substance.—With regard to its substance, the trunk in general is softer than the head, and harder than the abdomen, especially as to its upper surface; but in some cases, where it is not protected by the elytra, as in the rove-beetles (Staphylinus L.), the abdomen appears as hard as the trunk. Though usually not very different from the elytra in this respect, in Meloe, Lytta, and other vesicatory beetles, it is of a firmer consistence.
II. General Form.—In the Coleoptera Order the only part of the trunk that is visible on its upper-side is the prothorax: the mesothorax, with the exception of the scutellum, and the metathorax, being entirely concealed by it and the elytra; so that, with regard to shape, it may nearly be considered as merging in the prothorax. Below it is more visible, and may be stated as more or less quadrangular; in oblong beetles inclining to a parallelogram, and in shorter or hemispherical ones to a square. In the majority it is more convex below than above, except in the case of the hemispherical or gibbous beetles (Coccinella, Erotylus, &c.), in which the under-side is flat and the upper-side very convex. In the Diurnal Lepidoptera the trunk approaches to a cubical shape, in the Nocturnal it is more spherical. A similar difference obtains in the Hymenoptera and Diptera: in the bees, wasps and flies, the trunk approaching to the figure of a sphere; in the ants, Scoliæ, crane-flies, &c. to that of a cube. The upper part of it in many Ichneumonidæ, female ants, &c. is very elevated, forming an arch, and sloping towards the abdomen. In general it may be observed with respect to the remaining Orders, that the form of the trunk merges in that of the whole body, the tendency of which is often to a three-sided figure.
III. Proportions.—The trunk is usually longer and larger than the head and longer than the abdomen, but not wider: but there are exceptions to both these rules. In Colliuris, Mantis, &c., it is more slender; and in Atta megacephala and some neuter ants, it is shorter than the head; in Atractocerus, many Staphylinidæ, Phasma, the Libellulina, the Lepidoptera, and various Hymenoptera, it is shorter, and in the Mantidæ more slender than the abdomen. The greatest disproportion between it and the last part is exhibited by the genus Evania, parasitic upon the Blattæ, in which the abdomen appears merely as a minute and insignificant appendage of the trunk. The vertical diameter of this part, almost without exception, is greater than that of either head or abdomen. When we consider that it contains the muscles that move both the organs of flight and the legs, we see clearly the reason why the Creator gave it greater volume.
IV. Composition.—I lately intimated to you that the trunk, though resolvable into three segments, in most cases properly consists of only two primary ones. Whoever examines the perfect insects of every Order, except the Aptera[1591], will find this distinction strongly pointed out, not only by the different direction of the first pair of legs from that of the two last, but also in a large proportion by a deep incisure; and in all it is further manifested by the anterior segment having a motion distinct from that of the rest of the trunk, and separating readily from it; and this not only where it is large, as in insects that have a thoracic shield, but also in those in which the prothorax is less apparent: whereas the other two pedigerous segments have little or no distinct motion, will not readily separate from each other, and in some cases exhibit no pectoral suture between them. Sometimes, however, these two last segments are more prominently distinguished: in Lytta, Mylabris, and other vesicatory beetles, they are separated below by an incisure, or rather the first or mid-leg segment, is not nearly so elevated as that of the hind-legs. In some ants (Atta Latr.), in the neuters, there is no distinction of segments in the trunk; but in others (Formica Latr.) it follows the general law, and consists of three. In the Arachnida, with the exception of Galeodes, in which the head is distinct, and the three segments of the trunk may be traced, these parts together form only a single segment. Induced by these reasons, I consider the trunk as consisting in general of two primary segments, the manitrunk and alitrunk: the latter resolvable into two secondary ones.
* Manitruncus[1592].—The manitrunk, then, is the anterior section of the trunk, which bears the arms and contains the muscles that move them. This part has free motion, or a motion independent of that of the rest of the trunk. This indeed seems a necessary result of the direction and uses of the arms. It consists of an upper and lower part—the prothorax and antepectus.
i. Prothorax[1593].—The upper part of the manitrunk in the Coleoptera, Orthoptera, and Hemiptera, is by far the most conspicuous part of the trunk, but in the other Orders it is less so. With respect to it, insects may be divided into two classes—those that have and those that have not a prothorax. In the Coleoptera Order it is remarkable both for size and variations in its shape and sculpture. In the Orthoptera, though less various, it is almost equally conspicuous, especially in Blatta. In the Homopterous section of the Hemiptera, in many genera it has become extremely short; while in the Heteropterous section its dimensions are not much reduced. In the majority of the Neuroptera, likewise, it is comparatively large; in the Libellulina much shorter, and in the Trichoptera and Lepidoptera nearly evanescent[1594].—In the Hymenoptera and Diptera, with very few exceptions, the thoracic shield altogether disappears, at least if I am correct in an idea, which I shall hereafter explain, that the collar usually regarded as the analogue of the prothorax, is really a part of the alitrunk. In these last Orders, though there is no true prothorax, the manitrunk still remains under the form of an antepectus, bearing the fore-legs, and containing the muscles that move them.
The prothorax of insects may in general be considered with respect to its parts, margin, appendages, shape, sculpture, clothing, and proportions.
1. The prothorax, regarded as a whole, distinct from the antepectus or fore-breast, consists commonly of two pieces—the shield, or upper part[1595], and the ora, or under part[1596]. In the shield you are to observe its apex[1597], base[1598], sides[1599], limb[1600], and disk[1601]. The apex is the part next the head; the base that next the abdomen; the limb the circumference, and the disk the central part. In many Orthoptera and Heteropterous Hemiptera, the shield appears further to consist of two pieces, an anterior and posterior one. The ora is a continuation of the shield below the lateral margin, turned downwards and inwards towards the fore-breast and the legs, but separated from the former in most cases by a suture, as in Carabus L.; and in others merely by an impressed line, as in Blaps F.; but in Curculio and Cerambyx L., &c. there is no ora, the shield being without a lateral margin, and forming one piece with the antepectus. The part we are now considering varies in different genera. Sometimes it is very narrow, as in Scarites; at others very broad, as in Buprestis, Nepa, &c. In Lampyris, except L. italica, and affinities, it projects posteriorly into a lobe or tooth, which forms a right angle with the rest of the ora, and becomes the lower part of the cavity that receives the head; and in Dermestes this part is excavated into an anterior and posterior one which admits the antennæ and arms when folded for repose.
2. The margin of the prothorax is a ridge, either defining its sides or whole circumference. In many cases this margin is broad and dilated, but in others it is merely a thread or bead that separates the shield from the ora. Though generally terminating the upper surface, it sometimes, as in Staphylinus, dips below it. In many insects, however, as I just observed, the thoracic shield has no lateral margin whatever.
3. Various and singular are the appendages with which the prothorax of numerous insects is furnished. Many of these are sexual distinctions, and have been before described to you[1602]; but there are others common to both sexes, the most remarkable of which I shall notice.—Some are distinguished by a long horn which overhangs the head, as Membracis cultrata, ensata, &c.[1603]; in others it stands upright, as in Centrotus spinosus[1604]; C. Taurus has a pair of thoracic horns like those of a bull, only dorsal[1605]; in Ledra aurita they are flat, and represent ears[1606]; in some species of Tingis (T. Echii, Pyri, &c.) a kind of reticulated hood, resembling lace, is elevated from the anterior part of the prothorax, which receives and shelters the head[1607]. In Centrotus globularis and clavatus F., especially the former, the part in question is armed by a most singular and wonderful apparatus of balls and spines,—in one case standing erect[1608], and in the other being horizontal[1609],—which gives these animals a most extraordinary appearance. In many of the species here quoted the prothorax is producted posteriorly into a long scutelliform horizontal horn, which more or less covers the wings and abdomen; a circumstance which also distinguishes the genus Acrydium F. (Tetrix Latr.). This horn seems to have been sometimes regarded by Linné and Fabricius as a real scutellum, and sometimes only as a process of the prothorax: but that it is merely the latter will be evident to you, if you examine carefully any insect furnished with this appendage; for if you remove that part, you will discover the true scutellum and other parts of the trunk concealed beneath it. A very remarkable prothoracic appendage is exhibited by some species of Mantis. In general the part we are treating of in this tribe is very slender; but in M. strumaria, gongyloides, &c., it appears dilated to a vast width, and assumes, either partially or generally, a subrhomboidal form; but if it is more closely examined, it will be found that the form of the prothorax is really similar to that of the rest of the tribe, but that this part is furnished on each side, either on its whole length or anteriorly, with a large membranous flat subtriangular appendage resembling parchment[1610]. Perhaps this kind of sail may be useful to the animal in flight. In Prionus coriarius &c. its sides are armed with teeth, and in many Lamiæ, Cerambyces, and other Capricorn beetles, and often in various bugs (Pentatoma Latr.) with sharp fixed spines. But the prothorax has moveable as well as fixed appendages; of this kind are those spines (umbones), whose base is a spherical boss moving in an acetabulum of the thoracic shield of the Capricorn subgenus Macropus Thunb. If I might hazard a conjecture, I should say that these organs were given to this animal by an all-provident Creator, to enable it to push itself forward, when in the heart of some tree it emerges from the pupa, that it may escape from its confinement. Another kind of moveable appendages are attached to the thorax of Lepidoptera, usually in the form of a pair of concavo-convex scales covered externally with a tuft of hairs[1611]. M. Chabrier, who examined these scales in recent insects, describes them as vesicles, which appeared to him full of a liquid and of air, and capable of being alternately inflated and rendered flaccid; he regards them as accessories to a pair of spiracles, which he looks upon as vocal[1612], opening into the manitrunk just above the insertion of the arms. These organs are quite distinct from the tegulæ that cover the base of the primary wings of insects of this Order[1613], and are what, borrowing a term from Mouffet[1614], I have called in the table patagia, or tippets. Under this head I may include the caruncles at the anterior angles of the prothorax of a genus of beetles with soft elytra, named by Fabricius Malachius. When pressed, says De Geer of these insects, a red inflated soft vesicle, of an irregular shape, and consisting of three lobes, emerges from the thorax and from each side of the anterior part of the abdomen, which re-enters the body when the pressure is removed[1615]. M. Latreille seems to think that these vesicles have some analogy with the poisers of Diptera and the pectens of scorpions; and that they are connected with the respiration[1616].
4. We are next to say something upon the shape of the prothorax. The forms of the thoracic shield, especially in the Coleoptera, are so various, that it would be endless to aim at particularizing all; but it may be useful to notice a few of the most remarkable. The prothorax of Moluris, a darkling-beetle, approaches the nearest of that of any insect to a spherical form, from its remarkable convexity; in the wheel-bug (Reduvius serratus) it is compressed, and longitudinally elevated into a semicircular serrated crest: it is crested, also, in many Locustæ and Acridæ, in some having two parallel ridges; but, generally speaking, its surface is more depressed. In Necrodes it is nearly circular, in Blatta petiveriana semicircular, in Nilion and some Coccinellidæ crescent-shaped, in Carabus obcordate, in Cantharis and Sagra approaching to a square, in Languria to a parallelogram; in many Cimicidæ, Belostoma, &c., it is triangular, with the vertex truncated; it is trapezoidal in Elater, in Ateuchus rather pentagonal, and exhibiting an approach to six angles in some other beetles[1617]: but the prothorax most singular in form is that of some species of M. Latreille's genus Helæus[1618], as H. perforatus, Brownii, &c.: in these its anterior angles are producted, and curving inwards, lap at the end one over the other, so as to form a circular orifice for the head, which otherwise would be quite covered by the shield. Thus the upper portion of the eyes can see objects above, as well as their lower portion those below. I might enumerate many other forms, but these are sufficient to give you some notion of the variations of this part.
5. The prothorax is equally various in its sculpture; but since in the Orismological table almost every instance of it has its place, I shall here only notice it as far as it is common to the whole tribes, genera, or subgenera. The Scarabæidæ of Mr. W. S. MacLeay are distinguished by a small excavation on each side of this part, which, as has been before remarked[1619], furnishes an elevated base for an internal process with which the anterior coxæ ginglymate. In Onitis and Phanæus, to these excavations are superadded a pair impressed in the base of the prothorax, just above the scutellum; in Carabus L. a longitudinal channel divides the thoracic shield into two equal portions; and many genera of that great tribe have in addition, at the base on each side, one or two excavations or short furrows. Elophorus F. has on this part several longitudinal channels, alternately straight and undulated. Generally speaking, in Carabus L. the prothorax has no impressed points; but in one or two subgenera of Harpalidæ (Chlænia &c.) it is thickly covered with them. In numbers of Locusta Leach, the part we are considering is what Linné terms cruciate, being divided into four longitudinal portions by three elevated lines, the intermediate one being straight, and the lateral ones diverging from it both at their base and apex, so as to form a sinus or angle[1620]. In certain Acridæ K. (Locusta F.) there are only two of these lines or ridges, but notched or toothed; and in some of the genus first named only one[1621]; in Locusta Dux and affinities the prothorax has several transverse channels or rather folds[1622], with corresponding ridges on its internal surface.
6. With respect to the clothing of the prothorax, I have not much to say: in Coleopterous insects this part is commonly naked; but in some genera, as Byrrhus, Anthrenus, Dermestes, and many weevils (Curculio L.) it is partially or totally covered with hairs or scales. In the other thoracic Orders it is usually naked, but in some Neuroptera, the Myrmeleonina, &c., it is hairy; and in the Libellulina it is fringed posteriorly with hairs.
7. As to its relative proportions, the prothorax is sometimes rather wider than the rest of the trunk and the head, as in Onitis, Pasimachus, &c.; it is considerably narrower in Collyuris and Odacantha; and of the same width in those Scaritidæ with striated elytra[1623]. Again, it is sometimes of the same width with the elytra, but wider than the head, as in Hydrophilus, Dytiscus, &c.; in some instances it is of the same width with the head, and narrower than the elytra, for instance in Anthia and Brachinus. In most Coleoptera it is longer than the head and shorter than the elytra; but in Manticora, the vesicatory beetles, &c., it is shorter than either. In Gnoma longicollis[1624], it is nearly as long as the elytra; in many Staphylinidæ, Atractocerus, &c., longer; in Phanæus carnifex, bellicosus, &c., it is longer than the elytra and the rest of the body. With regard to itself, it is sometimes very wide in proportion to its length—Dytiscus, Helæus; at others very long in proportion to its width—Colliuris, Brentus, Mantis, &c. In Flata, and many other Homopterous Hemiptera, it is extremely short; extremely long in Gnoma; in Sagra and Donacia its width about equals its length; in Elater, Dytiscus, and many Heteropterous Hemiptera, it is narrowest before; in Languria it is every where of equal width; in Anthia, Carabus, &c., it is widest before; and, lastly, in the Scarabæidæ MacLeay it is usually widest in the middle.
ii. Antepectus[1625].—The antepectus, as was before observed[1626], in some tribes forms one piece, without any kind of separation, with the prothorax; but very often this is not the case. In Carabus L. it occupies almost the whole under-side of the manitrunk; but in Elater, in which the ora is very wide, the antepectus is merely the middle portion of that part. In Carabus F. &c. between the ora and the base of the arms is a convex triangular piece, distinguished from the rest of the antepectus by a spurious suture; and in Pentatoma and other Heteropterous Hemiptera a similar piece is observable, which terminates in a convex bilobed subtriangular sheath, receiving the base of the clavicle[1627]. This piece seems a prop to that part, and analogous to the scapula of the medipectus and parapleura of the postpectus. I shall say no more upon the antepectus, as it is seldom remarkable. In the mole-cricket, however, one peculiarity distinguishes it: it is in this of an elastic leathery substance, while the prosternum is hard, resembling a bone. In other instances these parts are both of the same substance.
1. The sternum or breast-bone of insects consists mostly of three distinct pieces; in this resembling the human sternum, which is described by anatomists as composed originally of three bones[1628]. Each of these pieces is appropriated to a pair of legs, and each of them at times has been called the sternum: thus in Elater the prosternum, in the Cetoniadæ the mesosternum, and in Hydrophilus the metasternum, have been distinguished by this name. Our business is now with the first of these pieces, the sternum of the antepectus or prosternum[1629]: this is the middle longitudinal ridge of the fore-breast, which passes between the arms, when elevated, extended, or otherwise remarkable. It is most important in the Coleoptera Order, to which my remarks upon it will be chiefly confined. In these it is sometimes an elevation, and sometimes a horizontal process of the fore-breast. If you examine the great Hydrophilus (H. piceus), at first you will think that there is only a single sternum common to all the legs; but if you look more closely, you will perceive between the head and the arms a triangular vertical process, with a longitudinal cavity on its posterior face, which receives the point of the mesosternum that passes between the arms[1630]: this vertical piece is the real prosternum, and not the other, which really belongs to the alitrunk. In this case the elevation of the prosternum is before the arms; in others it is between them, as you may see in a Chinese chafer (Mimela K.), which imitates the external appearance of a quite different tribe[1631]; in others again it is behind them, as in most of the Lamellicorn beetles. In the common dung-chafer (Geotrupes stercorarius), it is a hairy process, which, when the head is bent downwards, is received by a deep cavity of the mesosternum. The Dynastidæ MacLeay may always be known by a columnar prosternum rising vertically between the arms and the medipectus. Lastly, in other tribes there is a prosternal elevation both before and behind the arms, as in Cerambyx thoracicus, dimidiatus, and affinities. Of the second description, those that have a less elevated horizontal prosternum, the point in most is to the anus, but in some to the head: thus in Carabus L. it is generally a subspathulate flat piece, the point of which slides over the mesosternum, or covers it; but in Harpalus megacephalus Latr.[1632], one of this tribe, though similarly shaped, its point is to the head. These horizontal prosterna vary in their termination. In that of Carabus L. the apex is obtuse; in that of Elater, above described[1633], and Dytiscus it is acute; in Prionus lineatus, Spencii K., &c., it is bilobed; and in Buprestis variabilis, attenuata, &c., obsoletely trilobed. With regard to the other Orders no striking features of this part are observable, except in some Orthoptera. In Acrida viridissima K. (Locusta F.) it is represented by two long filiform vertical processes; and in Locusta Leach by a single conical horn[1634], mistaken by Lichtenstein for a process of the throat[1635]. In one instance, Gryllotalpa, this part is a long piece between the arms, shaped like the human thighbone or tibia, being more slender in the middle and widest at the ends, and which is of a much harder substance than the rest of the antepectus, and forms the lower termination of a singular machine which will before long be noticed. In many bugs (Cimicidæ), instead of being elevated, the three portions of the sternum are hollowed out into a longitudinal groove, in which the promuscis when unemployed reposes.
The most conspicuous and remarkable appendages of the manitrunk, are the brachia or arms. I shall not, however, enter into the full consideration of these, as they consist numerically of the same parts, till I treat of the legs in general. Here it will only be necessary to assign my reasons for calling them by a distinct denomination. In this I think I am authorized, not only by the example of Linné, who occasionally found it necessary to do this[1636], and more particularly by the ancient notion that this pair of organs in insects were not to be reckoned as legs[1637], but likewise from their different position and functions. They are so inserted in the antepectus as to point towards the head, whereas the other two pair point to the anus. With regard to their functions, besides being ambulatory, and supporting the manitrunk in walking, they are applied to many other purposes independent of that office,—thus they are eminently the scansory or climbing legs in almost all insects; in most Carabi L., by means of the notch and calcar[1638], they are prehensory legs; in Scarites belonging to that tribe, the Lamellicorn beetles, and the mole-cricket, they are fossorious legs, or proper for digging[1639]; in Mantis, Nepa, and some Diptera, they are raptorious, or fitted to seize and dispatch their prey[1640]: they are used also by many insects to clean their head, eyes, and antennæ, &c. For many of these purposes they cannot be fit without a structure different from that of the other legs, which renders it a matter of as great convenience in descriptions to speak of them and their parts under different names from those of the legs, as it is of the arms of man; on this account it is that I propose to give to the fore-leg and its part the names by which the analogous parts, or what are so esteemed, in the human species are distinguished;—when spoken of in common with the other legs, they may still be called the forelegs.
** Alitruncus. The alitrunk is the posterior segment of the trunk, which below bears the four true legs, and above the organs for flight or their representatives. In treating of this part we may consider its insertion or articulation, its shape, composition, substance, motions, and organs.
i. With regard to its insertion, or articulation with the manitrunk and abdomen, it may be observed that it is attached to both by its whole circumference by means of ligament; in the Coleoptera, Orthoptera, and Heteropterous Hemiptera being received by the posterior cavity of the prothorax, the shield of which in these Orders, especially the last, almost covers and conceals it; but in the remaining ones it is merely suspended to it. In the former also, especially in the Coleoptera, it seems more separate and distinct from the manitrunk than from the abdomen, and more independent of its motions than of those of the latter part: but in the Hymenoptera and Diptera its greatest separation is from the abdomen in both respects. In many insects, as in the Lamellicorn beetles, the mole-cricket, &c., the manitrunk terminates posteriorly, drawing a line from the base of the prothorax to the antepectus, in an oblique section; in other tribes, as in the Cerambyx L., the Predaceous beetles, &c., the section here is often vertical, but in the alitrunk the anterior one is always vertical, while the posterior, by which it articulates with the abdomen, in the Orders with an ample thoracic shield, is oblique, so that the pectoral portion is more ample than the dorsal.
ii. As to its composition, the alitrunk is usually much more complex than the manitrunk; for, besides the instruments of motion, it consists of numerous pieces. It may be regarded as formed of two greater segments, the first bearing the elytra, or the primary wings, and the intermediate legs; and the second, the secondary wings and the hind legs.
1. Collare[1641]. The first segment of the alitrunk is the middle piece of the whole trunk, and therefore, when spoken of per se, may be called the meditruncus. It consists primarily of an upper and lower part, which in the table are denominated the mesothorax and the medipectus. The first piece in the former that requires notice is the collar. I formerly regarded this piece, which is peculiar to the Hymenoptera, Diptera, and one tribe of the Neuroptera, as the representative of the prothorax in the other Orders, and this opinion seems at this time very generally adopted, but subsequent observations have caused me to entertain considerable doubts of its correctness. Many other Entomologists have thought it improper to distinguish these parts by the same name[1642]. Much, however, may be said on both sides of this question, and I shall now lay before you the principal arguments that may be adduced in defence of each opinion, beginning with those that seem to prove that the collar is the analogue of the prothorax. First, then, the collar, like the prothorax, is placed precisely over the antepectus, and being placed in the same situation, on that account seems entitled to the same denomination; especially as in some genera, for instance Chlorion F., it assumes the very semblance and magnitude of a thoracic shield, and is separated from the mesothorax by a considerable incisure. Again, in some cases that have fallen under my own observation, the collar is endued with some degree of motion distinct from that of the alitrunk, since in Pompilus and Chrysis the animal can make the former slide over the latter in a small degree. A third and last argument is, that no prophragm is formed from the collar: insects that have a thoracic shield are generally distinguished by having the anterior margin of the dorsolum deflexed so as to form a septum, called in the table the prophragm, which enters the chest and separates the cavity of the mesothorax from that of the prothorax; now in Hymenoptera this septum is a process of the piece behind the collar, and excludes it from having any share in that cavity. These arguments at first sight seem to prove satisfactorily the identity of the collar and prothorax. But audi alteram partem, and I think you will allow that the scale containing the claims of the collar to be considered as a piece sui generis, dips much the lowest. And, first, I must observe, that though in Hymenoptera the collar seems to replace the prothorax by its situation, yet it is in fact a part of the alitrunk; for, if the manitrunk be separated from the latter, the collar remains, in most cases, attached to it[1643], while the antepectus and arm, with the ligament that covers its cavity above, the real representative of the prothorax, are easily removed, and this in recent individuals: as a further proof of this, I must request you will examine a neuter Mutilla; you will see that in this the collar is not separated from the alitrunk in any respect, but forms one piece with it, while the antepectus is distinct and capable of separate motion: further, the action of the collar is upon the alitrunk, it being of essential importance in flight, whereas the prothorax is of no other importance than as a counterpoise to that part[1644]. A further argument to prove the distinction of these parts may be drawn from the case of Xylocopa, a kind of bee. In this genus the collar forms a complete annulus or segment of the body: now, if it really represented the prothorax, the under side of the segment, as in those Coleoptera in which no suture separates the upper from the lower part of the manitrunk[1645], should represent the antepectus, and have the arms inserted in it; but in the case before us there is a distinct antepectus bearing the arms received by the socket formed by this annulus. But the most powerful argument is the fact that some insects have both the prothorax and collar, a circumstance that completely does away every idea of their identity. If you examine the common hornet (Vespa Crabro), or any saw-fly (Tenthredo L.), you will find, as was before intimated, that the real covering of the cavity of the manitrunk is a ligamentous membrane, which properly represents the prothorax. In another genus of the same order (Xiphydria Latr.), the sides of the antepectus turn upwards and nearly form a horny covering distinct from the collar[1646], the ligamentous part being reduced to a very narrow line, and in Fœnus the dorsal fissure is quite filled up, so that in this the manitrunk is perfectly distinct, and exhibits both prothorax and antepectus of the usual substance. In Nomada likewise, N. Goodeniana K. was the species I examined, there is a short minute prothorax besides the collar. Next let us turn our attention to the Diptera; if you examine the common crane-fly (Tipula oleracea), you will find, first, a regular short prothorax, to which the antepectus, with the arms, is attached; and behind this also is a short collar embracing the alitrunk anteriorly. The next insects that I shall mention, as exhibiting both prothorax and collar, are the Libellulina. These are generally admitted to have the former of these parts[1647], but besides this they have also the latter, which is the most ample and conspicuous piece in the whole trunk[1648]; intervening, as the collar should do, between the prothorax and those parts of the trunk to which the wings are attached. There is one circumstance connected with the subject which should not be overlooked. In the Hymenoptera, usually under a lateral process of the posterior part of the collar, is a spiracle or respiratory apparatus; in the Diptera there is also one, though not covered by the part in question, in the same situation; now this you will find precisely so situated with respect to the second piece in the thorax of Tipula oleracea, proving that this piece is the real representative of the collar. Enough, I think, has been said to satisfy you that I have not changed my sentiments on this subject upon slight grounds. Probably traces of the part in question might be detected in the thoracic Orders in general, in connexion with some vocal or respiratory organ[1649]; but having had no opportunity, by an extended examination of living subjects, to verify or disprove this suspicion, I shall merely mention it, and conclude this head by observing, that the collar varies most in the Hymenoptera order, and that its most remarkable form is in Vespa, Cimbex, Dorylus, &c., in which it bends into an ample sinus that receives the dorsolum[1650].
