An Introduction to Entomology: Vol. 3 / or Elements of the Natural History of the Insects

by William Kirby; William Spence

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.