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.