2. Dorsolum[1651]. Where there is no apparent collar, the dorsolum (dorslet) is the first piece of the mesothorax, and where there is one, the second; it bears the elytra or other primary organs of flight. It varies in the different Orders, particularly with respect to its exposure. In Coleopterous insects it is most commonly, but not invariably[1652], covered entirely by the shield of the prothorax, the scutellum alone being visible; as it is also in the Orthoptera (with the exception of Mantis and Phasma, in the first of which it is partially, and in the latter intirely exposed), and the Heteropterous, and most of the Homopterous section of the Hemiptera. The scutellum is likewise covered in Gerris, Hydrometra, and Velia, and the whole of the back of the alitrunk by a process of the prothorax in Acrydium F., Centrotus, &c. But in the remaining Orders, and the tribe of Tettigonia in the Homopterous Hemiptera, the dorsolum is not hidden by the thoracic shield. It is usually less elevated than the scutellum; in Necrophorus, and some other beetles, however, the latter is most depressed. With regard to its substance, it is generally not so hard and rigid as the scutellum, but in most Coleoptera harder than in the other Orders in which it is covered; in the Hemiptera, except in Tettigonia, it approaches to membrane. As to shape and other circumstances, it varies in the different Orders. In the beetle tribes it has generally a sinus taken out of its anterior margin, and it approaches more or less to a trapezium; in Blatta it is transverse and somewhat arched; in Gryllotalpa it is nearly square, and distinguished besides on each side by a minute aperture, fitted with a tense membrane, which perhaps covers a respiratory apparatus. In the locusts it is more or less triangular, and in Mantis and Phasma long and slender. In the Hemiptera the dorsolum appears to consist of several pieces, variously circumstanced, separated by sutures, corresponding with which are as many ridges on the inside of the crust[1653]. In the Libellulina it is rhomboidal[1654]; in Panorpa nearly hexagonal; in the Ephemerina it is ample and oblong; in Sialis and the Trichoptera this part is represented by three subtriangular pieces, the scutellum constituting a fourth, with the vertices of the triangles meeting in the centre[1655]; in the Lepidoptera the part in question is large, and receives the scutellum into its posterior sinus[1656]. The Hymenoptera usually exhibit a very ample dorsolum, mostly subtriangular with the vertex rounded or truncated, and pointing in some (Vespa L.) to the head[1657], and in others (Apis) to the anus; in the Diptera, except in Tipula, the parts of the mesothorax are not separated by any suture, but only indicated by impressed lines or channels; in the genus last mentioned, however, the dorsolum is distinct, subrhomboidal, and received by an angular sinus of the scutellum, which last, I think, is not the part that has usually been regarded as entitled to that denomination; for this opinion I shall soon assign my reasons.
3. Scutellum[1658]. Some writers on the anatomy of insects, looking, it should seem, only at the Coleoptera and Orthoptera, have regarded the dorsolum and scutellum as forming only one piece[1659], and others have affirmed that the Lepidoptera and subsequent Orders have no scutellum[1660]. But as we proceed in considering the scutellum in all the Orders, we shall see that both these opinions are founded on partial views of the subject, and that all winged insects have a scutellum, more or less distinctly marked out or separated from the dorsolum. In the Coleoptera the scutellum is usually the visible, mostly triangular, piece that intervenes between the elytra at their base[1661], and which terminates the dorsolum. Some Lamellicorn beetles, &c. (Scarabæidæ MacLeay) are stated not to have the part in question (exscutellati): but this is not strictly correct, for in these cases the scutellum exists as the point of the dorsolum covered by the prothorax, though it does not intervene between the elytra: in others of this tribe, as Cetonia chinensis, bajula, &c., it separates these organs at their base, though it is covered by the posterior lobe of the prothorax: in Meloe F., the elytra of which are immoveable, there seems really to be no scutellum. Generally speaking, as was lately observed, but not always, it is distinguished from the dorsolum by being more elevated: this is particularly conspicuous in the genus Elater, in which it is a flat plate elevated from the dorsolum by a pedicle; in Sagra the latter part is horizontal, while the scutellum is vertical: and even in cases where the distinction is not so striking, these parts are separated either by a line, or some difference in their sculpture and substance. In this Order this part varies greatly, and often in the same tribe or genus, both in size and shape; being sometimes very large[1662], and sometimes very minute; sometimes very long, and sometimes very short; sometimes nearly round, at others square; now oval or ovate, heart-shaped, triangular, acuminate, intire, bifid, &c. In the Orthoptera, though less conspicuous, it still is present as a triangular elevation of the middle of the posterior part of the dorsolum, with the vertex either pointing towards the head, as in Blatta, or towards the tail, as in Locusta Leach[1663]. In the Heteropterous section of the Hemiptera (which, in columns of Mandibulata and Haustellata, appear to bear the same reference to the Coleoptera, that the Hymenoptera do to the Diptera, and the Homopterous Hemiptera to the Orthoptera[1664]) the part we are considering is mostly very large and conspicuous, quite distinct from the dorsolum, and in some (Tetyra F.) covering the whole abdomen, as well as the Hemelytra and the wings; it is most commonly, as in the Coleoptera, obtriangular[1665], but in the last-mentioned genus it often approaches to a pentagonal shape. Though usually so striking a feature in this tribe, in the aquatic bugs (Gerris &c.) it is covered by the prothorax. In some species of Reduvius F. (R. biguttatus, mutillarius, lugens, &c.) it is armed with one or more dorsal or terminal spines. In the Homopterous section, where the dorsolum, as in Tettigonia F., is not covered by the prothorax, the scutellum, which is merely a continuation of that part, bears some resemblance to a St. Andrew's cross, and terminates in a fork[1666]; in Fulgora, in which it is partly covered, it is merely the triangular point of the dorsolum: in the Cercopidæ, &c., whose dorsolum is wholly covered, the triangular scutellum is distinct from it; in Centrotus, Darnis, and Membracis, in which the prothorax is producted, and covers the abdomen more or less, the scutellum is a short transverse distinct piece. In the Lepidoptera, from the difficulty of abrading sufficiently the scales and hairs without injury, it is difficult to obtain a correct idea of the part in question; in the cabbage butterfly (Pieris Brassicæ) it appears to be triangular: in the humming-bird hawk-moth (Macro-glossum Stellatarum) it approaches to a rhomboidal shape[1667]; and in the eggar-moth (Lasiocampa Quercus) it is completely rhomboidal. In the Libellulina, in the Neuroptera Order, it seems to be represented by the posterior point of the dorsolum, which terminates in something like a St. Andrew's cross[1668]. In most of the other tribes of this Order the scutellum is a triangular piece, with the vertex to the head, received between two pieces of the dorsolum; in Psocus it is nearly like that of Tettigonia before described. In the Hymenoptera the scutellum is separated from the dorsolum, which it often embraces posteriorly, as the collar does in front, by a suture; it varies occasionally in shape in the different tribes, most commonly it is crescent-shaped, but in many Ichneumonidæ and others it is triangular[1669]; in the hive bee, &c., it overhangs the succeeding piece of the alitrunk; in Melecta, Crocisa, &c., it is armed with a pair of sharp teeth[1670]; in others (Oxybelus uniglumis, &c.) with one or more spines, and in some with a pair of long horns[1671]. Before I describe this part in the Diptera, it will be proper to assign my reasons for considering a different piece as its representative, from what has usually been regarded as such, and which at first sight seems the analogue of what I admit to be the scutellum in the Hymenoptera. The dorsolum, and its concomitant the scutellum, belong to the first pair of the organs of flight, which are planted usually under the sides of the former, and in the case of wings, by their Anal Area, connected either mediately or immediately with the latter. Now, if you trace the sides of the piece that I have considered as the part in question in Hymenoptera, you will find that they lead you not to the base of the lower but to that of the upper wings[1672], and in the saw-flies (Tenthredo L.) you will see clearly that the Anal Area of these wings is attached to a process of it, a proof that it belongs to the mesothorax, or region of that pair. But in the Diptera, the part that has been usually called the scutellum is not at all connected, either by situation or as a point of attachment, with the wing itself, but with the lower valve of the alula, which is with reason thought to be the representative of the secondary wing of the tetrapterous Orders. You may see this even in the common crane-fly (Tipula), in which there is a real alula, connected by means of a lateral process, terminating in ligament, with this supposed scutellum. If you examine further the same insect, you will easily find what I regard as the true one in the bilobed piece which receives the dorsolum, situated between the wings, and to the sides of which they are attached. In Asilus, Tabanus, &c., this part is transverse, and only distinguished on each side by an oblique impressed line; in the Muscidæ it is square, and marked by a straight transverse one.
4. Frænum[1673]. This appendage to the scutellum and dorsolum varies considerably in the different Orders, and in many cases, as you will see, is a very important part, being the process by which the former is mostly connected with the elytra or upper wings. In the Coleoptera, the elytra of which are nearly stationary in flight, and therefore less require any counteraction to prevent their dislocation, this part is commonly merely a process or incrassation of the under margin of the scutellum, which towards the base of the dorsolum is dilated to form the socket for the elytra. Its use as a countercheck in this Order is best exemplified in the common water-beetle (Dytiscus marginalis). This at the inner base of the elytra has a membranous fringed alula resembling those of Diptera; to the lower fold of this the extremity of the frænum is attached, which forms a right angle with the scutellum, and the upper fold is attached to the base of the elytrum[1674]. The object of this appendage is probably to prevent the dislocation of these organs, which seems to indicate that they are used more in flight than those of other beetles. The Blattæ also, in the next Order, have a winglet attached to the anal area of the tegmina. The frænum, as in the preceding Order, lies under the margin of the scutellum and dorsolum, but which here forms one uninterrupted transverse line; it is nearly vertical, and is attached to the alula. The structure is not very different in the other Orthoptera[1675], but the frænum is surmounted or strengthened by one or two ridges; in Mantis it runs from the scutellum in an angular or zigzag direction—but in all it is attached immediately to the tegmen. In the Heteropterous Hemiptera it is represented by the narrow bead adjacent to the scutellum on each side[1676], which dilates into a flat plate as it approaches the Hemelytrum, with the Anal Area of which it is connected. But the Homopterous section of the Order in question furnishes examples of the most remarkable structure of this countercheck, which proves that it is really, what its name imports, a bridle. If you examine the great lanthorn-fly (Fulgora laternaria), or any species of Tettigonia, &c., you will find adjacent to the scutellum or parallel with it, on each side a flat plate; and from the angle of that part in the first case, and from one of its processes in the last, you will further perceive a ridge or nervure which runs along this plate, in one forming an angle, and in the other being nearly straight, to the base of the tegmen, where it becomes a marginal nervure to a membrane that is attached to the posterior part of the base of the Anal and Costal Areas; and that this marginal nervure, like a trachea, consists of a spiral thread, or rather of a number of cartilaginous rings connected by elastic membrane[1677], and consequently is capable of considerable tension and relaxation, as the tegmen rises and falls in flight. In the Lepidoptera it appears to be a short piece overhung by the scutellum, which as it approaches the base of the wing is dilated. In the Libellulina, to go to the Neuroptera, it has the same kind of elastic nervure connected with the Anal Area of the wing which I have just described in the Homopterous Hemiptera; another nervure, in Æshna at least, appears to diverge upwards from the scutellar angle to the Intermediate Area[1678]: a structure little different distinguishes the rest of the Neuroptera, and even the Trichoptera. In the Hymenoptera this part varies somewhat; in the majority perhaps of the Order, as well as in the Diptera, it appears to be merely the lateral termination of the scutellum where it joins the wing; but in some tribes, as in Tenthredo L. (especially Perga Leach), Sirex L., and the Ichneumonidæ, a ridge, and sometimes two, runs from the scutellum to the wing; the upper one, where there are two, as in Perga, being the stoutest, and connecting with the Costal Area, and the lower one with the Anal.
5. Pnystega[1679]. We learn from M. Chabrier, that in the common dragon-fly, a space, consisting of three triangles, which immediately succeeds the frænum, affords attachment to no muscles, but merely covers aërial vesicles[1680]. This is the part I have called the pnystega[1681]. An analogous piece may be discovered in Phasma and Mantis in a similar situation; but I cannot trace it in Locusta Leach, or in the other Orders.
Having considered the parts that constitute the mesothorax, we will next say something upon those, as far as they require notice, that compose the medipectus or mid-breast. But first I must observe, in general of the medipectus and postpectus taken together, or the whole underside of the alitrunk, that though usually they are in the same level with the antepectus or under side of the manitrunk, yet in several instances, as the Scarabæidæ MacLeay, the Staphylinidæ, &c. they are much more elevated than that part; they are also usually longer, very remarkably so in Atractocerus, but in Elater sulcatus and many others they are shorter. These parts are also commonly rather more elevated than the abdomen,—much so in some, as Molorchus; but scarcely at all in others, as Buprestis, the Heteropterous Hemiptera, &c. In some of the latter (Tetyra F.) the abdomen seems the most prominent. Another observation relating to this part must not be omitted, namely, that though in many cases the medipectus and postpectus are perfectly distinct and may be separated, yet in others, as for instance the Lamellicorn beetles, the Hymenoptera and Diptera, &c., no suture separates them; so that though the upper parts, the mesothorax and metathorax, are separable, the lower ones just named are not so.
6. Peristethium[1682]. The first piece of the medipectus is what I have called, after Knoch, the peristethium[1683]. This immediately follows the antepectus; on each side it is limited by the scapulars, and behind by the mid-legs and mesosternum. Its antagonist above is usually the dorsolum. In the Coleoptera Order it varies occasionally, both in form and magnitude, but not so as to merit particular notice, except that both are regulated by the scapulars—if these are small, the peristethium is ample; and, vice versa, if they are large it is small. In all the following Orders, except the Hymenoptera, it is equally inconspicuous, but in them it is often more remarkable. I have a Brazilian species of Cimbex (C. mammifera K. MS.) which appears undescribed, in which this part swells into two breast-like protuberances, terminating posteriorly in membrane, as if it had separate motion: in the golden-wasps (Chrysis L.) it is anteriorly concave to receive the coxæ of the mid-legs; and in Stilbum, of the same tribe, it is armed with one or more conical obtuse teeth.
7. Scapularia[1684]. The scapulars are situated between the coxæ of the mid-legs and the base or axis of the upper organs of flight, and they seem to act as a fulcrum to each. In the Coleoptera Order they are most commonly quadrangular or subquadrangular, often divided diagonally, and sometimes transversely, by an impressed line; the posterior part, which is usually the most elevated and often has an uneven angular surface, is that which intervenes between the coxæ and elytra: where the former are short, as in the Capricorn beetles, the scapulars are long; and where they are long, as in the Petalocerous ones, the latter are short. The anterior part is that which forms the lateral limit of the peristethium, upon which it often encroaches: this part, in conjunction with the dorsolum above, and the last-named part below, forms the kind of rotula that plays in the posterior acetabulum of the manitrunk, as the head does in the anterior one. In the flower-chafers (Cetonia F.) the scapulars are very thick and elevated, and interpose between the posterior angles of the prothorax and the shoulders of the elytra, which is one of the distinguishing characters of that tribe: in this case the lower angle of the scapular connects with the coxa of the mid-leg, and the upper angle with the axis of the elytra; and the most elevated and thickest part of the scapular is about midway between the two. This robust structure seems to indicate that the scapular has to counteract a powerful action both of the leg and elytrum. In the Orthoptera the scapulars are usually divided into two parallel pieces, corresponding probably, though more distinct, with the two parts lately noticed of those of the Coleoptera: the upper side of the socket of the mid-leg is common to the base of both these pieces, but the articulation of the tegmen is chiefly with the anterior one. In the grasshoppers, locusts, &c. (Gryllus L.) in which tribe this leg is nearly opposite to that part, the scapular inclines but little from a vertical position[1685]; but in the praying-insects (Mantis), spectres (Phasma), and cockroaches (Blatta), in which the insertion of the mid-legs is behind that of the tegmina, it is nearly horizontal. In the Heteropterous Hemiptera the anterior part of the scapular is covered by the antepectus, and separated by a ridge, more or less pronounced, from the open part; the whole is of an irregular shape, and nearly parallel with the parapleura. In the Homopterous section it likewise consists of two pieces, and sometimes of more. Thus in Tettigonia F. it is bilobed, and between it and the coxa two small pieces are inserted[1686]. In some others, Iassus Lanio F., &c., it is not very unlike the scapular in Coleoptera, being subquadrangular and divided diagonally. In the Neuroptera this part and the parapleura are parallel, and placed obliquely[1687]. In the common dragon-fly (Æshna viatica) the former forms nearly a parallelogram[1688], which is not divided by any ridge or channel, but its lower half is separated into two unequal parts by a black longitudinal line, opposed to which on the inside is a ridge. The mid-leg in these is connected with the scapular by the intervention of a triangular transverse anterior piece, which in fact seems only marked by a black channel, to which also interiorly a ridge is opposed[1689]. In the rest of the Order it is divided longitudinally into two parallel pieces. In Panorpa the posterior piece is longer than the anterior and props the coxa behind; in Myrmeleon and Perla, &c., it appears to consist of three pieces. I have not been able to obtain a clear idea of them in the Lepidoptera, except that they have more than one piece. Hymenopterous and Dipterous insects for the most part have no scapular distinct from the peristethium; but in Cimbex, Perga, and other saw-flies, it seems represented by its posterior depressed and sometimes membranous part: in Vespa, &c. a small subtriangular piece, just below the base of the upper wing, is probably its analogue[1690].
8. Mesosternum[1691]. The central part of the medipectus, or that which passes between the mid-legs when elevated, protended, or otherwise remarkable, is called the mesosternum or mid-breast-bone. In the Coleoptera Order it exhibits the most numerous variations, and is usually the most strongly marked of any of the three portions of the sternum, affording often important characters for the discrimination of genera and subgenera. It may be said to be formed upon three principal types—the first is, where it is a process of the posterior part of the peristethium, and points towards the anus or the head;—the second, where it is a process of the anterior part of the mesostethium, and points only towards the head: in this case there is no suture to separate the medipectus from the postpectus;—the last type is where it is a ridge formed by a process both of the peristethium and mesostethium meeting between the legs; an example of this you will see in the common dung-chafer (Geotrupes Latr.). Upon the two first of these cases I shall offer a few remarks;—the last affording no variation need only be mentioned.
If you examine the terrestrial Predaceous beetles (Cicindela and Carabus L.) you will find that the peristethium is usually flat, terminating towards the postpectus in a kind of fork, the sinus of which receives the anterior point of the mesostethium—this is the mesosternum; but in the aquatic insects of this tribe, at least in Dytiscus marginalis, &c. the structure at first sight seems different, for apparently the prosternum is received by the anterior fork of the mesostethium; but if you proceed to separate the manitrunk from the alitrunk, you will find that the true mesosternum of the usual form is quite covered by this point, which curves towards the breast, is longitudinally concave to receive the point of the prosternum, and permit its motion in the groove. In some Heteromerous beetles, as the Helopidæ, &c. this part is anteriorly bilobed, so as to form a cavity which receives the point of the prosternum when the head is bent down: in Helops nitens (Tenebrio Oliv.) this sinus represents a crescent; in Cistela Ceramboides it is shaped like the Greek letter γ; in the Lady-bird (Coccinella L.) it assumes nearly the shape of a Saint Andrew's cross; in Spheniscus K.[1692] the mesosternum is wide, concave and wrinkled, with an anterior and posterior sinus; while in the analogous genus Erotylus[1693] it is convex anteriorly, and posteriorly more or less rounded; in Doryphora it is a long, robust, subconical horn, often standing at an angle of about 45°, overhanging the prosternum.
In the genus last named, though its mesosternum in its direction and appearance resembles that of many Petalocerous beetles, yet it is separated by an evident suture from the mesostethium; but in the last-mentioned tribe its representative is a process of the latter part: yet as the peristethium and mesostethium are separated by no suture, though in some cases a transverse channel, and in others merely a coloured line, marks the point where they may be considered as soldered together, in these cases the mesosternum may perhaps be said to be common to both. In this great family, which includes within its limits some of the most singular and wonderful in their structure and armour, as well as some of the most brilliant and beautiful of the beetle tribes,—the part in question, in a vast number of cases, will enable the Entomologist satisfactorily to trace its numerous groups, not only where it rises or stretches out into a horn or ridge, but even often where it is merely a flat space between the mid-legs. I shall notice some of its most striking variations in this tribe. In Phanæus festivus, and in Macraspis and Chasmodia MacLeay, it is elongated horizontally, with the apex curving upwards; in Anoplognathus it is horizontal, straight, and figures an isosceles triangle; in Cetonia suturalis, vitticollis, &c. it is very long, passing between the arms and nearly reaching the head; in C. marmorea, Lanius, &c. it is a lofty, robust, conical prominence; and in many Rutelidæ, especially those with striated elytra, it is pyramidal or four-sided; it varies also in its termination, particularly in the Cetoniadæ; and even where there is little or no elevation of it, as in the Scarabæidæ MacLeay, it is often terminated anteriorly by lines that vary in their angle or curvature. The genus Copris, as restricted by Mr. W. S. MacLeay, may from an inspection of this circumstance be divided into several families. Thus in C. Molossus and affinities its termination represents the letter ⅄ reversed, or a triangle surmounted by a mucro; in C. orientalis, &c., it ends in an acute-angled triangle; in C. lunaris, &c. in an obtuse-angled one; and in C. Iacchus, &c., in the segment of a circle.
The part we are considering is not so important in the other Orders. In the Orthoptera, however, it is occasionally remarkable. In Acrida viridissima (Locusta F.) attached to the anterior margin of the peristethium are two long triangular pieces which appear to represent this part; in the kindred subgenus, Conocephalus[1694], it is a single piece bifid at the apex; in Gryllotalpa it is a very elevated hairy ridge; and in Locusta Leach, it is a flat anterior process of the mesostethium. In the Heteropterous Hemiptera this part is often merely a portion of the channel in which the promuscis reposes; but sometimes, as in Edessa F., it is an elevated piece varying in its termination. In the remaining Orders, as far as I have had an opportunity to examine them, it can scarcely be said to exist separately from the medipectus, except that in Tipula Latr. a bipartite subtriangular membranous piece seems to be its analogue.
We are now to consider the last segment of the alitrunk, which, as a whole, may be denominated the potruncus; it bears the second pair of the organs of flight, and the last pair of legs. The upper side of this is the metathorax, and its lower side the postpectus.
9. Postdorsolum[1695]. The first external piece of the metathorax is the postdorsolum, which presents itself under very different forms and circumstances in the different Orders. In the Coleoptera it is intirely covered by the dorsolum and scutellum; it is generally more or less of a membranous substance, or partly membranous and partly corneous, which enables it to yield more to the action of the wings in flight; it is usually an ample transverse piece with tumid sides[1696]; but in the Scarabæidæ MacLeay, it is short though very wide; and in Cychrus, and probably other apterous beetles, it is extremely minute and almost obsolete. In the Orthoptera Order, I observe once for all, the part in question, as well as the postscutellum and postfrænum are mere counterparts of the dorsolum, scutellum, and frænum, except that in some cases they are larger[1697]. In the Heteropterous Hemiptera at first sight it would appear that all the parts of the metathorax were altogether wanting or absorbed in the ample scutellum; but if you remove this with care, you will find under it their representatives, its lower surface being hollowed out to receive them. The postdorsolum appears in these as a transverse obtusangular band; in the Nepidæ, Notonectidæ, &c. the three parts of the metathorax seem united into a single plate, emerging laterally from under the scutellum below the frænum; in which, however, some traces of a distinction between them may be discovered. In the Homopterous section the Fulgoridæ exhibit these pieces very distinctly, covered only at the base by the mesothorax: but in Tettigonia they are not so easily detected; they exist however as a narrow strip or band, almost concealed by that part. As to the Lepidoptera Order, in Pieris Brassicæ at least, the postdorsolum is represented by a pair of nearly equilateral triangles whose vertexes meet in the centre of the metathorax, and between which and the scutellum is a deep cavity; but in Macroglossum Stellatarum and Lasiocampa Quercus, there appears to be also a central transverse piece between them. In the Neuroptera there is no material or striking difference between the parts of the mesothorax and metathorax[1698]. In the Hymenoptera more variety occurs in this part. In the saw-flies, &c. (Tenthredo L.) the postdorsolum is a transverse piece covered by the scutellum; in the Ichneumonidæ it is smaller, but not covered; in the Vespidæ it is apparent, transverse, and with the postscutellum obtusangular[1699]; in Apis it is overhung by the scutellum. The Diptera exhibit some variations in this part. In Tipula it consists of three pieces placed transversely, the central one quadrangular, and the lateral ones roundish; in the Asilidæ and most others of this Order, with the postscutellum, it forms a segment of a circle[1700], sometimes armed with a pair of spines, as in Stratyomis F., and is what has been usually regarded as the real scutellum, though, as I have endeavoured to show, not correctly[1701].
10. Postscutellum[1702]. The postscutellum bears the same relation to the postdorsolum that the scutellum does to the dorsolum, but it is seldom, if ever, a distinct piece. In the Coleoptera it is represented by the longitudinal narrow channel that terminates the postdorsolum towards the anus[1703]: this usually figures an isosceles triangle with the vertex truncated or open; but in Copris the triangle is equilateral. In the other Orders it is little more than the central posterior point of the postdorsolum[1704].
11. Postfrænum[1705]. The part now mentioned is much more important than the preceding one, and must not be passed over so cursorily. In the Coleoptera it usually presents itself under the form of two large and usually rather square panels, the disk of which is convex, but the rest of their surface unequal, which are situated one on each side of the postscutellum[1706]; under the anterior outer angle of these is the socket or principal attachment of the secondary wings, and their basal margin is attached to their outer side; posteriorly behind the vertex of the postscutellum the postfrænum is crowned with a ridge or bead, below which it descends vertically or obliquely to the abdomen; this ridge often turns upwards, and proceeds towards the middle of the basal margin of the wing. In the Petalocerous beetles the part in question is usually more or less hairy; but in many others, as the rose-scented Capricorn (Callichroma moschatum), &c. it is naked. At its side you will commonly observe several plates and tendons (osselets Chabr.) connected inter se and with the base of the wing by elastic ligaments, which are calculated to facilitate the play of those organs. In the Orthoptera, Neuroptera, and Homopterous Hemiptera, the postfrænum does not differ materially from the frænum[1707]. In the Heteropterous section of the last Order it is usually a transverse ridge terminating the postdorsolum, with a bifurcation where it unites with the wing; but in Tetyra F. (at least so it is in Tetyra signata,) it is a nearly vertical piece, marked in the centre with an infinity of very minute folds, which probably by their alternate tension and relaxation let out and pull in the wings. Amongst the Lepidoptera it is not remarkable. In the Hymenoptera Order it is mostly represented, I think, by a double ridge or fork, sometimes however obsolete, but very conspicuous in the saw-flies, which laterally terminates the postdorsolum; the upper branch, usually the thickest, going to the anterior part of the base of the underwing, and the lower one to the posterior. You may observe something similar in the crane-flies (Tipula Latr.) and Asilidæ. A tendon proceeding from the point of the postscutellum forms a fork near its end, the upper branch of which connects with the anterior and the lower with the posterior valve of the winglet; the structure is a little, but not essentially, different in other Diptera.
12. Pleura[1708]. By this name I would distinguish the part which laterally connects the metathorax and postpectus. It includes in it the socket of the secondary wings. In the Coleoptera this is a two-sided piece lying between the postfrænum and the parapleura, with the upper side horizontal and the lower vertical[1709]—a tendon usually proceeds from its anterior extremity to the base of the wing. In the Orthoptera, Neuroptera, and other Orders, it is merely the longitudinal line of attachment of that part; but in the genus Belostoma Latreille, related to the water-scorpion, it presents a peculiar structure, being a deep channel or demitube, filled at its posterior extremity by a spiracle and its appendages[1710].
13. Metapnystega[1711]. This part, although in the table I have placed it as an appendage of the pleuræ, is not always confined to them, as you will soon see. It either covers aërial vesicles, or is the seat of a spiracle. In the Order Coleoptera it is of the former description. If you examine the metathorax of the common dung-chafer (Geotrupes stercorarius), in the horizontal part of the pleura you will see a sublanceolate or subelliptical rather membranous silky tense plate, with its point towards the head,—this is the part we are considering; something similar you will find in most beetles; but in some, as Callichroma moschatum, it is less conspicuous. This part, as far as I have observed, is not so situated in any other Order, except in some Heteropterous Hemiptera: in Belostoma the channel lately mentioned is filled up at its posterior end by a red organ with an anterior vertical fissure, terminating behind in a conical bag: in Notonecta the pleura has something of a plate like that of Coleoptera, but of a horny substance. In the Orthoptera and Neuroptera this part changes its situation, if it be indeed synonymous; and as the pnystega follows the frænum, so the metapnystega succeeds the postfrænum. In the Libellulina M. Chabrier found that this as well as the other covered aërial vesicles[1712], and it probably does the same in the other cases in which it occurs. In Mantis and Phasma in the Orthoptera it is very minute; but in Locusta Leach, it is more conspicuous under the form of a tense membrane, the surface of which is depressed below that of the abdomen: in Acrida viridissima K. it fills the sinus of the postfrænum, and is vertical, as it is in Æshna. It is worthy of remark that this piece bears some analogy to that below the ridge of the part just named in Coleoptera, which descends either vertically or obliquely to the abdomen[1713]. A similar space, though often nearly obsolete, may be seen in the Hemiptera and Lepidoptera. But the Orders in which this part is most conspicuous are the Hymenoptera and Diptera, and in these its aërial vessels are connected with a spiracle. In Tenthredo L. and Sirex L., what Linné named grana, from their situation, should be regarded as belonging to the pnystega, and whether there is any part representing the metapnystega I am not quite satisfied; perhaps the membrane at the base of the abdomen in Tenthredo, and the bipartite piece, apparently its first segment, in Sirex[1714], may be its analogues: but in the great majority of the Order, the convex or flat piece that intervenes between the postdorsolum and its adjuncts and the abdomen, and which bears a spiracle on each side, is the metapnystega[1715]. This part is often remarkable, not only for its size, but for the elevated ridges that traverse it, as in Ichneumon, Chlorion, &c. In the last genus it is of a pyramidal shape, with the anterior part horizontal and the posterior vertical; it is altogether vertical in Vespa, Apis, &c. Amongst the Diptera, in Tipula it is nearly horizontal, and shaped like a cushion; but in general in this Order it is vertical, and concealed under the postdorsolum[1716].
We are now to consider the parts that constitute the postpectus or under-side of the metathorax, and which bears the posterior pair of legs.
14. Mesostethium[1717]. This part in Coleoptera is terminated anteriorly by the peristethium, scapulars, and mesosternum, laterally by the parapleuræ[1718], and behind by the coxæ of the posterior legs[1719], which generally are inserted transversely between it and the abdomen. It is commonly very wide; but in Dytiscus L., Carabus L., &c., in which the coxæ and parapleuræ are dilated, it is proportionally reduced: its length is regulated by the distance of the intermediate and posterior legs; where these are far asunder, as in the rose-scented Capricorn (Callichroma moschatum), &c. it is long: but where they are near each other, as in the Scarabæidæ MacLeay, it is short; its width, however, generally exceeds its length. In shape it is generally subquadrangular[1720], though sometimes rhomboidal, and other forms of it occur. Between the hind-legs it generally terminates in a notch or bifurcation distinct from the metasternum, as in Hydrophilus, &c.; in Hister there is no notch, and in many Scarabæidæ it projects between the hind-legs in a truncated or rounded mucro; in the Vesicatory beetles, Meloe L., it is more elevated than the medipectus, towards which it descends almost vertically; in Dytiscus L., Carabus L., &c., this part is usually divided into two by a transverse sinuous channel, and in Elater by a longitudinal straight one. In many Orthopterous genera, Gryllotalpa, Acrida K., Locusta Leach, &c., the mesostethium consists of two pieces[1721]. It is remarkable that in many of these genera, in this part, as likewise in the medipectus and antepectus, are one or more perforations which appear to enter the chest, the use of which I shall explain hereafter. In the Libellulina, as I shall soon have occasion to shew, there is a peculiar arrangement of the legs and wings, in consequence of which this part is placed behind the posterior ones. In the remaining Orders, the mesostethium, though it exists, exhibits no peculiarities worthy of particular notice, except in some Aptera and Arachnida: thus, in Nirmus Anseris it is terminated posteriorly by a pair of transverse membranous appendages which cover the base of the posterior coxæ; in Scorpio it consists of two pieces, the pectines[1722] being attached to the sides of the posterior one.
15. Parapleura[1723]. The parapleura, speaking generally, is that piece of the postpectus which, intervening between the pleura, mesostethium, and scapulars, is attached by its posterior extremity to the coxæ of the hind-legs; by means of the pleura, from which it does not appear to be separated by any suture, it connects the secondary or under-wings with the hind-legs, as the scapular does the primary ones with the mid-legs; so that the direction of the parapleura depends upon the relative situation of the legs and wings. In Coleopterous insects its direction is horizontal, it being generally a narrow subquadrangular piece that runs straight from the posterior coxæ to the scapular[1724], and usually divided into two unequal portions by an elevated or impressed line. In the palm-weevil (Calandra Palmarum) this part is wider than usual; in Dytiscus marginalis,—in which genus, as likewise in Carabus L., the coxæ are incapable of separate motion,—it is nearly a right-angled triangle, and is divided longitudinally into two unequal portions. In the Orthoptera Order this part usually consists of two equal portions, and its direction is sometimes nearly horizontal, as in Mantis and Phasma; sometimes forming an angle with the horizon, as in Blatta; and sometimes nearly vertical, as in Locusta Leach. In the two first cases the wings are before the legs, and in the last their position is over them. In the Heteropterous Hemiptera it is parallel with the scapular, is divided into two unequal portions, and its direction is more or less inclined to the horizon[1725]. As to the Homopterous section—in Fulgora it is of a very irregular shape with an angular surface, and its direction from the leg to the wing is first nearly vertical and then horizontal: in Tettigonia it is almost vertical, and consists of two nearly equal portions. To come to the Neuroptera—in the Libellulina it consists of two pieces, like those of the scapulars, but smaller[1726], and its inclination is towards the head: in Panorpa also it resembles the scapulars both in form and other circumstances[1727]. In the remaining Orders it exhibits no very remarkable features.
16. Metasternum[1727]. The central part of the mesostethium when elevated or porrected, or otherwise remarkable, is called the metasternum. In the Coleoptera, in those cases, as we have seen above[1728], in which the medipectus and postpectus form one piece, its anterior point becomes the mesosternum; but in others, as the Predaceous and Capricorn-beetles, &c., it is received in a sinus or fork of that part, or meets it. It is usually neither so remarkable nor important as the mesosternum. In Bolbocerus K. it is a rhomboidal elevation: in Gyrinus a ridge; as also in many Hydrophili, in which it passes between the hind-legs to the abdomen, and terminates in a sharp point[1729]; and in Dytiscus its two diverging lobes cover the base of the posterior trochanters[1730]. In the Orthoptera Order this part is not remarkable; but in Acrida viridissima K. it consists of three triangular pieces, the lateral ones being erect, and the intermediate one horizontal: in Locusta Leach it resembles the mesosternum[1731]. In the Heteropterous Hemiptera the whole mesostethium is elevated, and terminates at both ends in a fork, the anterior one receiving the point of the promuscis, and the posterior one that of the epigastrium: in the Homopterous section, the Tettigoniæ F. have usually a distinct metasternal point between their hind-legs. In the remaining Orders there is no metasternum, or no remarkable one, except in one singular Hymenopterous genus, Evania, the parasite of the Blattæ[1732], in which there is a forked posterior process of the mesostethium with recurved points.
17. Opercula[1733]. By this term I distinguish those plates, before largely described[1734], which cover the drums of male Tettigoniæ F.; and likewise those called also by the same name by M. Chabrier[1735], which cover, in many cases, the vocal apparatus of the trunk of insects: those of Melolontha vulgaris he describes as situated below the wings, and between the two segments of the alitrunk[1736]; and if you take this insect and remove the elytra, the mesothorax and scapulars, under the latter and below the wing you will find an oval convex plate, which is probably the part he is speaking of;—but it is better exemplified, I think, in the common Dytiscus marginalis, in which it is very distinct as a convex subtriangular plate connected with the metathorax by membranous ligament, covering a kind of pouch, and appearing to open and shut at the vertex[1737].
I must here observe, with regard to the Aptera and Arachnida, that the trunk in them is much more simple than in those insects that are furnished with wings. In the hexapods, in the former Orders, though there are usually three pedigerous segments, there is no distinction of dorsolum, scutellum, &c. In the Scolopendridæ and Scutigera amongst the Myriapods, according to the acute observations of M. Savigny[1738],—on which, however, some doubt at present rests,—there is a remarkable formation, the whole thorax being represented by the single plate that follows the head, to the under-side of which are attached the first and second pair of palpi or pedipalpi, and the first pair of legs, representing the three pairs of legs of hexapods. In the Iulidæ the three segments that follow the head, each bear a single pair of legs, while all the rest bear a double one: from whence it should seem to follow, that these segments and their legs represent the trunk and legs of Hexapods. In the Octopod Aptera and the Arachnida the trunk consists of a single piece, not separated from the head, and sometimes not distinct from the abdomen.
V. Internal processes[1739]. Perhaps you will think that this head would be better considered when I treat of the Internal Anatomy of Insects; but as the parts included under it are really processes of the external integument of the trunk, it seemed to me best to treat of them under that head. They are of two descriptions; processes of the thorax or upper part of the trunk, and processes of the breast or its under part.
i. Processes of the thorax[1740]. These are the phragma, prophragma, mesophragma, and metaphragma. The first belongs to the prothorax, the second to the mesothorax, and the two last to the metathorax; each forming a kind of chamber of the under-side of each segment of the thorax.
1. Phragma. The phragm, or septum of the prothorax, is most conspicuous in the mole-cricket (Gryllotalpa), in which it is a hairy ligament attached to the inside of the upper and lateral margins of the base of that part: inclining inwards, it forms the cavity which receives the mesothorax. It is not, however, without a representative in many Coleoptera, though in these it is less striking, from its being smaller and taking a horizontal direction. In Elater, by means of some prominent points received by corresponding cavities of the vertical part of the base of the elytrum, it forms a kind of ginglymous articulation, which probably keeps them from dislocation in repose, and, by the sudden disengagement of these points from the cavities, assists the animal in jumping[1741].
2. Prophragma[1742]. This is a piece usually almost vertical, but in Elater horizontal; of a substance between membrane and cartilage, descending anteriorly from the dorsolum, and forming the first partition of the chest of the mesothorax; it is generally much shorter than the mesophragm. Though very visible in Coleoptera and the Heteropterous Hemiptera, in the other Orders it is less easily detected, and is sometimes obsolete. It may be observed here, that in the Hymenoptera, at least in the wasp, the hive-bee, the humble-bee, and the Diptera mostly, the interior of the upper-side of the alitrunk, instead of two, seems at first to be divided into four chambers, formed by septula: but as these ridges merely mark out the internal limits of the dorsolum, scutellum, postdorsolum, and metapnystega, the last but one of these being usually less distinct, they seem not analogous to the three partitions of the alitrunk in other Orders; so that in these the mesophragm at least seems to have no representative, and the prophragm and metaphragm include between them only one ample chamber. In the Diptera, wherever there is an external depression or suture there is a corresponding internal ridge or seam, so that the parts seem more distinctly marked out on the inside than on the outside of the crust.
3. Mesophragma[1743]. This piece also, which forms the middle partition of the upper part of the cavity of the alitrunk, dividing it into two chambers, is most conspicuous in Coleoptera. It is usually in them a vertical piece, resembling the prophragm in substance, but twice its height, of a quadrangular shape with a notch in the middle; it fills the sinus of the postdorsolum, the sides of which sometimes descend below it[1744]. In this Order the chamber that it forms with the prophragm is very small[1745], the motions of the elytra requiring no powerful apparatus of muscles; but that which it forms with the metaphragm, which is appropriated to the muscles moving the wings, is very large[1746]. In the Orthoptera the anterior chamber is larger than in the preceding Order, which proves that tegmina are more moved in flight than elytra. In the Heteropterous Hemiptera a remarkable variation takes place—the anterior being larger than the posterior chamber; which last, in fact, consists of two, one for each wing: in these the mesophragm towards the abdomen forms an angle, which in Pentatoma, &c., is acute; in Belostoma a right angle, and in Notonecta an obtuse one. In the two first the angle of the mesophragm sends two short diverging ridges to the metaphragm; and in the last only a single one: in this also the posterior chambers together are nearly as large as the anterior. From this structure it should seem that in flight the Hemelytra are more important than the wings. In the Homopterous section the anterior chamber is the smallest, at least in Fulgora candelaria; and the mesophragm is lofty and bipartite. In the Lepidoptera the anterior chamber is the largest, and the part in question conspicuous[1747]. In the Libellulina and Hymenoptera it is merely represented by a low ridge, and in the Diptera it seems evanescent.
4. Metaphragma[1748]. This, in many cases, is the largest and most remarkable of the three partitions of the upper portion of the cavity of the alitrunk, which separates it from that of the abdomen; it is attached to the posterior margin of the metathorax, and is nearly vertical: in substance it may be stated as rather firmer than the two preceding partitions. In the Coleoptera it is commonly of the width of the posterior orifice of the alitrunk; and its centre is cleft so as to form a deep sinus[1749] for the transmission of the intestines,—a circumstance which also, though less conspicuously, distinguishes the mesophragm[1750]: from this sinus it slopes gradually towards the sides, and is sometimes armed with an intermediate process on each side[1751]. This structure you will find exemplified in the common cock-chafer and many others of the Order. I have not, however, discovered traces of it either in the Silphidæ, Staphylinidæ, or the vesicatory beetles (Meloe L.); or even in such species of Carabus L. and Cicindela L. that I have examined; while in Dytiscus it is very visible. In the Orthoptera it is nearly obsolete; but in Locusta Leach, under the metapnystega, one on each side, is a pair of seemingly pneumatic pouches which may be mistaken for it. It is almost equally inconspicuous in both sections of the Hemiptera. As to the Lepidoptera,—in Pieris Brassicæ, it resembles in some degree, though in miniature, the metaphragm of the Coleoptera; but in Sphinx Stellatarum and Lasiocampa Quercus it has a sinus on each side, but no middle one. In Panorpa it nearly closes the posterior orifice of the trunk, but in the Libellulina it is a mere ridge. In some Hymenoptera, as Cimbex sericea, the drone-bee at least, &c., it is a large convex bifid piece. In the wasps, under the spiracle of the metapnystega on each side, as in the Locusta, is what I also take to be a pneumatic pouch, which might easily be mistaken for a metaphragm. In the Diptera Order this part is very conspicuous. If you remove the abdomen of any common Tipula, you will find that the posterior orifice of the trunk is closed above by a pair of oblong, vertical, convex, diverging plates;—do the same by any fly (Musca L.), and you will detect in the same situation a very large convex or gibbous one notched below, which occupies almost the whole orifice: this is the metaphragm.
5. Septula[1752]. These are the smaller ridges of the interior of the alitrunk, which afford a point of attachment to many muscles, and run in various directions both on the interior of the crust and of the metaphragm. These little seams are not to be found so generally in the other Orders; but very frequently, as has been before observed, where there is an exterior impression of the crust, or a suture, one of these forms its internal base.
ii. Processes of the pectus[1753]. We are next to consider the internal processes of the breast of insects: these consist for the most part of the endosternum, or internal sternum, and its branches. As the principal feature of this are the processes which rising from it serve as points of attachment to the muscles that move the legs, &c., I shall confine myself to them—they are, the antefurca, the medifurca, and the postfurca.
1. Antefurca[1754]. The first portion of the endosternum, or the internal prosternum, branches into the antefurca. In the Coleoptera a plate varying in shape and direction[1755] sends forth a pair of mostly vertical processes of a cartilaginous substance[1756], differing in height in different genera. In Carabus L. there is neither this plate nor its processes; but in Dytiscus the latter are very visible. A very singular and complex machine represents the part we are considering in that extraordinary insect the mole-cricket (Gryllotalpa Latr.). When we look at its prodigious arms and consider their office[1757], we may imagine that the requisite apparatus for moving them must be very powerful and peculiar. Their Creator has accordingly provided them with a machine for this purpose more than usually complex, extending from the prothorax to the prosternum; the former being its base, and the latter its vertex. The cavity of the manitrunk is divided longitudinally by a double cartilaginous partition surmounted by a bony frame, with an anterior condyle or tuberosity, with which the inner part of the base of the clavicle of the arm appears to ginglymate; and the manitrunk is preserved from the injury the powerful action of the arm might occasion, by the counteraction of this machine, to describe which fully, would demand more space than I can afford[1758]. I mentioned under the mesostethium, the apertures visible in the breast of Locusta Leach and Acrida K. Each of these apertures opens into an internal, tubular, horny, process, which arching off is attached at the other extremity to the sides of the trunk—a pair being appropriated to each segment; the first analogous to the antefurca, the second to the medifurca, and the last to the postfurca. In the medipectus and postpectus of Acrida viridissima there is only a single aperture, terminating in a single tube, which after rising vertically a little way sends off a branch on either hand to the sides of the trunk. Where there are three of these holes, as in the antepectus and medipectus of Locusta Dux, there are three of these processes, the intermediate one being vertical. In the subsequent Orders the processes of the endosternum are not sufficiently remarkable to require particular notice: my further observations upon them will therefore be confined to the Coleoptera Order.
2. Medifurca[1759]. This part, which belongs to the mid-legs, is in many cases more conspicuous than the antefurca. In Copris Molossus the endosternum of the medipectus is represented by a transverse zigzag ridge[1760] between the sockets of the mid-coxæ, from which proceeds a pair of branches wide at the base and growing gradually more slender to the extremity[1761], which is attached to the sides of the trunk; in Dytiscus marginalis a pair of slender, vertical, straight processes, fitted with a broad cartilaginous plate at their apex, rises from the endosternum, and sends forth a lateral one to the side of the medipectus: and lastly, in Carabus the medifurca is represented by a pair of subtriangular laminæ attached to the sides of the trunk.
3. Postfurca[1762]. This, which belongs to the hind-legs, is the most remarkable of the pectoral processes, and has been noticed by more than one writer[1763]. It is a kind of trident, the branches[1764] of which are acute, and on their upper surface longitudinally concave, elevated on a footstalk[1765] inclined towards the medifurca, consisting of two plates, a posterior one supporting the lateral branches, and an anterior or interior one forming a right angle with the other, supporting the intermediate one. This footstalk rises from between the posterior coxæ, which appear in the Lamellicorns to ginglymate with it at its base. The middle branch of the trident dips to the sinus of the medifurca. In Dytiscus marginalis the form is different; for the intermediate branch consists of two parallel pieces, and the lateral ones are dilated into broad vertical plates: the stalk of this is triquetrous, and a triple cartilaginous partition appears to go from its base anteriorly, the lateral ones diverging to the sides of the trunk, and the intermediate one running straight to the base of the medifurca.
It may not be without interest to state here some of the several objects and uses of this structure of the trunk. When our Saviour says to his disciples, "But even the very hairs of your head are all numbered"[1766]—he taught them that the attention and care of the Deity were not confined to the mighty and the vast, but directed to every atom of his creation—that he not only decreed the number and magnitude of the planets and planetary systems, and of their various inhabitants, but that the most minute and apparently insignificant part of each individual, both as to its number and form, was according to the law by him laid down; and whoever studies them with attention will find that insects furnish a very interesting homily upon this text; since in various instances I think I have made it clear, that parts seemingly of the least importance—as a hair, a pore, or a slight impression—have their appropriate use[1767]. At first, it would seem that the various pieces of which we have seen the second primary segment of the trunk of these animals to be composed, would be of little importance; but when we reflect that this multiplicity of parts is usually not to be found in those that have no wings, whether they be apterous sexes or tribes[1768], a suspicion arises in the mind that they must be of more consequence than their prima facie appearance seems to warrant:—and this is really the case. The manitrunk, which is destined principally to incase the muscles that move the arms, did not require to be so complex as the part that had to support the action of wings as well as legs. In those that have a large prothorax, as the Coleoptera, it may, indeed, be useful in flight as a counterpoise to the abdomen; and since when the wings descend it rises, and vice versa, it may be of some service by its vibrations[1769]; but for this it required no complexity of structure. But not so the alitrunk: it consists of parts much more numerous, and this number of parts is of great importance to the animal in its flight. All of them are so put together, being lined by a common elastic ligament[1770], as to be capable of a certain degree of tension and relaxation, which enables the animal to compress or dilate the trunk as its necessities require. To cause the elevation of the wings, it must be compressed or have its longitudinal diameter increased, and its vertical and transverse diminished: this compression is produced by the condensation of the internal air, which parts with some of its caloric, and by the action of the levator muscles. To cause the depression of the wings, it must be dilated, or have its longitudinal diameter diminished, and its vertical and transverse increased, which is effected by the rarefaction of the internal air, and the action of the depressor muscles[1771]. In some Orders, the Coleoptera, &c., this effect is promoted by the segments of the trunk, which are attached by loose ligamentous membranes, and received, one or more of them, into each other, which facilitates the above action[1772]. Thus much for the general use of these parts. I shall further here mention a partial one of two of them which seems indicated by a particular circumstance, and upon which a theory may be built. In some insects the primary and secondary wings or their analogues are placed before the legs, in others over the legs, and in others behind the legs: but whatever their position, the pieces which I have named the scapularia and parapleuræ invariably connect the one with the other; the former, the primary wings with the mid-legs, and the latter, the secondary wings with the hind-legs. This circumstance seems to prove that the wings by the intervention of these pieces have an action upon the legs, and the legs upon the wings; and this is further proved in one case by an observation of M. Chabrier with regard to Melolontha vulgaris,—that the levator muscles of the wings, by means of a long tendon, are attached to the lower part of the posterior coxæ[1773]. Now, more than one medical friend has suggested to me, that what are called the coxæ in insects are really analogous to the thighs of vertebrate animals[1774]: consequently these parts must represent the coxæ; whence it would seem that the wings are really appendages of the legs. It must, however, be observed, that were this opinion admitted, in the Aptera, Hymenoptera, and Diptera, or even in the prothorax of other insects, there would scarcely be any analogue of the coxæ at all distinct from the trunk itself, of which even in the other Orders these pieces are component parts. An instance occurs in the Strepsiptera K., and in which the arms are furnished with an alary appendage, and the metathorax has none[1775].
VI. Organs of Motion. We are next to consider those organs attached to the trunk of insects which are instruments of motion. These are principally those by which they are transported through the air, and those by which they move on the earth or in the water—their wings and their legs. I shall begin with the first, the wings[1776]. These are not formed precisely after any type at present discovered in vertebrate animals: in some respects they have an analogy to those of birds[1777]; in others, to the dorsal fins of fishes: but, perhaps, altogether they approach the nearest to those of the dragon or flying-lizard (Draco volans L.), which do not, as in birds, replace the fore-legs, are kept expanded by diverging bony rays, and are connected with the hind-legs[1778]. As the Divine Creator appears in his works to proceed gradually from one type of structure to another, it has been supposed by a learned physiologist of our own country, that in winged insects, four of the legs of the Decapod Crustacea are represented by the four wings[1779]: this opinion, however, is not yet fully proved; a remark which may also be applied to a more recent one of a celebrated French writer, who seems to think their origin and structure aërostatic, that they are auxiliary to the legs, and borrowed in part from the respiratory organs[1780]. Were I disposed to enter into these subtile speculations, I might here recall your attention to the analogy that, in their metamorphoses, exists between the Saurian Reptiles or lizard tribe and insects, and conjecture that the wings of the Draco are really representatives of the mid-legs of Hexapods, thus preparing to disappear altogether; but I shall content myself with throwing out this hint, which you are welcome to pursue. The organs of flight in general may be considered as to their number, kinds, and composition.
i. Number. The most natural number is four, for this obtains in the majority. In almost every Order, indeed, there occur instances of insects that have solely a single pair or none[1781].
These, however, are only exceptions to the rule; but in the Diptera, unless we consider the alulæ, the representatives of the secondary wings[1782], as a distinct pair, there are never more than two wings, and one instance is known in which an insect of this Order has none[1783]. Certain genera or individuals of the Tetrapterous Orders are also furnished with alulæ: besides Dytiscus, Blatta, Phalæna hexaptera, which have been before noticed[1784], they may be detected in miniature in Ammophila K. and affinities; these all may be regarded in some slight degree as insects with six wings.
ii. Kinds. Under this head we may consider the organs of flight as to their situation and as to their substance. As to their situation, usually the first pair are attached to the mesothorax, and the second to the metathorax; but in one instance, as has been before observed[1785], in the Strepsiptera K., the anterior pair belong to the manitrunk, and the posterior to the mesothorax. As to their substance, they take the several denominations of elytra, tegmina, hemelytra, and wings, for the most part according to its variations, as will be seen more at large hereafter. Under this head I shall only further observe, that in many instances the organs of flight appear to be mere abortions or rudiments, which serve to exemplify what has been more than once stated, that the CREATOR has seen it good to approach to new organs gradually as well as to new forms. Thus elytra are mere rudiments that do not serve to protect the wings in Atractocerus; tegmina in some species of Phasma, Acrydium, &c.; hemelytra in the bed-bug[1786]; wings in many female moths, in Cryptus hemipterus a Hymenopterous insect, &c.
iii. Composition. The structure of wings has been before explained to you[1787], and I shall again have occasion to allude to it; but here I wish to call your attention to a circumstance that has not hitherto, that I recollect, been adverted to; I mean that all kinds of organs of flight, and it may be traced as we shall soon see even in elytra, are divided longitudinally into three areas or folds; the first or external one I call the Costal Area[1788] from its beginning with the costal nervure; the second is the Intermediate Area[1789]; and the third is the Anal Area[1790].
Having made these observations with respect to the organs of flight in general, I shall now proceed to consider more at large the elytra, tegmina, hemelytra, and wings.
i. Elytra. These are the wing-covers of the Coleoptera Order, distinguished from tegmina by the absence of nervures, from hemelytra by the want of the membrane at the apex, and from both by their uniting in almost every instance at the suture. I shall consider them as to their substance; articulation with the trunk; expansion; parts; shape; appendages; sculpture; clothing; colours, and uses.
1. Substance. The firmness of the substance of elytra is usually regulated by that of the crust of the insect to which they belong; in hard insects they are hard, and in soft ones they also are soft. The most impenetrable ones that occur to my recollection are those of Illiger's genus Doryphora, and the softest and most flexile those of Telephorus, Meloe and affinities. With regard to individuals, they are mostly as hard as the prothorax, and harder than the back of the abdomen. Elytra also, as far as my observation goes, are never diaphanous.
2. Articulation with the trunk. This is by means of a process of the base of the elytrum which I call the axis[1791] or pivot, attached by elastic ligaments, and certain little bony pieces (osselets Chabr.) in the socket under the side of the anterior angle of the dorsolum[1792]. You may easily remove the elytra attached to the mesothorax from Geotrupes stercorarius, which will enable you to see the mode of articulation with little trouble[1793].
3. Expansion. It is by means of the bony pieces just mentioned that the organs in question are opened and shut[1794] under the action of the antagonist muscles. In opening for flight the two elytra recede from each other, and are elevated so as not to retain their horizontal position, which would interfere probably with the play of the wings, but form an angle with the body. When they return to a state of rest, the sutures usually meet and coincide longitudinally; but in some cases when closed, as in Necydalis, &c., they diverge from each other at the apex; and in Meloe, like the Orthoptera, to which that genus approaches, one laps over the other.
4. Parts. The parts to be considered in an elytrum are the areas, the axis, the suture, the margin, the epipleura, the base and apex, the angles, and the hypoderma. At first it should seem as if an elytrum was not like other wings divided into areas; but I think upon examination it will be found that, though often nearly obsolete, these are represented in it; for the epipleura[1795] with the recurved part of the external margin seems to me analogous to the Costal Area; the inflexed part adjoining the scutellum and often going beyond it to the Anal, and the rest of the organ to the Intermediate. All this you may see in the dung-chafer, Geotrupes stercorarius. The axis[1796] or pivot by which the elytrum articulates with the trunk is generally placed about the middle of its base, but nearer the scutellar than the humeral angle, and varies in length and shape in the different tribes, but not so as to merit particular notice; it may be regarded as composed of three parallel pieces, one belonging to each area, that of the costal being the longest. In many these pieces are marked by no line of distinction, but in Macropus, &c., they may be readily traced[1797]. The suture[1798] is the internal margin of the elytrum from the point of the scutellum to the end. In many beetles the right hand suture, looking from the anus to the head, has a lower ledge or margin, and the other, one more elevated, which when they are closed lies upon the former; in some Dynastidæ there seems a kind of ginglymous structure in this part, each suture being fitted with a kind of ridge which is received by a channel of the other; in these the suture is generally marked out by an adjacent channel: but the most remarkable structure of this part distinguishes the genuine species of the genus Chlamys, in which both the sutures, except at their base, are armed with little teeth, alternating with each other like the cogs of a mill-wheel. In apterous beetles the elytra are often connate, or have both sutures as it were soldered together. The margin[1799] or external edge of the elytra is generally formed by a bead or ridge, which, except in the case of the truncated ones, in which it is straight, curves more or less from the base to the apex; this ridge is often recurved so as to form a kind of channel between it and the disk of the elytrum, as may be seen in the Dynastidæ; in some there are two parallel ridges, as in Copris; in Silpha the margin is dilated; in Helæus and Cossyphus it is remarkably so and recurved, so that, in conjunction with those of the prothorax which are similarly circumstanced, they give the animal some resemblance to a small model of a barge. Though the margin of elytra is most commonly intire, yet in some beetles, as Gymnopleurus Illig., a sinus is taken out of it; in Cetonia it often projects at the base, and in Cryptocephalus in the middle, into a lobe; in Phoberus MacLeay it is denticulated, and in many Buprestes more or less serrulated; sometimes it terminates before it reaches the apex of the elytrum in a tooth, as in many Carabi Latr. The epipleura[1800] or side-cover is that part of the organ in question, below the margin, with which it usually forms an angle, being more or less inflexed, that covers the sides of the body. It varies in different tribes, being sometimes obsolete, as in the weevils (Curculio L.); in the Capricorn beetles it is very narrow; in Carabus, &c., dilated at the base; in many Heteromerous beetles, as Blaps, Pimelia, &c., it is very wide and conspicuous; in Cossyphus it stands out a little from the abdomen, so as to form a kind of fence round it. Its shape generally approaches that of a scythe, being incurved and growing more slender towards the apex[1801]; but it is sometimes straighter and shorter. In Geotrupes and many other Lamellicorns, the base of the elytrum is nearly vertical, forming a right angle with the rest of it; it is usually transverse and straight; but in Calandra Palmarum and many Cassidæ it slants to the scutellum; in Chlamys it is sinuate, and in Elater it has a deep cavity above the axis which receives the points of the phragma mentioned before[1802]. The apex of elytra is usually acute, the angle being formed by the confluence of a curving and straight line: but there are many exceptions; for instance, in Mylabris it is rounded; in Hister obliquely, and in Necrophorus transversely, truncated; in many Capricorns it is emarginate; in others, as Macropus longimanus, it is bidentate; in some Prioni, P. cinnamomeus, &c., it terminates in a mucro at the internal angle; and in Cerambyx Batus, horridus, &c., at the external; and, to name no more, in some species of Necydalis it ends in a long acumen. The scutellar angle in insects that have a large scutellum, as Macruspis MacLeay, is obliquely truncated to admit it, but where it is small it is generally rectangular, with the angle rounded; in Buprestis vittata it is obtusangular; and in Dytiscus marginalis, &c., it is emarginate. In Cassida spinifex, perforata, &c., the humeral angle is producted into an acute lobe that stretches beyond the head, and in C. bicornis and Taurus it forms a horn at right angles with the elytrum. In general it is either rectangular or rounded, with a prominence of the elytrum within it. The sutural and anal angles exist only where the elytra are truncated at the apex. In this case the sutural is generally rectangular, and the anal rather obtusangular or rounded. The Hypoderma is the fine soft membrane before noticed[1803] that lines the underside of the elytra, the use of which is probably to prevent injury to the wings from friction with their usually hard substance; this membrane is commonly of either a pallid or brownish colour; but in some insects, as Staphylinus hybridus, murinus, &c., Buprestis grandis, it is of a beautiful green or blue; and it exhibits the puncta, striæ, and other modes of sculpture of the elytra very distinctly, the pores of which usually perforate this membrane[1804]. Just under the shoulders of these organs you may observe an oblong and sometimes roundish spot, occasioned by the hypoderma in that part being particularly tense, and covering a cavity or pocket which appears to be connected with the axis by the hollow part, which I regard as representing the Costal Area; this pocket is evidently the analogue of a part in the wings noticed by M. Chabrier[1805], and named by me the phialum: from its connexion with the axis by a channel, this part in elytra should also seem destined to receive a fluid to add to the weight of the margin and its means of resistance.
5. Shape. The shape of elytra is various; taken together, in which case, in describing insects, they are denominated coleoptra, their most common form is more or less oblong, or forming more or less a considerable portion of an ellipse; taken separately, it inclines to that of an isosceles triangle, with the exterior side curvilinear: truncated elytra are generally quadrangular, sometimes presenting a trapezium, at others nearly a parallelogram, and at others a square. With regard to their proportions they vary considerably, but the most general law seems to be that the length shall exceed twice the width; in some, as Buprestis grandis, it is more than thrice; in many Staphylinidæ they are as wide as they are long and sometimes wider; they are generally narrower at the apex than at the base, but in some species of Lycus, as L. fasciatus, &c., the reverse takes place; in Telephorus they are nearly of the same width every where: with regard to their surface they are sometimes very convex, as in Moluris; at others very flat, as in Eurychora, Akis, &c.
6. Appendages. These, though not so remarkable as those of the head and prothorax of beetles, ought not to be overlooked. In many Capricorns, as Lamia Tribulus, speculifera, &c., the disk and sides are armed with short sharp spines; in others (Stenocorus, &c.) the sutural and anal angles or one of them terminate in a spine or tooth; sometimes the whole surface, as in Hispa atra, &c., is covered, like a porcupine, with a host of slender spines, or its sides defended by spinose lobes, as in H. erinacea: the humeral prominence is armed with a spine pointing to the head in Macropus longimanus, and forming a right angle with the elytrum in some Curculionidæ, as Rhynchites spinifex; but the most remarkable appendage of this kind is exhibited by Cassida bidens and its affinities,—from the centre of the sutures of the elytrum rise perpendicularly a pair of long, slender, sharp processes internally concave, which both apply exactly to each other, so as together to form a single horn which rises, like a mast from a ship, from the body of the animal[1806]. Besides the appendages here mentioned, the elytra exhibit a variety of tubercles and other elevations of various form and size, which it would be endless to particularize.
7. Sculpture. The sculpture of the organs in question is very various and often very ornamental: but as almost every kind of it will be noticed in the orismological tables, it will not be necessary to enlarge upon it here, especially since I have endeavoured upon a former occasion to explain how it may be useful and important as well as ornamental to the animal[1807]. I shall therefore only notice a few instances, amongst many, in which a particular kind of sculpture distinguishes particular tribes. Amongst those that are Predaceous the Cicindelidæ have elytra without striæ or furrows, while the majority of the subsequent terrestrial tribes of this section are distinguished by them: the Dynastidæ in the Lamellicorn section are remarkable for a single crenated furrow next the suture; in the weevil tribes the numerous species of the genus Apion are ornamented by furrowed elytra with pores in the furrows, which give them the appearance of neat stitching; in many of those beetles that have soft elytra, as the glow-worms (Lampyris), the blister-beetles (Cantharis, Mylabris), and still more in Œdemera, two or three slight ridges generally run longitudinally from the base to the apex, and are visible also on the under-side; as the furrows probably lighten a hard elytrum, these ridges may serve to strengthen a soft one, and it is by these that the first approach is made to the reticular structure of tegmina or the wing-covers of Orthoptera: Lycus palliatus, &c., in its elytra exhibits a direct resemblance of the reticulations of nervures.
8. Clothing. To what I have before said on this subject in general[1808] I shall here add a few remarks, which, though they more properly belong to elytra, may in many cases be extended to the whole body of a beetle. In various instances it happens that the beautiful markings of these organs, as in Macropus longimanus, whose elytra when denuded are black, are produced by short decumbent hairs; in some these variegations are the effect of scales resembling those of Lepidoptera, often of a metallic lustre; from these scales is derived all the brilliancy of the diamond-beetle (Entimus imperialis, Germ.); in some the scales are so minute as to resemble the pollen of flowers, as the white marks observable on the green elytra of the rose-chafer (Cetonia aurata).
9. Colour. The organs of flight in the majority of the Orders with respect to colour are usually the most gaily decorated part of insects; I therefore deferred the notice of that subject till I came to treat of them. In general the colour of insects is either inherent in the substance of their crust, or produced by the hairs or scales that either partially or totally cover it. To confine myself to the Coleoptera, of whose elytra we are treating, it may be observed, I think, in general, that the majority of those that feed upon putrescent substances, the saprophagous tribes of Mr. W. S. MacLeay, are commonly of a more dark and dismal aspect and colour than those which feed upon such as are living and fresh, denominated thalerophagous by the same learned author; this you may see exemplified in his Scarabæidæ and Cetoniadæ. Again, in the Predaceous beetles a similar contrast of colours is often observable. How brilliant and gay are the fierce Cicindelæ! those tigers of insects, as Linné calls them; how black as to colour, how horrible in aspect is their near relation the Manticora: what difference exists in the economy of these animals is not known, except, as I learn from Mr. Burchell, that the latter is subterraneous, whereas the former seek the sunbeam and fly rapidly. I shall now point out a few instances in which the colours of their elytra distinguish tribes or families. Amongst the Predaceous beetles a large family of the Cicindelidæ are distinguished by a middle angular white band, and several white dots on their green or brown elytra, as in C. sylvatica; a family of Brachinus, and the majority of Mylabris, Lamia capensis and fasciatus, &c., by black elytra, with yellow or red bands; Carabus violacea and affinities by the violet margin of these organs; Calliochroma Latreille by their sericeous, and Eumolpus by their metallic, lustre. These instances will be sufficient to turn your attention to this subject, which though not of primary importance in discriminating genera &c., is not without its use in a secondary view.
10. Uses. I must not quit this subject without saying something upon the ends which elytra seem designed to serve. Their first and most obvious use is the protection of the wings when unemployed, that they may not be lacerated or soiled, and rendered unfit for flight in the various retreats to which these animals betake themselves either for food, repose, or to lay their eggs; to promote this purpose more effectually, the wings are usually curiously folded and laid up under them; and where the elytra are very short, as in the Staphylinidæ, these folds are very numerous and complex. In some instances, however, as in Molorchus F., Atractocerus, &c., the wings are only partially protected by the elytra and not folded under them; probably they are less in danger of laceration from their peculiar habits than the generality. Another use is to protect the upper-side of the alitrunk, which for reasons before assigned is usually softer than the under-side, and also of the abdomen, often above nearly membranous, from the injury to which they would otherwise be exposed; in the latter part also the spiracles in Coleoptera are not covered by the inosculations of the segments, as is the case in most other Orders, and therefore probably require some covering when the insect is not flying. In the Apterous beetles this appears to be their principal use; where these organs are connate, or as it were soldered together, the back of the abdomen is a thin membrane; the appearance of two elytra in these cases is given, doubtless, for the sake of symmetry and beauty, a subordinate attention to which may be traced in all the works of creation. If we consider the bulk and weight of many flying beetles, we may imagine that they want some assistance, more than the extent and dimension of their wings seem to promise, to support them in the air, and to enable them to move more readily in it; and although it seems clear from the state of their muscular apparatus that elytra do not move much in flight, yet by giving a broad and concave surface to the air, for then they are usually nearly vertical, they may assist in some measure as sails, and help them in flying traversely and before the wind[1809].
ii. Tegmina[1810]. By this name the learned Illiger has distinguished the upper organs of flight of the Orthoptera and Homopterous Hemiptera[1811]. They may be considered under the same heads nearly as elytra.
1. Substance. Tegmina differ very materially from elytra in their substance, being generally more or less diaphanous, though in Blatta Petiveriana the dark parts are as opaque as elytra, and those of the Mantes that resemble dry leaves are only semidiaphanous. These organs are also of a less dense substance than elytra, something between coriaceous and membranous, which I shall express by the term pergameneous, as somewhat resembling parchment or vellum. Another circumstance relative to this head also distinguishes them,—they are not lined with membrane. In some instances, as in B. Petiveriana just named, they approach nearly to the substance of elytra, and in B. viridis, some Mantes, and Tettigonia, &c., they are little different from wings in their substance; but this does not diminish their right to be considered as tegmina, since their structure is altogether the same.
2. Articulation with the trunk. I observed above that the axis of elytra may be regarded as formed of three parts, one appertaining to each of the areas or their representatives[1812]; in tegmina, and indeed in wings in general, these parts are separate and may be more distinctly traced, the axis of the Costal Area being generally the longest, and that of the Intermediate often the shortest; these axes are suspended in the wing-socket by elastic ligaments, intermixed with hard bony plates, the principal one of which, called by M. Chabrier the humerus[1813], is connected both with the tegmen and the trunk, and in some a little resembles the head and neck of a swan. This structure permits the animal to move the lateral areas in some degree separately, so that each, especially the anal, shall form an angle with the intermediate; as the motion of the latter is not wanted, its axis often falls short of the base, or is obsolete, as in Blatta.
3. Composition. The three areas, traces of which we had discovered in elytra, are particularly visible in tegmina. If you take any cockroach (Blatta), you will at first sight see that in it they are divided into three larger portions by stronger nervures or folds; and if you also take a Mantis, or Locusta Leach, a Fulgora or Tettigonia, the same circumstance will strike you, only you will see that in these the intermediate portion terminates also in an axis; these are what I call the three areas. The external one or Costal is usually the longest and narrowest[1814]; the Intermediate one is commonly triangular, with its inner side curvilinear[1815]; and the interior one, or Anal area, in the Orthoptera is rather oblong; in Fulgora angular, and in Tettigonia it presents an isosceles triangle; with its vertex to the apex of the wing[1816]. The first of these may be defined as that portion of the wing that lies between the costal and postcostal nervures; and perhaps, in some cases, as in Mantis, for there is the fold of the tegmen, the mediastinal may be regarded as its limit; the Intermediate Area is that which lies between the postcostal or mediastinal nervure and the anal fold of the wing; and the Anal Area is the remainder. These areas may perhaps best be made out by tracing each to its axis. To study them carefully in tegmina and hemelytra is of considerable importance; for in them we find the first outline of the general plan upon which the wings of insects are constructed, and which, as we shall see hereafter, more or less enters into the composition of them all.
4. Position, and folding in repose. With regard to their position when not expanded, tegmina vary somewhat in the different tribes. In the Coleoptera we have seen that, except in a few instances, the elytra unite at their suture. Something like this takes place in Fulgora, Cercopis and affinities, in the Homopterous Hemiptera; in these, though the union is not near so exact, yet the tegmina do not lap over each other; they are usually more or less deflexed, with scarcely any portion in a horizontal position: in Tettigonia F., Chermes, Aphis, &c., the middle part only of these organs meets, from which point they diverge both towards their base and apex[1817]. In the Orthoptera the position is quite different, for one tegmen more or less lies over the other. In Blatta, in which the tegmina are nearly horizontal, the left hand one covers almost half the other[1818]: in the other tribes of the Order, with little variation, the Anal Area of the tegmen is horizontal, and covers the back of the animal, and the Intermediate and Costal are vertical and cover its sides; the former, however, in some cases, only forms the angle between them. Sometimes in these the right-hand one is laid upon the left, as in Acheta; and sometimes the reverse of this takes place, as in Acrida K. With regard to the folding of the tegmina, the most remarkable instance that occurs is that of Acheta monstrosa, in which the ends of both these organs and the wings, in repose, are folded like a fan, and then rolled up like a serpent[1819].
5. Shape. The shape of tegmina is various. In the Blattæ and some Mantes they are more or less oblong; in Mantis precaria, strumaria[1820], and others, they incline to elliptical; in Phasma grandis and Acheta monstrosa they are rather panduriform[1821]; in M. gongyloides they are semi-cordate[1822]; in Pterophylla trapeziformis they are rhomboidal[1823]; in Conocephalus erosus they are sinuated; in Locusta Leach they are usually linear or linear-oblong[1824]; in Pterophylla K. they generally terminate in a short mucro[1825]; and in some of those Mantidæ whose tegmina simulate arid leaves, in a recurved one[1826]. In the Homopterous Hemiptera the shape of these organs is less various. In the Fulgorellæ Latr. they incline to a trapezium, sometimes to a pentagon[1827]; in the Tettigoniæ F. they approach to an obtuse-angled triangle; and in others of the tribe they are nearly wedge-shaped[1828].
6. Neuration. The circumstance that most strikingly distinguishes tegmina from elytra is their neuration or veining; which adds much to their strength, without increasing their weight so much as to render them unapt for flight. To look at these organs in Blatta Petiveriana, you would imagine them at first to be deprived of this distinction; but if you observe them attentively, particularly their white spots, you will soon detect their nervures; and if you further examine their lower surface, you will find them very visible. The gibbous Blattæ also, Blatta picta and affinities, the analogues of Erotylus amongst the Coleoptera, have tegmina which, except at their apex, exhibit but faint traces of the nervures of their tribe, and approach to elytra besides by the innumerable minute impressed points that cover them. In the Orthoptera and some Homopterous Hemiptera the nervures may be divided into longitudinal ones more or less ramified, and traversing ones. In the Blattæ the traversing nervures cut the longitudinal ones nearly at right angles, but not at regular intervals, so as to cover the tegmen with quadrangular areolets; in Mantis precaria and affinities the longitudinal nervures of the Anal Area diverge from the base, and are traversed nearly as in Blatta, while those of the Costal diverge from the mediastinal nervure, but the traversing ones form innumerable irregular reticulations; in Mantis sinuata K.[1829] the whole tegmen has such reticulations but less numerous; in Locusta Leach it is regularly reticulated at the base, but the areolets of the apex are quadrangular; in the Mantes, with oblong wings, all are quadrangular; in Pterophylla K. the longitudinal diverging nervures are not numerous, and the traversing ones cut them into quadrangular and triangular areolets, besides which they are covered by innumerable impressed points, so as altogether to exhibit a most exact resemblance of the leaf of some evergreen: in Gryllotalpa the longitudinal nervures of the Anal Area rather converge towards the apex, are traversed by few transverse nervures, and those of the Costal Area which diverge from the mediastinal nervure by still fewer; the neuration of Acheta F. has been before described[1830]; I shall only observe here, that the constructors of stringed instruments of music might, perhaps, from the tegmina of the male, the nervures of which probably modulate the sounds which it produces, take a hint for giving the strings in them a serpentine or convolute direction, and so might produce something new in that department, corresponding with the serpents and French-horns in wind instruments. Of the Homopterous Hemiptera in the Fulgorellæ Latr., which are most analogous to the Orthoptera of all that tribe, the longitudinal nervures are more numerous and branching, more especially toward the apex of the tegmen, and are traversed as much by transverse ones, sometimes reticulating the wing with roundish areolets, as in F. laternaria, and at others with quadrangular ones, as in F. candelaria; in some of these however, as Otiocerus K., Flata F., &c.[1831], there are no traversing nervures; and these lead to the Cercopidæ and others in which the longitudinal nervures become few, and some are without any[1832], and these terminate those of this section of the Order in which the nervures in question are continued to the margin of the wing. We next come to those, Darnis, Centrotus, Membracis, &c., in which they are circumscribed a little within the apex by a traversing nervure, so that the tegmen ends in a margin of pure membrane, and thus some approach seems to be made to the Hemelytra, from Tettigonia, the most conspicuous genus of this tribe, in which the areolets, few in number, like those of Lepidoptera, are not formed, except the terminal ones, by traversing nervures, but by the ramifications of the longitudinal ones; in Chermes the Intermediate Area, which is connected with the base of the wing by a single nervure, is the only part that has any areolets[1833].
7. Colour. Orthopterous insects are seldom remarkable for tegmina of brilliant colours; there is in them none of that gilding or metallic lustre which so often distinguishes elytra: they are also frequently less ornamented in this respect than the wings, with which they usually form an agreeable contrast. Their reticulations and nervures, which are sometimes of a different colour from the rest of the tegmen, decorate them considerably: a remarkable circumstance belonging to this head attends the black tegmina of Blatta Petiveriana; one has four white spots, and the other only three; but as one laps over the other, the symmetry of the arrangement is preserved: the Homopterous Hemiptera are more distinguished in this respect, and some of the Fulgoridæ imitate the Lepidoptera both by their ocelli and spots: Fulgora laternaria, Candelaria, serrata, and Diadema, sufficiently exemplify this remark, as do several Flatæ likewise[1834].
We may observe here—that tegmina are more calculated for flight than elytra, both from their thinner substance, and from the angle that their Anal Area, and often the Costal, forms with the rest of the tegmen; a circumstance which, in wings, M. Chabrier thinks presents some facilities in that kind of motion.
iii. Hemelytra[1835]. The next species of wing-covers, which though varying in the substance of their base, terminate in a part distinct from the three areas, consisting in almost every case of mere membrane, peculiar to the Heteropterous Hemiptera, are called hemelytra, or half-elytra:—this term was also formerly employed, but certainly incorrectly, to denote tegmina. I shall consider them with respect to such of the particulars noticed under the former heads as apply to them, but without repeating them formally.
1. As to their substance, they must be separately considered with regard to their base and apex. In various instances the base, or part consisting of the three areas, is almost corneous, as in Cydnus Morio and bicolor, bugs not uncommon with us, and many others[1836]; in these cases it is lined with a hypoderma like elytra; and in many the points, which are impressed upon it, also perforate the hemelytrum, and seem to act as pores: but in Lygæus, Reduvius, Capsus, Miris, and the majority of the Heteropterous Hemiptera, the organs in question being soft and flexible, may be stated as rather resembling leather than horn;—on this account this part of a hemelytrum is denominated the corium. In Scutellera the portion covered by the scutellum is membranous; and in Acanthia paradoxa, and the cucullated species of Tingis, the wing-covers are entirely so. The apex of these organs is almost universally either membranous or coriaceo-membranous, on which account it is called the membrana. I say almost, because in Aradus and the Hydrocorisæ Latr., this part, though rather thinner than the rest of the Hemelytrum, is also coriaceous; in the latter tribe usually with a very narrow membranous edge; and in many Reduvii and Zeli there is scarcely any difference in the substance of the base and apex.
2. As to the articulation of Hemelytra with the trunk, it seems not strikingly different from that of tegmina: the point or base of the Intermediate Area, which falls short of that of the lateral areas, seems connected by a slender ligamentous piece, with its axis, which is thick; and I do not discern Chabrier's humerus shaped like a swan's head and neck[1837].
3. The composition of these organs differs from that of tegmina in more respects than one: in the first place, they consist, as was lately observed, of four instead of three areas; in the next, they appear to have, at least several of them, a part, which I suspect to be analogous to that above described in Coleoptera, supposed to represent the phialum of wings[1838]. I shall first speak of the areas. In some apterous species related to the bed-bug, Lygæus brevicollis Latr.[1839], &c., there is no trace of the usual areas, and the membrana is a very narrow strip; in L. apterus the former are very faintly traced out, but they are present in all those that are furnished with wings; whence we may conjecture that they are of the same importance in flight with the folds observable in those organs[1840]. The three basal areas may be said most commonly to present three isosceles triangles, the Costal one being narrow and curvilinear[1841], the Intermediate the most ample[1842], and the Anal one the narrowest and shortest[1843], with its vertex towards the apex of the Hemelytrum, while in the two former it is at its base. In Lygæus compressipes (Rhinuchus K. MS.) the Anal Area is cultriform; and in most of the Hydrocorisæ it has an angle in the middle of its posterior margin. The proportion that the membrana or apical area bears to the rest of the wing varies in the different tribes. In some, as before stated, it is obsolete, in others nearly so; in the majority, perhaps, it occupies about a third of the hemelytrum; in Lygæus compressipes, cruciatus, &c., full half; in Alydus calcaratus, two-thirds; in Reduvius, nearly three-quarters[1844]; and in Aradus depressus the corium,—divided, however, though indistinctly, into the three areas,—is driven to the base of the wing: two ends are answered by this structure—as this insect lives under bark, its thin hemelytra take less room; and as it flies, though it has only rudiments of wings, they are more fit to supply their place: the part we are speaking of usually runs obliquely from the vertex of the Anal Area to the base of the Costal.
4. As to their position and folding in repose, Hemelytra are usually nearly or altogether horizontal; but in Notonecta and Plea they are deflexed and cover the sides of the body; and the apical area of one wing precisely covers that of the other; where the scutellum does not intervene, as in Scutellera, Pentatoma, &c., the vertical angles of the Anal Area meet in the middle of the back, so as to exhibit the appearance of a cross. In Notonecta, in which the hemelytra are deflexed, at the apex of the membrana is a fissure which permits the two sides to form an angle with each other, and to apply exactly to the body. In Plea, in which there is no apical area, the posterior margins of the tegmina, as they ought rather to be termed, unite, but do not lap over each other. With regard to the appearance of something like a phialum, if you examine the hemelytra of most species of bugs on the underside, you will see that the costal nervure at the base is inflexed and covers a kind of channel; if you next take one of Belostoma grandis, where the structure is most conspicuous, or even the common Nepa cinerea, you will find in the same situation, adjacent to the inflexed costal nervure, a hollow tube running from the base of the wing, and terminating, after proceeding about one-fourth of its length, in a hollow cavity, which, as it is covered by a membrane, appears to me to be a collapsed pouch. This circumstance is worthy of further and more general investigation.
5. In their shape, with few exceptions, hemelytra more or less represent a wedge, being wider at their apex, where they are usually obliquely truncated, than at the base; but in Plea Leach they are obtusangular, with the angle in the sutural margin; in Notonecta, on the contrary, an obtusangular sinus distinguishes that part; in Naucoris they are curvilinear and every where of equal width; in Ranatra they are linear and straight; in Aradus they are oblong, usually with an external lobe or dilatation at their base: a remarkable instance of the intention of this is observable in a nondescript Brazilian species, in which the head, prothorax, and abdomen, are edged with a number of broad foliaceous appendages; if the base of the hemelytrum had not been furnished with a similar appendage, the symmetry of the whole body would have been destroyed by the hiatus between the prothorax and abdomen, as may be seen by removing the hemelytra; but by this compensating contrivance of Providence, the gap is filled, the above lobe fitting exactly into it.
6. The neuration of these organs will not occupy us long, since the corium or harder part, though in some species there are traces of nervures, is often without them. Those of the cucullated species of Tingis resemble many tegmina in being ornamented by them with a kind of network, which looks like the finest lace; in several Lygæi, Edessa, and some Reduvii, there are a few diverging longitudinal nervures which occasionally by a ramification here and there form an areolet[1845], but there are seldom any traversing nervures. The Apical Area is usually most distinguished by nervures, in some forming several areolets, as in Aradus, in others running parallel to each other, nearly to the end of this area, as in Belostoma grandis, where they are met by a traversing nervure; the object of this is doubtless to strengthen the membrane.
7. Both tegmina and hemelytra are most commonly naked, yet very short hairs are found on those of some species of Cercopis, and in many more instances in those of the latter description, as in Notonecta, several Lygæi and Reduvii, &c.
8. Colours in hemelytra are very various, and in many instances are peculiar to families; in certain Lygæi (L. Hyoscyami, &c.) black and red; in Lygæus compressipes and affinities a dingy black; in some Reduvii black with a large white spot;—but it is needless to enlarge further on this subject.
9. That hemelytra are used in flight is evident not only from the large space allowed for their muscles[1846], but likewise from a circumstance noticed by M. Chabrier, that in flight, in the Pentatomæ Latr., the corium of the hemelytrum is fixed to the wing[1846]; in which case both must describe the same arc.
iv. Wings. We are next to consider organs which are exclusively appropriated to flight, and therefore are properly denominated wings. These in the Orders that have elytra, tegmina, or hemelytra, are the pair that correspond with the secondary wings of the other Orders. It may be said, indeed, that in several instances both tegmina and hemelytra do not differ at all in substance or use from the wings that they cover. This is true; but as their structure in other respects is the same with that of those that are more solid and less apt for flight, it was convenient to consider them under the same name.
1. To begin with the articulation of these organs with the trunk; in general it may be stated that this, as in tegmina and hemelytra, is usually by the intervention of three axes, formed by the conflux of the nervures of the three areas at the base of the wing, which either immediately or by other pieces are implanted in the trunk, so as to receive from it the aërial and other fluids, necessary for its expansion and motions[1847]. Having given this general statement, I shall next apply it to the wings in some of the different Orders. If you carefully extract one from the stag-beetle (Lucanus Cervus) or any large species of the Dynastidæ, in the Coleoptera; the first thing that will strike you, upon examining the base, will be the plate before mentioned called by Chabrier the humerus, which is a stout transverse corneous piece, with a deep sinus towards the wing, filled with ligament: if you again follow the costal, mediastinal, and postcostal nervures, you will find them unite to form an axis, consisting of three parallel pieces, which connects by its intermediate internal piece with one end of this plate. The nervures of the Intermediate Area terminate also in a corneous axis at a greater distance from the base than the other two, which connects with Chabrier's humerus by means of the ligament of the sinus just named. Those of the Anal Area are received by a ligament attached to a transverse plate, widest at its anterior end, which connects with the posterior part of the said humerus; and at its posterior end is united to the postfrænum[1848], with which it forms a right angle. In the Orthoptera Order the structure is not very different, but the axes and other plates of the base of the wing are less distinct and rather cartilaginous; the nervures of the Anal Area often terminate in a transverse one that there forms the segment of a circle[1849]; the inner base of this circle is ligament connected with the postfrænum[1850]. In the Homopterous Hemiptera the three axes may be readily traced, but the humeral plate, with which they all are connected, is more irregular in shape, and in Fulgora longitudinal, with an angular surface; in this Order the nervure, in some cases consisting of cartilaginous rings[1851], in which the frænum and postfrænum terminate in the tegmina and wings, is attached posteriorly to the ligament of the Anal Area. In the Heteropterous section the three axes are evident, but the humeral plate is not easily made out. In the Libellulina the axes of the Costal and Intermediate Areas are the coloured broad plates at their base, formed by the dilatation of their nervures; that, however, of the Anal is not dilated, but forms one nervure, in the primary wing, with the frænum, and in the secondary with the postfrænum. Having given you this clue to trace the axes in those tribes in which they are most conspicuous, it will assist you in searching for them in the remaining Orders, in all of which they may be traced, except perhaps in those minute Hymenoptera whose wings have solely the costal nervures; probably in these there is only one axis. In the Lepidoptera and Hymenoptera a circumstance connected with the present head is observable, which is not to be discovered in the other Orders: these are the tegulæ or base-covers, which appear intended to defend the base of the anterior wings. They are concavo-convex scales, which in the Lepidoptera are large and of an irregular shape[1852], but in the Hymenoptera are smaller and semicircular[1853].
2. Wings, with regard to their substance, may generally be termed membranous; but they vary in this respect, some being much thicker than others, either partially or totally: in spotted wings, as in those of many Libellulina, Tettigoniæ F., &c., the dark opaque parts are denser than those that are transparent: in several Orthopterous insects, as in Phasma, some Mantes, &c., the Costal Area or covering part of the wing is of a substance equally firm with that of the tegmen. This is a compensating contrivance, that where the latter is shorter and smaller than the former, its membranous part, when folded, may be protected from injury. Another similar contrivance of Divine Wisdom is exhibited by those Pterophyllæ K. (Locusta F.) whose tegmina resemble the leaves of plants (Pt. laurifolia, &c.); in these the tip of the wings when folded being longer, is not covered by the tegmina, and therefore exposed to injury; to prevent which this small piece, while the whole wing, as far as covered by those organs, is membranous, is of the same substance with them[1854]. The wings of most Coleoptera, Orthoptera, Hemiptera, and Thereva coleoptrata, in the Diptera, &c., are of a firmer substance than those of the other Orders; in many Locustæ Leach, Fulgoræ, &c., they are nearly as firm as the tegmina; and in Ascalaphus italicus, except at their base, the secondary wings are less membranous than the primary. M. Chabrier has observed[1855] that the wings of insects in general diminish in thickness from their base to their apex, and from their anterior to their posterior margin.
3. I should have had, it is probable, but little original matter to communicate under the head of the composition and neuration of wings, had M. Jurine, who has written so ably on those of Hymenoptera, undertaken a survey of the organs of flight in every Order of insects: but as his views were confined to only two of the Linnean Orders, it is not wonderful that his system and set of terms should fail where a generalization is necessary; and I may stand acquitted of presumption and conceit if I attempt to substitute a system and body of terms more universally applicable. Had the plan of this able Entomologist led him to pay attention to tegmina and hemelytra, their division into three longitudinal areas would have immediately struck him; and having acquired this outline of the greater natural divisions, he would have applied it to the Orders that have wings only, and having discovered that it is to be traced in all, the result would have probably superseded my labors. Had his life been longer spared, perhaps something of this kind would have been effected by him; but as he, alas! is gone, and no abler hand seems to have undertaken the task, I will do what I can to give you satisfaction on this subject[1856]. You have already got a tolerably good idea of these areas from what has been said upon the subject under tegmina and hemelytra; but I shall now more particularly state to you how they are circumstanced in wings. I shall first explain the general law as to their limits. The Costal Area[1857] is all that longitudinal portion of the wing that lies between the anterior margin and the postcostal nervure; the Intermediate Area[1858] is all that longitudinal portion of the wing that lies between the postcostal and the anal nervures; and the Anal Area[1859] is all that longitudinal portion of the wing that lies between the anal nervure and the posterior margin. But there are other helps to enable you to distinguish the areas in the different Orders. The Anal Area in all Orders forms the posterior fold of the wing; in Coleoptera turned under when in repose; in Orthoptera folded like a fan; in Lepidoptera, in some Papilionidæ, forming an arch over the abdomen. Again, in Blatta, the Costal Area is distinguished chiefly by longitudinal nervures; the Intermediate by oblique ones; and the Anal by radiating ones; and in both this tribe and the Mantidæ this last Area is marked out from the Intermediate by a marginal notch, which is not present in Phasma, but is found in both sections of the Hemiptera. In Locusta Leach the notch is between the Costal and Intermediate Areas: in Phasma the nervures of the Intermediate Area are branches of the externo-medial, while those of the Anal, as they do in all the Orthoptera, diverge from the base of the wing: in many, as in Pterophylla K., the part of the wing lately alluded to, that is longer than the tegmen, and of the same substance, points out the limit of the Costal Area; and in others this part terminates in a segment of a circle and is differently reticulated at the apex from the Intermediate: in the Homopterous Hemiptera and the Libellulina, in which the areas at first seem indistinct, they may generally be easily traced by following them from the axes. The separation of the Costal from the Intermediate in the remaining Orders seems less easy on account of the branching of the nervures: in the rest of the Neuroptera and the Lepidoptera, if the posterior branches of the postcostal nervure are not included, you will have a narrow Postcostal Area, which in most cases forms an angle more or less prominent, in Corydalis almost a right angle, with the Intermediate: in Hemerobius and affinities this part is distinguished by areolets formed by transverse nervures, while those of the rest of the wing are longitudinal[1860]: but if the posterior branches are included, the Costal Area will be more ample: a similar observation applies to the Hymenoptera and Diptera; in these, in all cases, the areolets adjoining the anterior margin, which follow the stigma, should be regarded as belonging to the Area in question[1861]. In those tribes of the former Order, whose wings are without nervures, the areas are often marked by folds.
M. Chabrier has observed that in Coleoptera the specific weight of the margin of the wing, and its means of resistance, are augmented by a liquid which is introduced, at the will of the animal, into a long pocket under the brachial, here called the costal and mediastinal nervures, covered by a supple membrane, which in a state of repose becomes flaccid[1862]: it is easily detected, being of a paler colour than the nervures between which it lies; this is what I call the Phialum; we have before seen that it exists also in Elytra and some Hemelytra[1863]; but I have not detected it in any other wings.
I have before given you a sufficiently full account of the alulæ or winglets of Diptera[1864]; and shall here only observe that they are not confined to one particular tribe, as has been usually imagined; but though sometimes extremely minute, simple, and not easily detected, are an universal distinction of the Order.
Having thus endeavoured to elucidate the larger Areas into which wings appear to be divided; I shall next say something on the smaller ones produced by the intersection or ramification of the nervures; these had been named areolets (areolæ) several years before M. Jurine's work, in which he calls them, I think improperly, cellules (cellulæ), was published; I therefore retain the prior term. The general structure of the nervures of the wings of insects having been before explained[1865], I shall not here repeat what I then said; but there is a curious circumstance connected with it, particularly visible in the wings of certain Hymenoptera, that I must not pass without notice. If you examine attentively with a microscope against the light the wing of any Nomada or Andrena, you will discover little transparent points in some of the smaller transverse nervures that form the middle areolets, in which the nervure becomes white and looks as if it was interrupted, though in substance it seems continued: these little points, somewhat resembling minute air bubbles detained in the tubes, are what M. Jurine, who first discovered them, has, on that account, named bullæ, which he thus further describes:—"When the tube (of the nervure) arrives at the spot where a bulla is to be formed, it extends itself on all sides in minute threads in the upper membrane of the wing, losing its colour and tubular structure, which it resumes immediately after the formation of the bulla[1866]." But if you look closely at them you will find that there is always a slight fold of the wing that cuts the nervure exactly at the bullæ, and if the fold changes its direction they accompany it; their object, therefore, is clearly to relax the tension so as to admit a little motion where the fold is; consequently, rather than bullæ (bubbles), they should be denominated articulations. A similar construction, but on a larger scale, may be observed in the wings of Coleoptera[1867] and some others, as Psocus, where the folds traverse the nervures. I shall next make a few observations on the principal nervures; and first a word upon their names. M. Jurine, being of opinion that a striking analogy exists between the wings of insects and those of birds, in which M. Chabrier seems to agree with him, has named the nervures in the anterior margin of the wings of the former, radius and cubitus, as corresponding with the bones so named in the fore-arm of the latter, and the plate which often terminates these nervures in Hymenoptera, he names the carpus; it may look like presumption to differ from two such weighty authorities, but as their observations seem to have been too limited, in one case to the Hymenoptera and Diptera only; and in various Orders there is nothing analogous to the stigma or carpus, and all the other nervures of an insect's wing have no analogue in that of a bird, but more especially as M. Latreille seems to think with me on this subject[1868], I have retained Linné's term for the marginal nervure, and for most of the others have adopted those of the great French Entomologist just mentioned. I shall here only further observe,—and it seems to me an observation of prime importance, in the determination of the question of the analogy of the wings of insects,—that they are not, as in birds, the fore-leg converted into an organ of flight, but, like the wing of the Draco, an organ superadded to the legs; and, further, that the connection is not with the fore-legs, but, as has been before observed[1869], with the two posterior pairs.
The Costa[1870] is usually the strongest of the nervures, and that upon which the wing seems to be built; but in some cases, as in Blatta, Scutellera, Cynips, &c., it is represented by the mere membrane of the anterior margin; in some Coleoptera, as in Geotrupes, Dytiscus, &c., its structure, except at the base, appears to be annular or nearly so, at least a vast number of corrugations, running transversely, are observable on its upper and lower surfaces; it is thus capable of greater tension and relaxation, and more flexile. The stigma or carpus[1871], though most conspicuous in the Hymenoptera Order, may be traced in some Coleoptera, Heteropterous Hemiptera, the Libellulina, &c.; but it has no representative in the Orthoptera, Lepidoptera, Trichoptera, &c. The mediastinal is usually a very slender nervure, placed between the costa and postcosta, sometimes terminating in the former[1872], and at others in the latter[1873]: in the Orthoptera, Lepidoptera, &c., however, and some others, it is a very conspicuous and principal one[1873]; in the Hymenoptera it is obsolete, merging in those nervures[1874]. The Postcosta is the principal nervure of the wing in Scutellera, but in Staphylinus it is wanting; in Chalcis sispes it is the only true nervure of that organ, the others being represented by spurious ones[1875]. The externomedial and internomedial are sometimes distinct at their origin, but more frequently are branches from a common stem.
Having made these general remarks, I shall now consider particularly the neuration of the wings in the different Orders, beginning with the Coleoptera. The first thing that strikes the physiologist in surveying a wing belonging to an insect of this Order, is the general arrangement of the nervures[1876]; which are so placed that the required degree of tension may be given to every part of this organ: thus some are nearly straight[1877]; others run in a serpentine direction[1878]; others are forked with one branch recurrent and another proceeding onwards[1879]; others again are insulated, or do not originate from the base of the wing, or from other nervures, but are merely placed to strengthen an open space of it[1880]: these nervures are also usually broader and more substantial than those of the wings of the subsequent Orders. Another striking circumstance with regard to them is that the nervures form few or no closed areolets, except in the Costal Area, where they are inconspicuous; in Dytiscus marginalis, indeed, and Tenebrio Molitor one or two may be found, but in general there are none. In many of this tribe the postcosta, which terminates at the joint of the wing, becomes recurrent, so as to form a hook, which perhaps represents the stigma, as in Dynastes[1881]; in Creophilus K., a rove-beetle, there is no hook but a broad plate adjacent to the costa. In the Strepsiptera Order the neuration is extremely simple, the nervures, except one insulated one, diverging from the base of the wing[1882]: in this respect, as well as in the form of that organ, an approach is made to the Orthoptera. In the Dermaptera this approach is still more evident; in the common earwig[1883], the diverging nervures become numerous; between each is an insulated one, taking its origin in the middle of the wing, and running to the margin; a little nearer to the latter all the nervures are dilated into a plate; those of the anal area are angular[1884], and the exposed part of the costal is as hard as the elytra. The neuration in the Orthoptera Order may be called radiate, the longitudinal nervures for the most part diverging from the base of the wing like rays: in some few instances[1885], but not often, I believe, an insulated nervure intervenes between each; traversing or connecting nervures, cutting the longitudinal ones in various directions, ornament these wings with an infinity of areolets, causing them to resemble fine gauze or beautiful lace or net-work; very often these areolets are quadrangular, sometimes rhomboidal, frequently nearly circular, and differing occasionally, as has been before observed[1886], in the different areas: it sometimes occurs that there are no traversing nervures[1887], when the wing of course is without areolets. In the Heteropterous Hemiptera the type of neuration, as to the wing, seems borrowed from the Coleoptera, a further proof that these are the analogues of that Order amongst the Haustellata Clairv. In these the nervures usually are few and dispersed, and seldom form any closed areolets. If you examine any Scutellera, Pentatoma, or Lygæus, you may trace the uncinated, forked, serpentine, and insulated nervures of Coleopterous insects; in Gerris and Velia there is an approach to the neuration of some Homopterous species, and in Belostoma &c. the wing is reticulated by spurious nervures. In the Homopterous section there are several types of neuration; thus the Fulgoræ resemble the Orthoptera in this respect; while the Tettigoniæ F., &c., approach nearer to the Hymenoptera and Diptera, and have their apical areolets circumscribed within the margin by a traversing nervure; in Flata, &c., the areolets are mostly formed, not by traversing nervures, but by the branching of the longitudinal ones; in this respect they are not unlike the Lepidoptera. In this last-named Order there are some variations with regard to their neuration—thus, amongst the butterflies in Urania, &c., there is no closed areolet in any of the wings, and almost all the nervures diverge from the base[1888]; in Morpho, &c., there is only one in the primary wing[1889]; in Heliconia, &c., there is one in both wings; amongst the moths, in the Bombyces L., this is divided into two, and in Cossus labyrinthicus Don. into three areolets: in some butterflies (Lycæna) there is one insulated nervure[1890], and in others (Hesperia) there are two[1891]; in these two last, and Heliconia, Urania, &c., the end of the Costal Area is divided into several areolets by oblique nervures[1892], which gives them some analogy to the wings of many Neuroptera; and at the base of this Area, in Morpho, is a roundish areolet[1893]. In this Order the externo-medial and interno-medial nervures coalesce into one, and are only represented separately by their first and third branches[1894]. In the Neuroptera Order the general type of neuration is borrowed from the Orthoptera; but in Osmylus, Termes, &c., there is an approach to that of Flata in the Homopterous Hemiptera, and in Psocus to others of that section; in the second of these genera the nervures, except those of the costal margin, are spurious.
I now come to the Order in which M. Jurine has laboured with so much success, I mean the Hymenoptera; and I only regret that his labours were directed to so small a portion of the Class Insecta, and in that portion only to a part of the upper wing; I say only a part, because all those areolets of the posterior part of the wing, in some cases amounting to five[1895], that lie behind his cubital cellules, are not employed by him as diagnostics, and are left without a name. By dividing the areolets of the Intermediate Area of these wings into three portions, the basal, medial, and apical[1896], I have endeavoured to remedy this defect, and by naming each set of areolets in the middle portion, as you will see in the Orismological Definitions, under the term Areolets, you will find it easy to describe any given areolet and its place in the wing; those of the base may be called the anterior, intermediate, and posterior, where three occur; and the first and last of these terms will suffice where there are only two; the apical areolets, or those that are open to the margin, may be called, first, second, and third in the order of their occurrence, reckoning from the anterior or costal margin.
In this Order it is curious to trace the progress of neuration in the wings of different genera. Thus in Psilus only the costal nervure and the stigma are to be traced[1897]; in Chalcis the postcostal and stigma[1898]; in Codrus and Leucospis the costal, postcostal, stigma, and a nervure representing the externo-medial and interno-medial coalescing into one[1899]; in Omalus the basilar areolets appear[1900]; in Crabro both basilar and medial[1901]; in Cynips basilar, medial, and apical[1902]; and in Hylotoma the wing is filled with its greatest complement of areolets[1903]. The medial areolets of the Intermediate Area, as you will see in the definitions, form three distinct series; these may be called the protomesal, deuteromesal, and tritomesal, reckoning from the postcostal areolets; the first of these corresponds with the cubital cellules of Jurine. These series may be expressed, according to the number of their areolets; by figures, the protomesal standing first. They vary much in this respect in the different genera. Thus in Cyclostoma K.[1904], reckoning the didymous areolet as two, the numbers will stand 4:2:1; in Hylotoma, &c., 3:2:1[1905]; in Aulacus, &c., 2:2:1[1906]; in Bracon, &c., 2:1:1[1907]; in Chelonus, 2:0:1[1908]; in Cynips erythrocephalus Jur., 2:0:0[1909]; in Formica, 1:1:1[1910]; in Oxybelus, 1:0:1[1911]; in Chrysis, 0:1:1[1912]; and in Cynips Rubi K., 1:0:0[1913]. The most natural number is 3:2:1. The next in importance to the medial areolets of the Intermediate Area are the apical, or those open to the margin; the most usual number of them, excluding the postcostal areolets which belong to the Costal Area, is three; but in Sirex there is an approach to four[1914]; in Evania there are only two[1915]; and in Philanthus there are none[1916]; in many, as Prosopis, Nomada, Andrena[1917], though there is the usual number, they are incomplete and do not reach the margin. The basal areas are of little importance in assisting to determine genera; they are most commonly two in number, but in Cynips, &c., there is only one[1918]. The shape and other circumstances of the areolets vary considerably in different genera and species: upon these however I shall not enlarge further, but proceed in the next place to consider very briefly the wings of the Diptera Order as to their neuration. These are not so easily made subservient to a general plan. The basilar areolets are now reduced considerably in length, occupying merely the base of the wing[1919]; the medial are become less numerous and important[1920]; and the apical, in a variety of instances, are the most conspicuous[1921]; in some wings, as in those of Penthetria, the Intermediate Area has no nervures or areolets, or only spurious ones; in Psychoda the nervures diverge from the base almost without branching, so as to form no closed areolets[1922]; in many, the lower medial areolets are very long, resembling the basilar in Hymenoptera[1923]; these are often crowned by a single small one, as in the Stratyomidæ, Tipula, &c., from which numerous branches proceed to the margin[1924]; but in Musca two large ones approach the margin, the anterior one having an angle open to it[1925]; in the Hippoboscidæ almost the whole of the wing is occupied by the apical areolets[1926]; though in some cases they are incomplete[1927].
4. I am next to consider the position of wings in repose and their folding. The most important object of this is that when unemployed they may occupy less space, be less in the way of the insect, and be most effectually protected from injury. Another end is also served by this structure,—that wings can thus be very ample, and present a large surface to the action of the atmosphere without incommoding the insect when it has not occasion to use them.
With respect to this head, insects may be divided into two classes—namely, those whose wings in repose are covered by wing-cases harder than the wings themselves, and those that have no such protection. In the former the wings, though the rule admits several exceptions, have more folds than in the latter. As the different mode of folding the wings has been assumed for a characteristic of the earlier Orders, I shall explain to you with as much brevity as possible how each is circumstanced in this respect, beginning as usual with the Coleoptera.
There are two principal folds of the wing in this Order, which may be named the anal and the apical: the former is when the Anal Area or part of it is folded on the under surface of the base of the wing; this fold is always more or less longitudinal: the latter, the apical fold, is by means of the commissura or joint of the postcosta lately mentioned: which in Hister, Staphylinus, &c., for obvious reasons[1928] is nearer the base of the wing; in Necrophorus in the middle; in Dynastes Aloeus beyond the middle[1929]; in Tenebrio Molitor near the apex; and in Dytiscus marginalis there appears to be no joint at all; but the fact is, that in this insect the postcosta,—the termination of which really forms the joint, the costa itself being only flexible at that point,—stands at a greater distance from the latter at its end. Well, at this joint the above fold is made, the apex of the wing, being first folded longitudinally, turning under and inwards, and forming an angle, more or less acute, with the joint or costal margin, so that the fold is not quite but nearly transverse: this at least is the case in Geotrupes stercorarius and other Lamellicorns: in Staphylinus, &c., there are several transverse and longitudinal folds, and thus the wing is more easily packed under the short elytra; in Molorchus, Necydalis, &c., in which it is left uncovered, except at its base, the anal fold takes place, and the apical in some degree; a short portion near the apex forming an obtuse angle with the margin; in Atractocerus the wing appears to be only longitudinally folded; and in Buprestis vittata only the anal fold is to be detected. Besides these transverse and longitudinal folds these organs, in many beetles, have an infinity of fine corrugations, which ramify like the nervures of the tegmina of Flata[1930], &c., proceeding from the Costal Area or the disk of the wing to the posterior margin; the object of these plicatures is doubtless to present a more ample surface to the action of the atmosphere in flight[1931]. When all these folds have been made in a Coleopterous wing, the apex of the one at its posterior margin crosses or rests upon that of the other[1932].
In the Dermaptera[1933], at least the common earwig, there is a triple transverse fold of the wing, and besides this it has numerous longitudinal ones like those of a fan, each of the diverging nervures representing one of the sticks. In the Strepsiptera the folds are only longitudinal; a circumstance which, besides the form and neuration of the wing, sufficiently attests that its station is more near the Orthoptera and Coleoptera than the Diptera. We next come to the Orthoptera[1934]; in these the folds in general are longitudinal; and those of the Anal Area in particular, either in whole or in part, exact counterparts of a fan: wherever there is a straight nervure, there is usually a fold or a tendency to it; this is the case even with the short oblique ones observable in the Intermediate Area of Blatta: in this tribe the Anal Area, or a considerable portion of it, is folded under the rest of the wing, and the whole lies on the back of the animal, so that in this wing there are only two primary folds; but in those with a narrower body, as Phasma, &c., there are more, and the Anal Area, folded like a fan, lies horizontally on the back; the Costal is vertically applied to the sides, and the Intermediate is between both, as in the tegmina[1935]. In Gryllus Latr., Gryllotalpa, &c., when the wings are folded, the end of the Anal Area projects so as to present the appearance of two tails[1936]; and in that remarkable Chinese animal Gryllus monstrosus, in which these tails are very long, they are convolute like those of some quadrupeds[1937]. It is to be observed that in the secondary folds of these wings the angles of the folds are surmounted by a nervure.
In both sections of the Hemiptera Order, as in the Coleoptera, the Anal Area is turned under the wing and lies over the back of the insect; this is the only primary fold, but besides there are several longitudinal semifolds or secondary ones, in which one part of the surface forms an obtuse angle with another; and in Tettigonia, &c., these folds ramify in the wings as well as in the tegmina at the margin: a number of semifolds also, sometimes transverse and sometimes oblique, run in pairs from each side of every nervure of the disk of both tegmina and wings in the genus last named, the use of which has been before mentioned[1938].
We now come to those Orders that have four membranous wings: first, I shall consider the Lepidoptera. With respect to the position of their wings in repose some variations take place. In the majority of the day-fliers (Papilio L.), when the animal reposes the wings are applied to each other by their upper surface so as to be vertical; but in the skippers (Hesperia), the secondary wings assume a horizontal position, while the primary are vertical but applied to each other. In the Crepuscular tribes (Sphinx L.) the upper wings are incumbent on the lower, and deflexed. In the night-fliers (Phalæna L.) the types of position are various. In some Attacus, Saturnia, Noctua, &c., the wings cover each other, and are a little inclined from a horizontal position; in Gastropacha, Odenesis, and some other Bombycidæ, they are deflexed, and the anterior margin of the under wing projects beyond that of the upper: in some of the Tineæ L., as Crambus, the wings are convoluted, and in others, Galleria, they are applied close to the sides of the body, and being elevated at the apex, terminate, to use a French term—en queue de coq: in Noctua, Geometra, &c., the wings usually cover the abdomen, and are nearly horizontal. With regard to the folds of their wings, the Anal Area of the secondary is the only part that has any striking one; in Papilio Hector and affinities it turns up so as to defend the sides and part of the back of the abdomen; in Morpho Teucer it turns down, and meeting that of the opposite wing, forms a semitube which receives and shelters that part below. In the Crepuscular and Nocturnal Lepidoptera this fold, especially in the former, is very slight. With respect to semifolds in the Diurnal, there is one originating in the disk, between each of the nervures, that goes to the margin of the wing; likewise the under wings, particularly of many Noctuæ, Arctiæ, &c., have many longitudinal semifolds.
In the Neuroptera Order several variations take place with regard to the position of these organs in repose: thus, in Æshna, Libellula, &c., they continue expanded; in Argion they are applied to the body; in Myrmeleon the upper are horizontally incumbent on the lower; in Hemerobius they incline to the horizon. With regard to their folds in Æshna, &c., the longitudinal nervures alternately form the summit or the bottom of a semifold, as do those branches that terminate in the posterior margin; this kind of plicature may be observed, but in a less degree, in Ascalaphus, Myrmeleon, &c.; in Panorpa every nervure is the ridge of a slight fold; in Termes, on the contrary, it forms its bottom. In the Trichoptera, the under wing being much more ample than the upper, the Anal Area forms a fold under the wing, and there seem longitudinal secondary folds besides.
We now come to the Hymenoptera. In this Order the wings, as to their position in repose, are usually incumbent upon each other, and cover the abdomen; in the Vespidæ, however, they are placed parallel to the body, but do not cover it. Before I notice the plicature of these wings, I must recall your attention to what I lately observed[1939] with regard to Jurine's bullæ (bubbles), but which are really the joints of the nervures, as they are to be found only where the folds pass; and where they exist they are an index by which the folds, or rather semifolds, may be traced. I counted eleven of these little joints in the upper wing of Andrena cineraria; sometimes, however, instead of a bulla, a nervure stops short to admit the fold. Wings in this Order have often three longitudinal semifolds more or less conspicuous; these you may trace in the saw-flies (Tenthredo L.), whose wings Linné terms tumidæ, by which term he would indicate the elevation of the whole surface produced by this structure; in the under wings of these, and Scolia, Bembex, &c., the Anal Area is turned under the wing, as in many preceding tribes[1940]: in Sirex, &c., that Area of the upper wing turns upwards, forming an acute angle with the rest of the organ; the same circumstance distinguishes the under wing in the Ichneumonidæ. Several apical semifolds, marked by a pellucid streak, distinguish Tiphia F., and in Bombus, Bembex, &c., an infinity of branching ones, like those before described in Coleoptera, corrugate the apical margin. In the Vespidæ the upper wings are folded longitudinally into three nearly equal portions, but in the under ones the Anal Area only forms the fold.
In the Diptera Order, as to their position when at rest, the wings are mostly incumbent one on the other; but in Psychoda they are deflexed, so as to form a kind of penthouse. With regard to their plication, in some, Tipula oleracea, &c., a slight oblique semifold runs from the stigma to the apical margin, and the Anal Area has two, as it has in many Muscidæ, itself forming nearly a right angle with the rest of the wing; besides these it is corrugated with minute transverse semifolds, which are observable also in several other Dipterous insects; in many Stratyomidæ they are oblique, and run from the disk to the posterior margin; and in Asilus, Bombylius, &c., they are wavy.
5. We are next to say something upon the shape of wings: this, though apparently extremely various in the different Orders and tribes, may I think be traced in every wing to one original prototype, a triangle with the largest angle rounded and subtended by the anterior or costal margin: in some, as the Coleoptera, Orthoptera, &c., this type of formation is a right-angled triangle[1941]; and in others, as in the Hymenoptera, Diptera, &c., the majority of the Neuroptera, &c., it is an obtusangled one[1942]; it may be further observed, that in receding from these forms wings very often assume that of the half or quadrant of some regular figure, as we shall see when we consider those of the different Orders. Another general observation I shall first mention,—that these organs are universally narrowest at their base and widest at the apex, provided we consider as the apex the termination outwards of the three Areas; otherwise we might say that wings in the Coleoptera, Orthoptera, &c., were wider at the base than at the apex[1943]. The wings in the former Order, and in several of the Heteropterous Hemiptera, as Gerris, Velia, &c., may in general, as to their shape, be termed semicordate or semiovate[1944]; in the Dermaptera they incline to an oval figure[1945]: in the Strepsiptera, Orthoptera, most Homopterous and many Heteropterous Hemiptera, they approach to the quadrant of a circle; in a considerable portion of the Lepidoptera the two under wings, if united at their posterior margin, approach a circular form; the upper ones vary a little from the prototype of the under ones, forming an obtusangled triangle[1946]; in many Neuroptera the primary wings may be called oblong or linear-oblong, while the secondary betray more evidently the right-angled or obtusangled triangle; in the Hymenoptera this latter form is every where conspicuous, with little deviation, except in the rounding of the angles[1947]; and, finally, in the Diptera this form shades off again into an oblong, ovate, or linear shape, the wing being most commonly attenuated at the base into a kind of footstalk[1948]. Some singular variations with respect to the termination or marginal processes of the wings are exhibited by many Lepidoptera; thus in Attacus Atlas, &c., the primary wings are falcated or hooked at their apex[1949]; and in great numbers both wings are there scolloped into alternate bays and capes, if I may so speak, varying in depth and length[1950]. There is usually a sinus between every pair of nervures, each of which terminates in the adjoining prominence, as a fold does in the sinus[1951]. Where present, in the primary wings there are eight of these sinuses, and in the secondary, where they are most usual, seven; some are remarkable for the long tails which distinguish their secondary wings; those in Papilio are usually an elongation of the fifth, from the anterior margin, of the prominences before mentioned, into a spathula-shaped diverging process, varying in length and width[1952]: but in P. Ulysses it does not diverge; and in P. Podalirius it is linear. They are found also in other subgenera; thus in Urania Patroclus there are two; in U. Riphæus three; in Erycina Cupido five; and in E. Endymion six of these tails; in some, as in E. Dorylas, the whole wing seems to form the tail; in others again, as in Hesperia Proteus and Bombyx Luna, it is an elongation of the anal angle. Other wings in this Order are divided into lobes resembling feathers, as you may see in Pterophorus hexadactylus, &c.[1953]
6. We are next to consider the clothing of wings: these, in the Orders in which they are covered by elytra, tegmina, or hemelytra, are generally naked, except that the spots in those of Fulgora laternaria, serrata, &c., and the whole wing in Flata, Aleyrodes, and others, are covered with a kind of farinaceous powder; but in all the remaining Orders, hairs or scales are more or less implanted in these organs: as the Lepidoptera are the most remarkable for the clothing of their wings, I shall leave them till the last, and begin with the Neuroptera. If you lightly pass your finger over the wing of any dragon-fly (Libellula F., Æshna F.), from the apex towards the base, you will find that the longitudinal nervures are, as it were, serrulated with very minute bristles, which point towards the extremity; if you next move the finger across the wing, from the posterior to the anterior margin, a similar circumstance will strike you. M. Chabrier conjectures that, amongst other uses[1954], these hairs may contribute to fix the atmospheric fluid when the wings are depressed in flight, while it glides over them as they rise[1955]; in Ascalaphus, Myrmeleon, Nemoptera, Hemerobius, &c., the nervures are more visibly bristled; the bristles diverging on each side from the longitudinal ones, but all pointing towards the apex from the connecting or transverse ones; in Panorpa, besides these bristles, short hairs, pointing the same way, are thickly planted in the membrane of the wing; and in Hemerobius the margins of the wing are fringed; in the Ephemerina, Corydalis, &c., the wings are naked. In the Trichoptera Order, as their name imports, they are covered with minute decumbent hairs, less easily seen but still existing in the secondary pair. In the Hymenoptera in general the wings are covered with minute hairs or bristles; but in Tiphia, Scolia—with the exception of S. Radula and affinities in which they are hairy—and others, the wings are nearly naked; in Pompilus, Pepsis, &c., the hairs are infinitely numerous and very short; in the Sphecidæ, Mutilla, &c., they are more distinct, longer, and less numerous; in the humble-bee (Bombus) and many others the apex of the wing is darkened by a large number of more conspicuous hairs, each of which seems to spring from a minute tubercle: as these tubercles are in a part of the wing that is strengthened by few nervures, they may probably be intended to supply their place, in giving firmness and tension to this part. The wings of Diptera, under the present head, may be viewed with regard to the hairs that are implanted in the membrane of the wing, in its nervures, and in its margin. In the first view, in Stratyomis and immediate affinities the wing is nearly naked; but in Xylophagus, Beris, and the great majority of the Order, the membrane of the wings is thickly planted with innumerable very minute bristles, not to be seen but under a powerful lens, often black, and seemingly crowning a little prominence, and giving the wing an appearance of the finest net-work. As to the clothing of the nervures, the costal, in Anthrax, Bombylius, &c., is often remarkably bristly at the base, with hairs intermixed; in Œstrus Ovis, in the inner margin or edge of this nervure, is a single series of bristles, or rather short spines, like so many black points; in Œ. Equi the whole costa is covered with short decumbent hairs or bristles; in Musca pagana F., just at the apex of the costal areolet, that nervure is armed with a spur or diverging bristle larger than the rest, which is also to be found in many others of the Muscidæ, some of which have two and others more of these spurs. The little moth-like midges (Psychoda Latr., Hirtæa F.) at first appear to have the whole surface of their wings covered with hairs; but upon a closer examination it will be seen that they are planted in the nervures, from each of which they diverge, so as under a lens to give it a very elegant appearance[1956]. This fly has its wings beautifully fringed with fine hairs, the third circumstance to be attended to under this head; in the Tipulidans, and many others of this Order, the apex and posterior margin are also finely fringed with short hairs. Some Dipterous insects make a near approach to the Lepidoptera in the covering of their wings: in the common gnat, when the wings are not rubbed, the nervures are adorned by a double series of scales, and the marginal fringe also consists of them[1957]; and in a Georgian genus, which appears in some degree to connect Culex with Anthrax &c., there are scales scattered upon the membrane as well as upon the nervures; besides, its antennæ[1958] and abdomen are also covered with them.
The Order, the clothing of whose organs of flight excites the admiration of the most incurious beholder, is that to which the excursive butterfly belongs, the Lepidoptera. The gorgeous wings of these universal favourites, as well as those of the hawk-moths and moths, owe all their beauty, not to the substance of which they are composed, but to an infinite number of little plumes or scales so thickly planted in their upper and under surface, as in the great majority entirely to conceal that substance. Whether these are really most analogous to plumes or scales has been thought doubtful. De Geer is inclined to think, from their terminating at their lower end in little quills and other circumstances, that they resemble feathers as much as scales[1959]; Reaumur on the contrary suspects that they come nearer to scales[1960]. Their substance, approaching to membrane, seems to make further for the former opinion, and their shape and the indentations that often occur in their extremity, furnish an additional argument for the latter. Their numbers are infinite; Leeuwenhoek found more than 400,000 on the wings of the silk-worm moth (Bombyx Mori)[1961]; and in those of some of the larger moths and butterflies the number must greatly exceed this. You will observe however that in many Lepidoptera the wings are partially, and in some instances generally, transparent: thus in Hesperia Proteus, a butterfly before noticed for the long tail that distinguishes its secondary wings, there are many transparent spots; in Attacus Atlas, one of the largest of moths, and its affinities, there is as it were a window in each wing formed by a transparent triangular space; in A. Polyphemus, Paphia, &c., the pupil of the ocellus is transparent, which in the former is divided by a nervure. In several of the Heliconian butterflies, and in Zygæna F., &c., the greater part of both wings is transparent, with scales only upon their nervures, round their margin, or forming certain bands or spots upon them; in Parnassius Apollo, Mnemosyne, &c., the scales are so arranged as not wholly to cover the wings, which renders them semidiaphanous; and in some (Nudaria) the wings are intirely denuded. With regard to size, the scales vary often considerably in different tribes; in Heliconia they appear to be more minute than in the rest; and in Castnia they are the largest and coarsest; the extremity of the wings of Lepidopterous insects in general is fringed with longer scales than their surfaces, and even those of the last in the same wing; sometimes vary in magnitude. The little seeming tooth that projects from the middle of the posterior margin in the upper wings of Notodonta, a subgenus of Bombyx L., is merely produced by some longer diverging hairs. The shape and figure also of scales are very various—some being long and slender; others short and broad; some nearly round; others oval, ovate, or oblong; others spathulate; others panduriform or parabolical; some again almost square or rhomboidal; many triangular; some representing an isosceles triangle, and others an equilateral one; lastly, some are lanceolate and others linear; again, some have a very short pedicle and others a very long one: with regard to their extremity; some are intire, without projecting points or incisions, while others are furnished with them: of these some terminate in a single long mucro, others have several shorter ones; some are armed with teeth, varying in number from two to thirteen in different species[1962]. Many other forms might be enumerated, but these are sufficient to give you a general notion of the infinite variety of this part of the works of the Creator. I must next say a word or two upon their arrangement on the wing. In most instances this is in transverse lines, which sometimes vary a little from a rectilinear course, and the extremity of the scales of one row reposes on the base of those of the succeeding one, so that in this respect their arrangement is like that of tiles in a roof: in some cases it is not so regular: thus the minute scales on the wings of Parnassius Apollo, and others with subdiaphanous wings, are arranged without order; in Pieris and other Diurnal Lepidoptera, and many of the Crepuscular and Nocturnal, there appears to be a double layer of scales on both sides of the wing; the under layer usually consisting of white ones. If you denude the wings of any butterfly, which you may easily do by scraping it lightly on both sides with a penknife, you will be amused to trace the lines in which the scales were planted, consisting of innumerable minute dots: the lines of the under side, in some cases, so cut those of the upper side, as by their intersection to form lozenges. With regard to the position of the scales on the wing, they usually lie flat, but sometimes their extremity is incurved: in the beautiful Argynnis Vanillæ a very singular appearance of numerous transverse ridges is produced by the extremity of those scales that cover the longitudinal nervures of the primary wings, except at the base, being recurved.
But though the general clothing of the wings of Lepidoptera consists of these little scales, yet in some cases they are either replaced by hairs or mixed with them. Thus, in the clear parts of the wings of Heliconians, Attaci, &c., short inconspicuous hairs are planted; in a large number of the Orders the upper side of the Anal Area of the secondary wings is hairy; in several Crepusculars (Sphinx Phœnix, &c.), where there is a double layer as before mentioned, the upper one consists of dense hairs, except at the apex, and the lower one of scales; and in most of them the scales of the primary wings are piliform, and the secondary are covered by what approach very near to real hairs; many of the Attaci are similarly circumstanced: the four wings of A. Cytherea are also covered externally with hair.
7. Before I conclude this long diatribe on the organs of flight of insects, I must not omit some notice of the infinite diversity of colours with which their wings are often variegated and adorned by the Creator, who loves to delight us by the beauty, as well as to astonish and awe us by the immensity and grandeur of his works. Though the wings in every Order exhibit instances of brilliant and beautiful colouring, yet those of the Lepidoptera in this respect infinitely excel them all, and to these, under this head, after noticing a few in the less privileged Orders, my observations will be confined. Although in the Coleoptera the wings are seldom distinguished by their splendour; yet those of some Cetoniadæ, as Cetonia africana, are extremely brilliant, and resemble those of many Xylocopæ in the lovely violet hue that adorns them: amongst the Orthoptera some Pterophyllæ, and in the Homopterous Hemiptera some Fulgoræ, emulate the Lepidoptera in the ocelli that give a kind of life to these organs[1963]; and a vast number of the destructive tribe of locusts (Locusta Leach) are remarkable for the fine colours and gaiety of their wings[1964]; in the Neuroptera numerous Libellulinæ emulate the Heliconian butterflies by their maculation; and in the genus Ascalaphus, which represents the Lepidoptera by its clubbed antennæ[1965], many also have the resemblance increased by the painting of their wings, so that some Entomologists have actually considered some of them as belonging to that Order[1966]; the wings of the Xylocopæ, before alluded to, sometimes add to the deep tints of the violet—which also prevail in the wings of several Diptera—towards their extremity the most brilliant metallic green or copper varying,
"As the site varies in the gazer's hand,"
and even those wings that consist of clear colourless membrane are often rendered extremely beautiful from the reflection of the prismatic colours. I should undertake an endless task did I attempt to specify all the modes of marking, clouding, and spotting, that variegate a wing, and all the shades of colour that paint it, amongst the Lepidopterous tribes; I shall therefore confine myself to a few of the principal, especially those that distinguish particular tribes and families. Of whole coloured wings—I know none that dazzle the eye of the beholder so much as the upper surface of those of Morpho Menelaus and Telemachus: Linné justly observes that there is scarcely any thing in nature that for brightness and splendour can be paralleled with this colour; it is a kind of rich ultramarine that vies with the deepest and purest azure of the sky; and what must cause a striking contrast in flight, the prone surface of the wings is as dull and dark as the supine is brilliant, so that one can conceive this animal to appear like a planet in full radiance, and under eclipse, as its wings open and shut in the blaze of a tropical sun: another butterfly, Papilio Ulysses, by its radiating cerulean disk, surrounded on every side by a margin intensely black, gives the idea of light first emerging from primeval obscurity; it was probably this idea of light shining in darkness that induced Linné to give it the name of the wisest of the Greeks in a dark and barbarous age. I know no insect upon which the sight rests with such untired pleasure, as upon the lovely butterfly that bears the name of the unhappy Trojan king (P. Priamus); the contrast of the rich green and black of the velvet of its wings with each other, and with the orange of its abdomen, is beyond expression regal and magnificent. But peculiar beauties of colour sometimes distinguish whole tribes as well as individuals. What can be more lovely than that tribe of little butterflies that flit around us every where in our summer rambles, which are called blues, and which exhibit the various tints of the sky? Lycæna Adonis of this tribe scarcely yields to any exotic butterfly in the celestial purity of its azure wings: our native coppers also, Lycæna dispar[1967], Virgaureæ, &c., are remarkable for the fulgid colour of these organs; in Argynnis the upper side of their wings is tawny, spotted with black, while the under side of the secondary ones is very often adorned by the appearance of silver spots. How this remarkable effect of metallic lustre, so often reflected by spots in the wings of butterflies, is produced, seems not to have occupied the attention of Entomologists. M. Audebert is of opinion that the similar lustre of the plumes of the humming birds (Trochilus) is owing to their density, to the polish of their surface, and to the great number of little minute concave mirrors which are observable on their little beards[1968]. But these observations will not apply to the scales of the wings of butterflies, which are always very thin and generally very flat: in some instances, as in Morpho Menelaus, there appears more than one very slight channel upon a scale; but this takes place also in others that reflect no lustre. Their metallic hues must therefore principally be occasioned by the high polish of their surface and the richness of their tints. It is the purity of the white, in conjunction with their shining surface, contrasted with the dull opaque colour of the under side of the secondary wings, that causes the spots that decorate those of the Fritillaries (Argynnis) to emulate the lustre of silver. In Papilio the Trojans are distinguished by the black wings with sanguine spots, and the Greeks by the same with yellow spots; but these have proved in some instances only sexual distinctions[1969]. In the Danai candidi L. the colour of the tribe may be described as sacred to the day, since every shade, from white or the palest yellow to full orange, is exhibited by them. The yellows prevail also in those Noctuæ, the trivial names of which Linné made to end in ago, as N. Fulvago, Citrago, &c. I must not conclude this part of my subject without noticing one of the most striking ornaments of the wings of Lepidoptera, the many-coloured eyes which decorate so large a number of them. Some few birds, as the Peacock and Argus Pheasant, have been decked by their Creator very conspicuously with this almost dazzling glory; but in the insects just named it meets us every where. Some, as one of our most beautiful butterflies, Vanessa Io[1970], have them both on the primary and secondary wings; others, as Noctua Bubo[1971], only on the primary; others again, as Smerinthus ocellata[1972], only on the secondary: in some also they are on both sides of the wing, as in Hipparchia Ægeria[1973], and in others only on the upper side, as in Vanessa Io; in others again only on the under side, as in Morpho Teucer[1974]: in some likewise they are very large, as in the secondary wings of the same butterfly: and in others very small, as in those in the wings of the blues (Lycæna). Once more, in some they consist only of iris and pupil, as in Hipparchia Semele, and in others of many concentric circles besides, as in Morpho Teucer, &c.
v. Legs[1975]. We are next to consider those organs of motion affixed to the trunk, by which insects transport themselves from one place to another on the earth or in the water, and by which also they perform various operations connected with their economy[1976]. In treating of them we should consider their number; kind; substance; articulation with the trunk; position; proportions; clothing; composition; folding; and motions.
1. Number. Having before very fully explained to you the number and kind of the legs of insects in their preparatory states[1977], I shall now confine myself to the consideration of these organs in their perfect or last state; beginning with their number. Insects, properly so called, as I formerly observed[1978], in this state, including the anterior pair or arms, have only six legs, none exceeding or falling short of this number; but in several of the Diurnal Lepidoptera (Vanessa, &c.) the anterior pair are spurious, or at least not used as legs, the tarsi having neither joints nor claws[1979]; this in some cases is said to be only a sexual distinction[1980]. In Onitis, Phanæus, and some other Scarabæidæ McL., the arm has either none or a spurious tarsus or manus[1981]; which in the first of these genera is also a sexual character. From both these instances we see that walking is only a secondary use of forelegs in the insect tribes. Besides insects proper, a whole tribe of mites (Caris Latr., Leptus Latr., Astoma Latr., Ocypete Leach) have only six legs; the rest, and the Arachnida in general, have eight; in the Myriapods, Pollyxenus has twelve pairs; Scutigera has fifteen; the terrestrial Glomerides (G. Armadillo, &c.) sixteen; and the oceanic (G. ovalis) twenty; the oriental Scolopendræ Leach, twenty-one; Polydesmus has usually about thirty pairs; Craspedosoma, fifty; Geophilus electricus at least sixty; in Iulus terrestris there are more than seventy; in I. sabulosus nearly one hundred; in I. fuscus, 124; and in I. maximus 134 pairs or 268 single legs. But with respect to the Geophili, Iuli, &c., it is to be observed, that the number of pairs varies in different individuals; and the circumstance that has been before mentioned[1982], that these animals keep acquiring legs in their progress to the perfect state, instead of losing them, renders it difficult to ascertain what is the natural number of pairs in any species.
2. Kinds. Upon a former occasion I gave you a sufficiently full account of the kinds of legs[1982], and I have also assigned my reasons for giving a different denomination to the anterior legs under certain circumstances[1983]; I shall not therefore enlarge further upon this head.
3. Substance. The substance of the legs is generally regulated more or less by that of the rest of the body, only in soft-bodied insects they seem usually more firm and unbending. Each joint is a tube, including the moving muscles, nerves, and air vessels.
4. Articulation with the Trunk. M. Cuvier has observed that the hip (coxa), which is the joint that unites the leg with the body, rather inosculates, in its acetabulum, than articulates in any precise manner[1984]; but this observation, though true of a great many, will not apply universally, for the legs of Orthopterous insects, and of most of the subsequent Orders, are suspended rather than inosculating. Even in many Coleoptera a difference is observable in this respect. I have before mentioned that what are called the puncta ordinaria, which distinguish the sides of the prothorax of many Scarabæidæ and Geotrupidæ, form a base for an elevation of the interior surface with which the extremity of the base of the clavicle, which plunges deep into the breast, ginglymates[1985]; this structure may also be found in other Lamellicorns, as the stag-beetle (Lucanus) and Dynastes, that have not those excavations; in these last it is an elevated ridge forming a segment of a circle with, it should seem, a posterior channel, receiving a corresponding cavity and protuberance of the clavicle. With regard to the mid-leg, in Copris, the coxa is emboxed in a nearly longitudinal cavity of the medipectus, and the coxa of the hind-leg anteriorly is suspended to a transverse cavity of the postpectus, but posteriorly it is received by a cavity of the first segment of the abdomen; so that it may be regarded as suspended anteriorly, and inosculating posteriorly.
In some tribes of this Order, as the Weevils (Curculio L.) and Capricorns (Cerambyx), the coxæ of the four anterior legs are subglobose[1986] and extremely lubricous, and are received each by a socket that fits it, and is equally lubricous. In the bottom of this externally, and in the head of the coxa, is an orifice for the transmission of muscles, nerves, and bronchiæ; but the coxa is suspended by ligament in the socket. This structure approaches as near the ball and socket as the nature of the insect skeleton will permit; the high polish of the articulations acts the part of synovia, and the motion is in some degree rotatory or versatile, whereas in Copris, &c., lately mentioned, it seems to be more limited, and is probably, at least in the mid- and hind-legs, only in two directions; in the middle pair, probably, from the coxæ being in a position parallel with the breast, opposite to that of the hind pair. In Dytiscus L., Carabus L., and some other beetles, the coxæ, especially the posterior pair, appear to be fixed and incapable of motion. In many insects these coxæ seem to belong as much to the abdomen as to the trunk. We have just seen this to be the case in Copris, &c.; and in the Lepidoptera, if the former be separated from the latter, the legs will be detached with it.
4. Location. We are now to consider the location and position of the legs, both in general and with respect to each other. And first, as has been before stated, we may observe that, in the hexapods with wings, the arms belong to the manitrunk, and are attached to the antepectus on each side the prosternum; and the two pair of legs to the alitrunk, the mid-legs being attached to the medipectus, between the scapularia and mesosternum; and the hind-legs to the postpectus, between the parapleura and the posternum; and further, that the arms are opposed to the prothorax: the mid-legs to the mesothorax and the primary organs of flight; and the hind-legs to the metathorax and the secondary organs of flight; though in some cases the wings appear to be behind the legs and in others before them: thus, in Panorpa, the former are nearer the head than the latter; but in the Libellulina the reverse of this takes place, the legs being much nearer the head than the wings: in both cases, however, the scapularia and parapleuræ run from the legs to the wings, but in an oblique direction; and in Panorpa these pieces assume the appearance of articulations of the legs. In most of the apterous hexapods they appear to be attached laterally between the thorax and the pectus[1987]; but in the flea (Pulex) they are ventral. In this tribe the arms are usually stated to be inserted in the head[1988]: but I once succeeded in separating the head of a flea from the trunk, and these organs remained attached to the latter[1989]. As to the Octopods and Arachnidæ, in the mites (Acarus L.) they are lateral, and in their analogues, the spiders (Aranea L.), they emerge between the thorax and the breast, which last they nearly surround; in the Phalangidæ the bases of the coxæ approach near to each other, being separated only by a narrow sternum; in their antagonists, Chelifer and Scorpio, they apply to each other, the anterior ones acting as maxillæ. In the myriapods the legs of the Chilopoda Latr., and some Chilognatha, as Glomeris, are inserted laterally, a single pair in a segment; but in Iulus L. their attachment is ventral, the coxæ seem to spring from a common base, and there are two pair to each segment[1990], except the three first, which bear each a single pair.
I shall next consider how the legs are located with respect to each other. To render this clear to you I shall represent each of the variations, which amount in all to twelve in the hexapods that have fallen under my notice, by six dots.
1.
In this arrangement the legs are all planted near to each other, there being little or no interval between the pairs, and between the legs of each pair. It is exemplified in the Lepidoptera, Blatta, and many Diptera.
2.
Similar to the preceding, but the anterior pair are distant from the two posterior; exemplified in the bees (Apis) and most Hymenoptera; Chironomus; Scutellera; Pachysoma K.[1991]
3.
Like the last, but the posterior pair is distant from the two anterior. Examples: Silpha, Necrophorus, Telephorus, &c.
4.
Similar to the last, but the legs of the posterior pair are more distant from each other than the four anterior. Ex. Curculio L.
5.
The legs of each pair near each other, but the pairs distant. Ex. Gibbium.
6.
Both the legs of each pair and the pairs distant. Ex. Blaps, &c.
7.
Anterior pair distant from the two posterior, and the legs of the middle pair rather more distant from each other than those of the other pairs. Ex. Scarabæus McL.
8.
Like the preceding, only the legs of the middle pair are at a much greater distance from each other. Ex. Copris McL.
9.
Legs of the two posterior pairs distant. Ex. Hister, Scaphidium.
10.
Like the preceding, but the posterior legs more distant than those of the middle pair. Ex. Lygæus.
11.
Like the last, but the legs of the anterior pair also distant. Ex. Velia.
12.
The arms distant, intermediate legs more distant, posterior legs close together. Ex. Byrrhus L.
5. Proportions. In general the legs of some insects are disproportionally long and slender, as in Phalangium Opilio and some species of Gonyleptes[1992]: those of others are disproportionally short, as in Elater, &c. With regard to their relative proportions, the most general rule is, in Hexapods, that the anterior pair shall be the shortest and most slender, and the posterior the longest and thickest; but there are many exceptions: thus, in Macropus longimanus, Clytra longimana, &c., in the male the arms are the longest; again, a thing that very rarely occurs, in the same sex of Podalirius retusa the intermediate legs are the longest[1993]; but in Rhina barbirostris and many weevils they are the shortest: in Saperda hirtipes Oliv.[1994] the hind-legs are disproportionally long: with regard to thickness, they are in general extremely slender in Cicindela, and in the Scarabæidæ very thick. In Goliathus Cacicus the arms are more robust than the four legs[1995]; in Gyrinus the latter are more dilated than the former; in many Rutelidæ, and particularly in the celebrated Kanguroo beetle (Scarabæus Macropus Franc.) the hind-legs are much the thickest; in a new genus of weevils from Brazil (Plectropus K.), the intermediate pair are more slender than either the arms or the posterior pair.
6. Clothing. The hairs on the legs of insects, though at first sight they may seem unimportant, in many cases are of great use to them, both in their ordinary avocations and motions: but as most of these were sufficiently noticed when I treated of the sexes of insects[1996], I shall not here repeat my observations, but confine myself to cases not then adverted to. Some insects have all their legs very hairy, as many spiders, the diamond beetle (Entimus imperialis), or at least a species very near it and common in Brazil[1997], &c.: in others they are nearly naked, as in the stag-beetle. In the Crepuscular Lepidoptera (Sphinx L.) and some of the Nocturnal ones (Bombyx L.) the thighs are much more hairy than the rest of the legs: and in Lucanus, Geotrupes, and many other Lamellicorns, &c., the anterior ones have a yellow or golden spot at their base, composed of decumbent hairs, which prevent them from suffering by the violent friction to which they are exposed in burrowing. In most Petalocerous beetles the tibiæ are set with scattered bristles, and sometimes the thighs. The Tiger beetles (Cicindela) are similarly circumstanced: but the bristles, which are white, are generally arranged in rows. In Dytiscus, Hydrophilus, &c., the four posterior tarsi; and in Notonecta the posterior pair, and also the tibiæ—are fringed on each side with a dense series of hairs, which structure assists them in swimming[1998]. The tarsi, especially the anterior pair, in a certain family of Lamia F. (L. papulosa, &c.[1999]), are similarly fringed, only the hairs curl inwards; and the hand in Sphex and Ammophila, but not in Pelopæus and Chlorion, is fringed externally with long bristles.
7. Composition. With regard to their composition, both arms and legs generally consist of five pieces, which Entomologists have denominated—the coxa or hip—the trochanter—the femur or thigh—the tibia or shank—and the tarsus or foot. Where the structure and use of the fore-leg is different from that of the four hind-legs, I propose calling these pieces by names corresponding with those which anatomists have appropriated to the arm in the higher vertebrate animals: thus, as you will see in the table, I call the whole fore-leg the brachium or arm; and the coxa becomes the clavicula or collar-bone; the trochanter, the scapula or shoulder-blade; the femur, the humerus or shoulder; the tibia, the cubitus or arm; the tarsus, the manus or hand. But let me not lead you to suppose that the pieces, either in the arms or legs of insects, which are there named after certain others in vertebrate animals, precisely correspond with them—by no means—since that is a very doubtful point; and some of them, as the trochanter, clearly do not. Many gentlemen skilled in anatomy, as I have before observed[2000], have thought that what is regarded as the coxa in insects really represents the femur: but there are considerable difficulties in the way of this supposition, several of which I then stated. I shall not however enter further into the subject, and take the above names; since this application of them is so general and so well understood, except with regard to the fore-leg, under certain circumstances, as I find them. I shall now consider them in the order in which I have named them.
a. Coxa or Clavicula[2001]. The coxa is the joint that connects the leg with the trunk of the insect. With regard to their shape, the most general form of the four anterior is more or less that of a truncated cone: in the Staphylinidæ, however, they tend to a pyramidal or four-sided figure; as do the whole six in the Trichoptera: in numbers of the weevils and capricorns they are subglobose; in the Lamellicorns they are mostly oblong, and not prominent: the posterior pair in the Coleoptera are generally flat and placed in a transverse position, and more or less oblong and quadrangular: in Elater, &c., they are cuneiform: in Haliplus Latr. they are dilated, and cover the thigh[2002]: in Buprestis, Copris, &c., they have a cavity that partly receives it: the corresponding part, the clavicle, in the arm of Gryllotalpa, is very large and remarkable; viewed underneath it is triangular, and trifid where the trochanter articulates with it: in that of Megachile Willughbiella the clavicle is armed with a spine[2003]. As to their proportions, the most general law seems to be, that the anterior pair shall be the shortest and smallest, and the posterior the longest and largest. In some instances, as in Buprestis, the two anterior pair are nearly equal; in others (Mantis, Eurhinus K.), the anterior are the longest, in the former as long as the thigh, and the four posterior the shortest: in the Trichoptera, Lepidoptera, &c., all are nearly equal; in Mantis the two posterior, and in Phengodes the intermediate pair are the largest; but in Necrophorus they are the smallest:—though almost universally without articulations, in Galeodes the clavicle consists of two and the coxa of three[2004].
b. Trochanter or Scapula[2005]. This is the second joint of the leg: and if the coxa is regarded as the analogue of the thigh in vertebrate animals, this should seem to represent the patella or rotula, vulgarly called the knee-pan. Latreille and Dr. Virey consider this articulation as merely a joint of the coxa[2006]; but if closely examined, especially in Coleopterous insects, you will find it so fixed to the thigh as scarcely to have separate motion from it, and in many cases it seems to be merely its fulcrum; but I am not aware that any instance occurs in which it has not motion separate from that of the former joint.
As to its articulation with the coxa,—in the Coleoptera it appears to be of a mixed kind; for it inosculates in that joint, is suspended by ligament to its orifice, and its protuberances are received by corresponding cavities in it; and its cavities receive protuberances, which belongs to a ginglymous articulation. I have observed two variations in this Order, in one of which the motion of the thigh and trochanter is only in two directions, and in the other it is nearly versatile or rotatory. The Lamellicorns afford an example of the first, and the Rhyncophorous beetles or weevils of the second. If you extract from the coxa the thigh with the trochanter of the larger species of Dynastes McL., you will find that the head of the latter is divided into two obtuse incurving lobes or condyles: that on the inner side being the smallest and shortest, and constricted just below its apex: and that under this is a shallow or glenoid cavity, terminating posteriorly in a lubricous flat curvilinear ridge. If you next examine the trochanter in articulation with the coxa, you will perceive that the head of the former inosculates in it, that the lower condyle is received by a sinus of the coxa, which also has a lubricous very shallow cavity corresponding with the ridge, in which it turns; and in the head of the coxa, on the lower side, is an external condyle, which is received by a sinus common to both, of the head of the thigh and of the exterior side of the trochanter[2007], in which it likewise turns: this last condyle has also an internal protuberance, which appears to ginglymate with a cavity of the trochanter: from this structure the leg is limited chiefly to a motion up and down upon two pivots, or to fold and extend itself. You will find an articulation very near this, but on a smaller scale, in the stag-beetle. In the other kind of articulation, which admits of freer motion, the head of the trochanter is prolonged, and the process terminates in a short interior condyle, which appears to work in a corresponding cavity of the interior of the coxa; and the base of the process is encompassed by a ridge with a cavity behind it, which is received by another of the lower part of that piece, and admits a corresponding ridge—a structure that allows a rotatory motion. In the hind-legs of this tribe the motion is chiefly limited to folding and extending; in Carabus, &c., also the head of the trochanter is nearly hemispherical, and the articulation approaches ball and socket. In most of the other Orders, the Hymenoptera excepted, there is little or no inosculation, the trochanter being simply suspended by ligament to the coxa as well as to the thigh; its connection with the latter is similar in Coleoptera; but in Cicindela, &c., it inosculates in it. The part we are considering varies in its position with respect to the thigh: in the hind-legs of Carabus, &c., it forms a lateral fulcrum on the inner side of that part, and does not intervene between its base and the coxa; the muscles from the latter entering the former, not at the bottom of the base, but at its side: but in the four anterior legs it forms their base, as it does in all the legs in Apion, and in all the Orders except the Coleoptera, cutting them entirely off from contact with the coxa: in the Lamellicorns they cut off part of the base obliquely, but so as to permit their coming in contact with the condyle of the coxa, as before mentioned. In the Ichneumonidæ and some other Hymenoptera the trochanter appears to consist of two joints particularly visible in the posterior legs[2008].
As to size in general,—the part in question is smaller than the coxa; but in Notonecta it is larger, and in the dog-tick (Ixodes Ricinus) longer than that joint. It exhibits few variations in its shape or appendages worthy of particular notice. In general, in the Coleoptera it is triangular or trigonal; but in Carabus L., in the hind-leg it is oblong or rather kidney-shaped; in that of Necrophorus[2009] it terminates in one or two teeth or spines, varying in length in the different species: in the other Orders it is not remarkable in this respect.
c. Femur or Humerus[2010]. The femur or thigh is the third, and usually the largest and most conspicuous joint of the leg. In the hypothesis before alluded to[2011] it is considered as the analogue of the tibia of vertebrate animals. With regard to the articulation of this part with the trochanter, it has been sufficiently explained under that head, and that with the tibia I shall treat of when I come to that joint. As to the size of the thighs, and their relative proportions to each other and to the remaining joints of the leg, the most general law is, that the anterior pair shall be the shortest and smallest, and the posterior the longest and largest. With respect to the remaining articulations, most commonly the thigh is longer and larger than the tibia, and the tibia than the tarsus. But there are numerous exceptions to both these rules. With respect to the first, we may begin by observing that the increase of the magnitude of the thigh, from the anterior to the posterior pair, is usually gradual: but in many jumping insects, and likewise many that do not jump, the posterior pair are suddenly and disproportionally thicker than the rest[2012]. Again, in many insects the anterior pair are the longest and thickest, as in Macropus longimanus, Bibio, Nabis, &c.: in others, the intermediate exceed the rest in magnitude, as in Onitis Aygulus, cupreus; Sicus flavipes, &c.; in many Lamellicorns all the thighs are incrassated and nearly equal in size: but in some, as Ryssonotus nebulosus McL.[2013], the intermediate pair are rather smaller than the rest. With respect to the second rule—in some, as in the male of Macropus longimanus, the anterior tibia, though more slender, is longer than the thigh; in Hololepta maxillosa it is longer and more dilated; in Lamia marmorata, or one related to it from Brazil, the intermediate pair are longer; in Ateuchus gibbus and others of that tribe the posterior thighs are smaller than the tibiæ: and, to mention no more; in Callichroma latipes the posterior tibia is wider than the part last named. Again, the tarsi are as long as either tibia or thigh in many of the larger Dynastidæ, as Megasoma Actæon, &c.; longer than either in Melolontha subspinosa F.; and in Tiphia, Scolia and affinities, often as long, or longer than both together.
As to shape,—the thigh, especially in the fore-leg, varies considerably: most generally it is flat, linear, and a little thicker where it is united to the tibia, on the outer side convex, and concave next the body; but in many it is gradually thicker from the base to the apex: in some Cerambyces (C. thoracicus) it is clavate; in others of this genus and Molorchus they may be called capitate; in Pterostichus they are rather lanceolate; in Onitis Sphinx the humerus is triangular, and the intermediate thigh rhomboidal; in Bruchus Bactris it is bent like a bow; and in some Brazilian Halticæ it is nearly semicircular. The humerus in Phasma is attenuated at the base; in Empusa gongyloides it is at first ovato-lanceolate, and terminates below in a kind of footstalk[2014]; in Phasma flabelliforme it is dolabriform[2015]; in Mantis often semioval or semielliptical, and thickest at the inner edge, which affords space for two rows of spines with which it is planted. In Phyllium siccifolium all the thighs are furnished on both sides with a foliaceous appendage nearly from base to apex[2016]: in a species of Empusa (E. macroptera), the four posterior ones are so distinguished only on their posterior side[2017]: others of this last genus, as E. gongyloides, have an alary appendage on both sides at the apex of these thighs[2018]; and another family, as E. pauperata, have only one on the posterior side[2019]. The thighs of no insect are more remarkable for their elegant shape,—tapering gradually from the base to the apex, where they swell again into a kind of knee,—than the posterior ones of the locusts (Locusta Leach); each side of these thighs is strengthened with three longitudinal nearly parallel ridges, and the upper and under sides are adorned by a double series, in some coalescing as they approach the tibia, of oblique quadrangular elevations resembling scales[2020].
I shall next say a few words upon the spines and other processes which arm the thigh. Those moveable ones of Mantis which help to form a fearful instrument of destruction, have just been mentioned, and similar ones, but less conspicuous, arm the intermediate thighs of Sicus flavipes: other appendages of this kind are for a less destructive purpose—to keep the tibia when folded in its place. This seems to be the use of the serratures and spine that arm the thigh of Bruchus Bactris, or the Hymenopterous genera Leucospis, Chalcis, &c.; in Onitis Aygulus a short filiform horn arms the humerus, and a longer crooked one that of many species of Scaurus[2021]. In many Stenocori the thighs terminate in two spines, and in Gonyleptes K. the posterior ones are armed internally with very strong ones; with which, as the legs converge at their knee[2022], they may probably detain their prey. The knee-pan (Gonytheca) of the thigh, or the cavity at its end, which receives the head of the tibia, is very conspicuous in the weevils; but in no insects more than in Locusta[2023], in which tribe it deserves your particular attention.
d. Tibia or Cubitus[2024]. The tibia or shank is the fourth joint of the leg, which according to the hypothesis lately alluded to is the analogue, in the anterior leg of the carpus or carpal bones, and in the four posterior ones of the tarsus or tarsal bones of vertebrate animals. This may be called the most conspicuous of the articulations of the leg; for though it is generally more slender and often shorter than the thigh, it falls more under the eye of the observer, that joint being more or less concealed by the body: it consists in general of a single joint; but in the Araneidæ and Phalangidæ it has an accessory one, often incrassated at its base, which I have named the Epicnemis[2025].
With respect to the articulation of the tibia with the thigh—we may observe that in general it is by means of three processes or condyles, two lateral and one intermediate, of the head of the former joint[2026]: the lateral ones are usually received by a cavity or sinus of the gonytheca of the thigh[2027]; and upon these the tibia turns, with a semirotatory motion up and down as upon a pair of pivots: at the same time the mola or head of the latter joint, which has often a flexure so as to form an elbow with the rest of it, inosculates in the gonytheca, and is also suspended by ligament to the orifice through which the muscles, nerves, and bronchiæ are transmitted: so that in fact the articulation, strictly speaking, belongs exclusively to none of the kinds observable in vertebrate animals, but partakes of several, and may properly be denominated a mixed articulation,—a term applicable in numerous instances also to the other articulations of the legs of insects. In the different Orders some variations in this respect take place,—I will notice some of the most remarkable. In no Coleopterous insects is the structure more distinctly visible than in the larger Lamellicorns. In Copris bucephalus, for instance, if you divide the thigh longitudinally, you will find on each side, at the head, that it is furnished with a nearly hemispherical protuberance, perforated in the centre for the transmission of muscles, and surrounded externally by a ridge, leaving a semicircular cavity between them[2028]: if you next examine the tibia, after having extracted it, you will find on each side, at the base, a cavity corresponding with the protuberance of the thigh which it receives, having likewise a central orifice, and surrounded by a semicircular ridge corresponding with the cavity in the thigh in which it acts: below this ridge another cavity, forming a small segment of a circle, receives the ridge of the thigh[2029]. You will observe that the ridge of the tibia represents the lateral condyle lately noticed: in the Dynastidæ this is more prominent, and often forms a smaller segment of a circle. In these also the protuberance of the thigh is more minute, and its ridge is received by a cavity of the tibia nearly semicircular[2030]; in Geotrupes Latr. the articulation is not very different, though on a reduced scale; in Calandra Palmarum the lateral condyles of the tibiæ are flatter and broader[2031]; and the articulation not being quite so complex, this joint is kept steady by an intermediate process observable in the gonytheca[2032]. From the above description it appears that the dislocation of the tibia is effectually prevented in the Lamellicorns by the protuberance and ridge of the thigh working in their corresponding cavities, while the condyle of that part turns with a rotatory motion in the cavity of the thigh. In the Orthoptera Order the tibia is suspended by a ligament, in the gonytheca the lateral condyles, which are very prominent, working in a sinus of that part[2033]. The subsequent Orders exhibit no very striking variations from these types of articulation, I shall therefore not detain you longer upon this head.
With regard to the proportions and magnitude of the joint we are considering,—the most general law is, that the anterior pair should be shorter and more slender than the intermediate; and the intermediate than the posterior; and that all the tibiæ should be shorter and more slender than the thighs, and longer and thicker than the tarsi. Various exceptions, however, to this rule in all these cases might be produced; but I shall only observe that in all those insects in which the fore-legs are calculated for digging or seizing their prey, as in the Petalocerous beetles, the Gryllotalpa, Mantis, &c., this joint of the leg is usually much enlarged and more conspicuous than the others.
As to its figure and shape—most commonly the tibia grows thicker from the base to the apex, as in the majority of Coleoptera, Hymenoptera, &c.; in the Orthoptera, Neuroptera, &c., it is generally equally thick every where. Another peculiarity relating to this head observable in it, is its tendency to a trigonal figure: this, however, though very general, is not universal;—thus, in some Orthoptera, as Pterophylla K., its horizontal section is quadrangular; in others, as Locusta Leach and many other insects, it is nearly a circle; in some scorpions it is almost a hexagon. The superficial shape also of this joint in numerous instances is more or less triangular, but it sometimes recedes from this form:—thus, in Callichroma latipes it is a segment of a circle; in some Empides it is clavate; in Onitis Sphinx, dolabriform; in the Orthoptera, Neuroptera, &c., it is usually linear; in some Lygæi it is angular[2034]: but the most remarkable tibiæ in this respect are those of such species of this last genus as have the posterior ones winged or foliaceous, so that they resemble the leaf of some plant—the tibia being the rachis, and the wing (which in some species is veined) representing the leaf itself. This structure is exemplified in Lygæus compressipes, phyllopus, foliaceus, &c.[2035] Under this head I must say a few words upon the flexure of this joint, which in some cases merits notice. I have before mentioned its bend at the knee[2036] or base: the apex also is sometimes incurved—in the anterior one of the male of Macropus longimanus so as almost to form a hook[2037]: in Lygæus Pharaonis the posterior pair are flexuose[2038]; in Bruchus Bactris, Leucospis, and several species of Chalcis, these tibiæ curve so as to adapt themselves to the bend of the thigh when folded. The notch on the inside of the anterior pair, in a large majority of Carabus L., armed above by a spur[2039], a structure which probably assists them in seizing and detaining their prey, may also here be introduced: in the generality it is a little removed from the apex of the joint in question; but in Pamborus it is very near to it, and in Cychrus, Carabus, &c., it becomes obsolete. I may mention here also a singular character which distinguishes the cubit of both sexes of Gryllus campestris, domesticus, &c. At the base there is an aperture which passes through the joint—anteriorly it is oval, and posteriorly elliptical and much larger, and on both sides is closed by a tense membrane.
The most striking peculiarities as to the clothing of his joint have been chiefly noticed under the sexual characters of insects[2040], but some appear not to be of that description. In Sphæridium Leach, while the thighs and tarsi are naked, the posterior tibiæ are remarkably beset with stiff bristles; in Empis pennipes they are thickly fringed on both sides; in Scarabæus McL. only externally, and in Dytiscus serricornis internally; in Necydalis barpipes K. this fringe is longer at the apex; and in Saperda hirtipes Ol. the same tibiæ at that part are adorned with a large brush, like that observable in the antennæ of some Lamiæ[2041].
I must next call your attention to the teeth, spines, and spurs with which the tibiæ of insects are sometimes armed. With regard to teeth, you have doubtless often observed those that distinguish the cubitus of the arm of most Lamellicorn beetles: these vary in number from one, as in Trox suberosus, to seven, as in Geotrupes autumnalis; but the most universal number is three: in some species of Geotrupes, as G. stercorarius, &c., the third tooth from the apex, and those that follow it, may be called double. These teeth, in their cubit or anterior shank, doubtless assist these insects in burrowing. The four posterior tibiæ in this tribe are also distinguished by a kind of teeth which occupy their whole diameter, and resemble so many steps. I have before noticed the remarkable cubit of the Gryllotalpa, and likewise that of Scarites, Pasimachus, &c., in which some of the teeth are prolonged into spines[2042], which are the next description of tibial arms that I mentioned. Spines are of two kinds—those which are merely processes of the crust of the tibia, and those that are implanted in it, and seem to have a gomphosis or perhaps an amphiarthrosis articulation[2043]. An instance of the first kind may be seen in the hind-legs of some grasshoppers[2044] (Locusta Leach), the Rutelidæ, &c. though in others they are implanted:—of the second, in the cubitus of the Mantidæ, and of all the tibiæ of the dragon-flies (Libellulina McL.)[2045];—and of both kinds in the hind-legs of Acrida K., those which arm the upper angles of the tibiæ being processes, and those of the lower being implanted. The term spine I think ought to be restricted to the first kind; the second ought rather to be denominated spurs (calcaria), and may perhaps be regarded as in some degree synonymous with those most important appendages of the joint in question, that are implanted in or near their apex, which have been hitherto distinguished by this last denomination, and which I am next to consider. But though I have not altered a term generally adopted, I must here express my opinion that they ought rather to be considered as minute toes or fingers, and that the denomination best agreeing with their functions, as accessories to the main toe, would be digituli: this is proved particularly by a character peculiar to those of many species of the genus Cimbex amongst the saw-flies, in which these organs are furnished with a sucker or pulvillus (as they are also in Œnas a kind of blister beetle), as well as the joints of the tarsi[2046]; which makes it evident that they are applied by the animal to surfaces, and assist it in walking or climbing; and in general it may be observed that in most insects their principal use is connected with these motions, and with burrowing. This circumstance tends to prove that the generality of insects (for all have not these organs) have really a didactyle or tridactyle hand or foot; and the hypothesis so often alluded to—that the cubitus or tibia, &c., is really analogous to the carpus or tarsus in vertebrate animals[2047]—seems to receive no small confirmation from it; since, if the spurs be really analogous to fingers or toes, the part they articulate with cannot be the tibia, &c. Though the parts in question did not escape the notice of Reaumur, Linné, De Geer, Latreille, &c., yet they have not been employed in the determination of tribes, genera, &c., except by the author last named, but perhaps adopted from Bonelli[2048], in the subgenera Zabrus and Pelorus: in many instances, however, they afford excellent subsidiary characters, sometimes common to a whole Order, and at others distinguishing its various subdivisions. With regard to their number—I have noticed many variations which I will now state to you, first observing that I shall express them by three figures, the first representing the number of spurs on the anterior leg, the second that of those on the intermediate, and the third on the posterior; and where there are spurs, as in the Trichoptera and Lepidoptera, on the middle as well as at the end of the tibia, I shall express it by one figure over another, the upper one representing the number of the middle spurs. If you make an examination yourself, it will be proper to remind you that these little organs are extremely liable to be broken off, but the socket in which they were planted is usually very visible. The most natural number is represented by 2:2:2; this you will find very prevalent in the Coleoptera Order, as in the Predaceous and numerous other beetles: in the Orthoptera and Hemiptera Orders, however, I have not discovered an instance of it; but in all the rest it more or less occurs: next to this number—tibiæ with obsolete or no spurs seem most prevalent, particularly in the Hemiptera; not a single instance of an insect furnished with them occurring to me in the Heteropterous section; and it is doubtful whether there are any in the Homopterous.—Having stated the most universal structure in this respect, I will next consider the Orders seriatim. Amongst the Coleoptera though the numbers 2:2:2 are most frequent in occurrence, yet there are numerous exceptions. Thus, in the Lamellicorns, 1:1:1 represents the calcaria of one tribe of the Scarabæidæ McL. formed of the genus Scarabæus McL.; 1:2:1 represents those of another tribe of that family, including the subgenera Ateuchus, Copris, Phanæus, &c.; 1:2:2 again forms the character in this respect of Aphodius and the great majority of the Lamellicorns; while 2:2:2 is confined in this section to Æsalus F. and Melolontha chrysomeloides Schranck (Psephus McL. MS.). In the other tribes of Coleoptera other numbers occur. Thus, 0:1:1 characterizes Hylœcetus; 0:1:2 Mordella; 0:2:2 Macropus; 1⁄1:2:2 Harpalus, and all those Carabi L., except Zabrus, that have a notch in their anterior tibiæ; ½:2:2 Zabrus. In the Orthoptera Order it is not easy to distinguish the real spurs from the implanted spines that frequently arm the legs: these in Blatta are extremely numerous, even at the apex of the tibiæ; but I cannot distinguish any that can be regarded as true analogues of the former: the most natural number of spurs in this Order is represented by 0:0:4; this you will see in all the Locusts; in Acrida, Conocephala, Pterophylla; and in Truxalis, Pneumora, &c.; in Phasma there are none. In Mantis, if the terminal process of the cubitus is excluded, it will be 0:2:2; in Gryllotalpa, admitting the terminal teeth of that part[2049] as analogues of spurs, the number is 4:4:4; in Tridactylus Latr. 0:0:5[2050]; in Gryllus Latr. 3:3:5; in Gryllus monstrosus, 4:4:6. In the whole Hemiptera Order I have discovered no instance of an insect furnished with the real spurs: for though in Tettigonia F., Cercopis, &c., there are implanted spines in the posterior tibia, and several at the apex, there are none of them clearly analogous to real spurs. In the Lepidoptera the most general arrangement appears to be 1⁄0:2:2/2; and next to this, 1⁄0:2:2. In this Order most commonly there is no spur at the end of the cubit, but one resembling a thumb[2051] arms its middle; in Pieris, &c., this thumb is not present, so that the number is 0:2:2; in Agarista Leach, Erebus, &c., you will find 1⁄0:2:4, the posterior calcaria being all terminal; and in Attacus Atlas, all these organs are obsolete except the thumb. In the Neuroptera the most general arrangement is 2:2:2; but in the Libellulina, although the legs are very spinose, there are no spurs. In the Trichoptera K., in Phryganea rhombica and affinities, the number of them is expressed by ½:½:½; and in those with long antennæ, P. atra, &c., by 2⁄2:2⁄2:2⁄2. In the Hymenoptera the number 1:2:2 is most prevalent; and next to this, as in Apis L., 1:1:2. In the Ichneumones minuti L. the spurs are 1:1:1; in Atta Latreille, a kind of ant[2052], 1:0:0. In the Diptera it is often difficult to distinguish the spurs from the spines; but the number most universal is, I think, 2:2:2; in Tipula it is 1:2:2; in the Tabanidæ 0:2:0; and in Culex, Limonia, &c., there are none. Amongst the insects with more than six legs, most commonly the tibiæ have no spurs; but in the Araneidæ each is armed with two, a circumstance which also distinguishes the corresponding joint of the pedipalpi.
These little organs inosculate each in an appropriate socket of the end, or in many cases of the middle of the tibia; and that part of their head or base that is received by it, is often constricted for the purpose: from hence it follows that they are capable of some degree of motion, but in some insects, as those on the four posterior legs of Scarabæus sacer and its more immediate affinities, and those at the end of the cubit of Gryllotalpa, they are immoveable, and appear almost processes of the joint to which they belong. They are commonly sharp, of a subtriquetrous figure, with the lower side flat: where there are two, the outer one is usually the longest; and in general the spurs on the hind legs are longer than those on the four anterior: but there are exceptions—thus, in Acanthopus Latr. the intermediate spurs are the longest; and in Cicindela the anterior are longer than the former; in Blaps mortisaga those on all the legs are nearly equal in length. They vary sometimes in shape—those on the middle of the cubit of many Lepidoptera, which may be regarded as a kind of thumb[2053], are of a lanceolate shape; in Meloe the external posterior one is flat and obtuse; in Œnas Latr. it is obconical, concave at the extremity, and apparently furnished with a sucker; in Ateuchus smaragdulus the anterior, and in Copris Carolina the posterior is forked and emarginate; in Sirex the former is hooked and winged; in Lamprima it is triangular and dilated; in Aphodius analis it is dolabriform; in Dynastes retusus and Juvencus the spurs are bent like a bow. In many Hymenoptera, as the Sphecidæ, they are pectinated[2054], with a series of minute parallel spines—a structure which assists the animal in burrowing[2055]; in Acanthopus Latr. they are armed with little teeth or spines[2056]; in the hive bee the spur of the cubit is furnished with a membranous appendage which I have called the velum[2057]; and in a subgenus related to Saropoda Latr. (Ctenoplectra K. MS.), the interior spur of the posterior leg is crescent-shaped, fixed transversely, and fitted on the inner side with a membrane, the edge of which is finally pectinated.
e. Tarsus or Manus[2058]. This is the last portion of the leg, usually supposed to be analogous to the hand or foot of vertebrate animals; but, according to the hypothesis so often alluded to, rather the representative of their jointed finger or toe. In treating of this part I shall consider its articulation with the tibia, and of its joints inter se; the number of those joints; their proportion and shape; their parts and appendages.
I seem to have observed three kinds of tarsal articulation. The first is a species of enarthrosis or ball and socket, the joints terminating in a globular head, perforated indeed for the transmission of muscles, &c., and which is received by a corresponding cavity of the tibia or preceding joint, as may be seen in many weevils (Curculio L.[2059]). This admits of some rotatory motion.—The second is a mixed articulation between enarthrosis and ginglymus, when at the base of the ball a deep transverse channel receives a corresponding ridge of the tibiæ or preceding joint: this may be found in Rutela and probably many other Lamellicorn beetles; and something very similar in the Predaceous ones.—The third kind is where there is little or no inosculation, and the joints are scarcely more than suspended: this takes place in the Orthoptera, Neuroptera, &c.; but in Blatta and the hind legs of Mantis there is some approach to the foregoing kinds.
We are now to consider the number of joints of the tarsus, which varies considerably in the different Orders, and in one has been assumed as a clue for a subdivision of it into sections[2060], which, though not perfectly natural, is very convenient, and has been adopted by most modern Entomologists. In treating of this head, I shall use those denominations that have been employed by M. Latreille and others to express the variations of the number of the tarsal joints in the Coleoptera, but shall apply them to insects in general. Insects in this view, therefore, may be called pentamerous; heteromerous; tetramerous; trimerous; dimerous; or monomerous.
Pentamerous insects are those which have five joints in all their tarsi. This is the most universal, and may be called the natural number of these joints. More than half the Coleoptera belong to this section; in the Orthoptera—the Blattidæ, Mantidæ, and Phasmidæ; all the Lepidoptera except those butterflies called tetrapi (Vanessa, &c.); all the Trichoptera, Hymenoptera, and Diptera; in the Neuroptera—Ascalaphus, Myrmeleon, Hemerobius, Corydalis, &c.; and in the Aptera—Pulex[2061].
Heteromerous insects are those in which the number of these joints varies in the different pairs of legs[2062]. These variations, like the spurs, may be expressed by three figures, the first representing the anterior tarsus, the second the intermediate, and the third the posterior. I begin with 5:5:4. This number represents those beetles that have been exclusively regarded as heteromerous by modern Entomologists—of this description is the Linnean Tenebrio, Meloe, &c., now subdivided into numerous genera; they have five joints in the two anterior pair, and four in the posterior. The tarsal joints of the aquatic genus Hydroporus (a singular anomaly in the Order to which they belong) are expressed by 4:4:5, thus reversing the number in the preceding tribe: other Heteromerous genera are to be found amongst the Hemiptera. Thus, in Ranatra the numbers are 2:1:1; in Sigara and Nauceris 1:2:2; in a new subgenus between Belostoma and Naucoris (Xiphostoma K. MS.), brought by Dr. Bigsby from Canada, 3:2:2; in the Lepidoptera the butterflies called tetrapi (Vanessa, &c.) may be expressed by 1:5:5. Amongst the Aptera and Arachnida there are three remarkable genera, which if their pedipalps are included may be deemed Heteromerous. I mean Phrynus, Thelyphena, and Galeodes;—in the former the numbers will be *:4:4:4, the asterisk denoting more than ten; in the second, 8:4:4:4.; and in Galeodes (in which the first pair of pedipalps are not chelate, the mandibles performing their office) the numbers are 1:1:3:3:3.[2063]
Tetramerous insects are those in which all the tarsi consist of four joints; these in the Coleoptera are next in number to the pentamerous—indeed a very large proportion of them strictly speaking are really of the latter description, since in Linné's four great genera, Curculio, Cerambyx, Chrysomela, and Cassida and some others, the claw-joint (ungula) consists of two articulations, one very short, forming merely the ball at its base[2064], which inosculates in the socket of the preceding joint, and the other constituting the remainder: if you carefully separate these two pieces, you will find that the last inosculates in the summit of the ball, and is moved by appropriate muscles[2065]. This structure probably permits the readier elevation and depression of this joint. In the Orthoptera the tetramerous genera are those which Linné called Tettigonia amongst his Grylli (Locusta F.); Acheta monstrosa also, and in the Neuroptera, Raphidia belong to this section.
Trimerous insects are those whose tarsi consist of only three joints. Amongst beetles the Lady-birds (Coccinella
L.) are remarkable for this structure, but in them
the claw-joint is also biarticulate, so that strictly speaking they are tetramerous; in the Orthopterous Order the migratory locusts (Locusta Leach) belong to this section, as likewise Gryllus Latr. and Gryllotalpa Latr.: in the first of these genera is an appearance of there being more joints in the tarsus, because there is more than one cushion below the first[2066]. To this section also belong the great majority of the Hemiptera, excluding only those tribes that connect the two sections of the Order constituting the two Linnean genera Nepa and Notonecta; the Libellulina likewise belong here, as do also the Scorpionidæ and Scolopendridæ.
Dimerous insects are those that have two joints in all their tarsi. Such are the Pselaphidæ in the Coleoptera Order[2067]; in the Hemiptera—Belostoma and Notonecta; in the hexapod Aptera—Pediculus; in the octopod—the Acari of Linné; in the myriapod—Iulus; and in the Arachnida—the Araneidæ.
Monomerous insects are those which have only a single tarsal joint. Only one Coleopterous and also one Hemipterous genus is so distinguished: the first is Dermestes Armadillus De Geer[2068], and the second the common water-scorpion, Nepa Latr. Among the Aptera we find Nirmus, Podura, Sminthurus, &c., that belong to this section.
To the above sections another may be added for those insects whose tarsi have more than five joints, which may be denominated Polymerous. Here belong the genera Gonyleptes K., Phalangium and Scutigera Latr. In the first the number of joints varies from six to eleven, and in the two last they far exceed that number, amounting in some species of Phalangium to more than fifty, and becoming convolute like the antennæ of Ichneumons[2069].
I am next to notice the proportions and shape of the tarsus and its joints. The most general law is, that it shall be shorter and more slender than the tibia; but it admits of several exceptions—thus, in Megasoma K.[2070], in all the legs; in Agrostiphila McL. MS.[2071] in the intermediate, and in Amphicoma lineata in the posterior pair the tarsi are the longest; in Trichius Delta these last are longer than the thigh and tibia together. In some insects the tarsi are disproportionally short, as in Cassida, the Pselaphidæ, Locusta Leach, &c. Though generally more slender than the tibia, in several instances they are as thick or thicker, or more dilated, as in most of the tetramerous beetles, which being climbers require a dilated tarsus. Again, comparing the three pairs of this joint with each other, the most general rule is, that the anterior should be the shortest, and the posterior the longest: but in some, as the Capricorn beetles, &c., they are nearly equal in length; in others, as Lytta marginata, the anterior pair, and in Rhipiphorus the intermediate, are the longest; in Trichius Delta these last are the shortest. With respect to thickness, the anterior tarsi, except in many males[2072], are not very strikingly different from the rest.
With regard to the proportion of the joints of the tarsus to each other,—according to the most general law, the first is the longest, the last next in length, then the second and third, and the fourth is the shortest. In Gonyleptes K. and other Phalangidæ the first is almost thrice the length of all the rest taken together; but there are numerous exceptions to the rule. In the female Carabi the first joint is not longer than the last, and in the males not so long; and in Hydrophilus, &c., it is the shortest of all. Again, the second joint is longer than the three following ones in Dasytes ater[2073]; and than the last in Cicindela sylvatica: the third joint is shorter than the fourth in Lampyris ignita: it is longer than the first in Donacia, many Melolonthidæ, &c. Once more, the fourth joint, usually the shortest of all, is longer than the second and third in Anthia, &c. Lastly, the claw-joint, usually the second in length, in the Eproboscidea Latr. (Hippobosca L.) is very long and large, while the four first joints are so extremely short as to be scarcely distinguishable from each other: it is the shortest of all in Colymbetes, &c.; it is of the length of the third in Cicindela sylvatica, of the fourth in C. sexguttata. Though commonly the slenderest joint of all, particularly so in Raphidia, in many Heteromerous and Lamellicorn beetles it is the largest, conspicuously so in Mellinus tricinctus. Sometimes, as in Buprestis chrysis, &c., all the tarsal joints are nearly equal in length and thickness.
We are next to say something upon the shape of the tarsi and their joints. In general we may first observe that their upper surface is commonly more or less convex, and the lower flat or concave: in insects that are swift runners, as the terrestrial Predaceous beetles, they are usually slender and filiform[2074]; in those that swim, as Dytiscus, the two posterior pair taper nearly to a point from the base to the apex[2075]; in some that climb, as Buprestis, they are rather flat and linear; and in others (the Weevils, Curculio L.) they grow gradually wider towards the claw-joint[2076]; sometimes, as in Mordella Latr., the four anterior tarsi are of this shape, and the posterior pair setaceous. In Gyrinus the four posterior are flat and triangular; and in that extraordinary insect Gryllus monstrosus the tarsi are foliaceous and lobed[2077]. In many males and some others the anterior pair or hands are of a different shape from the two posterior: thus, in several Carabi they are lanceolate; in Staphylinus, Creophilus, &c. in both sexes they are often nearly circular, like those of male Dytisci[2078]. With regard to the shape of individual joints it may be said in general that they are rather triangular, with an anterior sinus for the reception of the succeeding joint: the first joint usually departs most from this form; in the bees it is commonly much larger than the rest, especially in the last pair of legs, and nearly forming a parallelogram[2079]; in Euglossa it is trapezoidal; in the majority nearly linear or filiform. With regard to their termination—in Brachycerus and some ants (Ponera, Myrmica, &c., Latr.) the three first joints; in Dascillus, Lycus reticulatus and affinities, the third and fourth; and in the great majority of the Tetramerous insects the penultimate joint is bilobed; although in most Predaceous beetles this joint is entire or simply emarginate, yet in Colliuris it terminates in a single oblique lobe; and in Lebia, Drypta, &c., it is nearly bipartite. I must now advert to the Ungula or claw-joint: it is usually clavate or thickest at the end and curved; but in the Asilidæ it is shaped like a vase or cup; in Phanæus, in the four posterior tarsi, in which the claws are obsolete, it is thickest at the base and sharpest at the extremity[2080]; it usually forms an angle with the rest of the tarsus, rising upwards, which enables the insect to move more easily without hindrance from the claws, and also more readily to lay hold of any object it meets with; but in the Lamellicorn beetles and many other insects it is in the same line with it. As in the beetles last mentioned this joint is often inserted in the extremity of the preceding one; but in Œdemera it articulates with the middle of its upper surface; and in Lycus and a numerous host of Tetramerous beetles it springs from its base, just behind where it diverges into two lobes.
I shall next call your attention to the different kinds of appendages with which the tarsi are furnished. They are seldom armed, like the tibiæ, with teeth, or spines, or horns; but something of the kind occasionally distinguishes them. In Phileurus, Oryctes, and several other Dynastidæ, the first joint is armed at the apex externally with a considerable mucro; in the fore-leg of Dasytes ater a similar process is prolonged into a crooked horn[2081]. But the most important appendages of the tarsi are the claws which almost universally arm their extremity, and which appear clearly analogous to those of birds, quadrupeds, &c., though probably differing as to their substance[2082]. Some few, however, are without them; this, as I lately observed, is the case with Phanæus with respect to the four posterior legs; the anterior ones of Vanessa amongst the Lepidoptera, and all those of Stylops and many Acari L., are also without them: this is likewise the case with the first pair of legs, or the second of the pedipalps of Galeodes. In this genus these organs consist of two joints[2083]. With respect to number they vary in different tribes, but not so much as the calcaria: these variations may likewise be represented by three numbers. The most natural is two in all the tarsi, exhibited by the Predaceous beetles and the great majority; 2.2.1. are to be found in Hoplia, Anisonyx, &c.[2084]; 1.2.2. in Belostoma; three in all the legs in the Araneidæ[2085]; in Meloe[2086], Elater, &c., each claw is double or consisting of two, which makes four in each leg; and in many Hippoboscidæ there are six[2087]; in Nepa and the Myriapods there is only one. In most insects, perhaps, the claws are simple or undivided[2088]; but in Galeruca, Melolontha subspinosa[2089], &c., they are bifid at the apex; as is the exterior claw of the four posterior legs in Chasmodia and Macraspis[2090] McL., and of all in Melolontha horticola; in Serica brunnea McL. the claws are all cleft at the extremity, but the internal tooth is broad, flat, and obtuse[2091]; in Melolontha vulgaris and Pelidnota punctata McL.[2092], the claws are armed with an internal tooth near the base[2093]. In the Araneidæ, which have three claws, the two external ones are furnished with several parallel teeth, which the animal uses to keep separate the threads of its web, and probably for other purposes[2094]; and some Predaceous beetles, as Lebia and Cymindis, have both their claws similarly furnished[2095]. These organs vary in their relative proportions: thus, in Anoplognathus the inner claw is much smaller than the other[2096]; and in Elater sulcatus, fuscipes, &c., it is represented by a mere bristle; in Hoplia, in the anterior tarsus it is not half the length of the outer one[2097]; in Areoda and Pelidnota McL. this last is the smallest. They vary also in length—in Rynchænus, Ascalaphus, &c., they are very short; in the Lamellicorns, Galeodes, &c., very long; and in Myrmeleon longer than the claw-joint. With regard to their curvature they generally form the segment of a circle; in many Asilidæ they are crooked like the claws of the eagle[2098], and the posterior one of the Hopliæ is bent like a hook[2099]; they most commonly diverge from each other; but in the Rutelidæ, Anoplognathidæ, &c., they are perfectly parallel, and in the former often inflexed[2100]. With regard to other appendages of the part we are treating of, if you examine the stag-beetle and many other Lamellicorns, you will find between the claws a minute but conspicuous joint terminated by two bristles which seem to mimic the ungula and its claws; these parts are what are denominated in the table the palmula, plantula, and pseudonychia: in the stag-beetle these are long[2101]; in the Melolonthidæ short[2102]; and in many Cetoniadæ they resemble an intermediate claw.
The most remarkable of the appendages of the tarsi are to be looked for on their under side or sole (solea), and are the means by which numbers of insects can overcome atmospheric pressure and walk against gravity. Many of these have been fully described in a former letter[2103]; but much that relates to them was there omitted, which I shall now detail to you. Four kinds of pulvilli, as I would call these appendages, are found in the sole of insects, upon each of which I shall make a few remarks.
The first is a cushion or brush composed of very thickly set hairs or short bristles: examples of this you will find in the majority of Tetramerous and Trimerous beetles. In Chrysomela, Timarcha, &c., there is one of these cushions on each of the three first joints; in Prionus, Liparus, &c., there is a pair; and in Coccinella on the two first; in others (Balaninus Nucum, &c.) a pair only on the penultimate joint; in Calandra Palmarum, Rhina barbirostris, &c., that joint has an intire cushion; in Eurynotus muricatus K.[2104] the three first joints of the four anterior tarsi are similarly circumstanced, but the cushions resemble sponge[2105].
The second kind of cushion is a vesicular membrane capable of being inflated. This distinguishes the tarsi of Thrips[2106], and many Acari L.[2107]; likewise those of Xenos[2108]; and also of many Orthoptera fully described on a former occasion[2109], though the fact of their capacity of inflation has not been ascertained, belong to this section.
The third kind of covering of the sole is when the three or four first joints of the tarsus each terminate in one or two membranous lobes or appendages: of the first description is Priocera K., in which the lobes are involute[2110]; and of the second Rhipicera Latr.[2111], in which there is a pair on each joint, in the Brazil species set with very fine hairs.
The fourth and last kind are what may with the utmost propriety be denominated suckers, since their use as such is clearly ascertained. These are not only to be found in a large proportion of the Diptera, in some of which there are two of them, as in the Asilidæ[2112]; and in others three, as the Tabanidæ[2113]; but also in many of the subsequent Orders: thus, in the Heteropterous Hemiptera, in Scutellera and Pentatoma, but not the Reduviadæ, and in the Neuropterous genus Nymphes Leach there is a minute one under each claw. It is discoverable between the claws in many Hymenoptera, as Apis[2114], Vespa, &c. But the genus that exhibits to the curious Entomologist the most singular and elaborate apparatus of this kind is Dytiscus Latr.; and the examination of the under side of the hand of any male of this genus will almost compel the most inattentive observer to glorify the wisdom and skill of the Allfather so conspicuously manifested in the structure of these complex organs. For this part in these, instead of two or three pedunculate cups as in the insects just mentioned, is composed of a vast number, some large and some small. If you take a male specimen of the common D. marginalis, you will find that the three first joints of the hand are very much dilated, so as to form a plate or shield nearly circular, fringed all round with stiffish hairs; if you next examine the under side of this plate with a good magnifier, you will discover at the base, where it is united to the cubit, two circular cups, the external one more than three times the size of the other, with an umbilicated centre[2115]; besides these two larger cups the rest of the shield is covered by a vast number of minute ones of a similar construction[2116]: the larger cups are nearly sessile, but the smaller are elevated upon a tubular footstalk[2117]; the three first joints of the intermediate tarsi are also dilated, but not into an orbicular shield, and thickly set with minute pedunculated suckers[2118]. The structure varies however in different species. Thus in D. limbatus the shield is triangular with the smaller suckers at the base, and two rows of larger oblong ones, concave but not umbilicated, at the apex; in another Brazilian undescribed species (D. obovatus K. MS.) the shield is oblong and quite covered with suckers like those last mentioned; in D. sulcatus (Acilius Leach) almost the whole plate is occupied by a very large sucker, above which, at some distance in the inner side, are two smaller ones, while the extremity of the shield is covered by minute ones elevated on long footstalks: the central umbilicated elevation of the large one, which nearly fills its cavity, is in this species beautifully radiated. The male of Colymbetes transversalis has also an orbicular shield, but the suckers are much less strongly marked. The use of this organ has been before sufficiently explained[2119].
A few words will be necessary upon the folding of the legs in repose. When insects walk, the thigh is usually in an ascending position, rising above the horizontal line, the tibia forming with it rather an obtuse angle, and the tarsus nearly a right one with the tibia; but in the Myriapods, as far as I can unravel their swift many-footed motions, these angles in walking do not take place; in repose however, in many insects, the coxa forms an angle with the thigh below the horizontal line and with the tibia above it, and the tibia and tarsus continue in the same line, and point downwards nearly vertically; in others, as in the Tetramerous beetles, the last-mentioned joints form an angle with each other and turn upwards, the tibia having an external oblique cavity to permit this; but the insects most remarkable for packing close their legs are those carnivorous genera Dermestes, Anthrenus, Byrrhus, &c. In the last-mentioned genus there are cavities in the under side of the trunk, in each division of the breast, and at the base of the abdomen, to receive the legs when folded; the coxæ have also a cavity to receive the base of the thigh. In the anterior legs this last part has a longitudinal one on its upper side, and in the four posterior on the under, which receives the tibiæ, which at the inner edge are straight, and at the outer curvilinear, and the tarsi are turned up and received by the concave part, on the anterior side of the first pair and the posterior side of the two last of the tibiæ, so as to lie between it and the body: when the legs are close packed, the animal looks almost as if it had none. I have observed that when Dytisci repose on the water, the posterior legs are turned up and laid over the elytra, and curved towards the head.
vi. Pectines. I must next say a few words upon a remarkable organ, which seems in some degree supplementary to the legs, by which the Creator has distinguished the genus Scorpio, called from its parallel teeth, set in a back, their pecten or comb[2120]. This back consists of two or more articulations, is attached by its anterior extremity to the sides of the posterior piece of the mesostethium, and is marked by a longitudinal furrow or channel. The teeth, which vary in number in the different species, and in the same species at different periods of its growth, are usually ovato-lanceolate, or obtusangular, furnished on their exterior edge with what appears to be a longitudinal sucker, and supported between their bases, or at the base, both within and without, by triangular, conical, or subglobose props. With regard to the use of these organs, it has not been clearly ascertained. Amouroux states that he has seen the animals use them as feet, and he conjectures that by them they may fix themselves and turn upon them as on a pivot, when they have to make a retrograde movement[2121]. M. Latreille, from their having branchial pouches immediately under them, seems to think that they are connected with respiration[2122]. This may be true; but from the suckers just described, I am inclined to think with Amouroux, that they are useful to the animal in its motions, and that like the suckers of the Gecko, flies, &c., they enable it to support itself against gravity and to climb perpendicular surfaces.
Whether the five obtriangular plates, elevated on a pedicle, which are found arranged in a series on the under side of each of the jointed coxæ of the posterior legs in Galeodes, are at all analogous to the pectens of scorpions, has not been ascertained[2123]. M. Leon Dufour watched them very attentively in one species (G. intrepidus), but he could observe no motion in them[2124].