Though size forms a pretty accurate distinction between insects and the great bulk of vertebrate animals, it affords less assistance in separating them from the invertebrate classes, which are of every size, from the monstrous bulk of some Cephalopoda (cuttle-fish) and Mollusca (shell-fish, &c.) to the invisible infusory animalcule: but external characters, abundantly sufficient for this purpose, may be drawn from the general covering, substance, form, parts, and organs of the body. As I shall enter into pretty full details upon this subject when I come to treat of the external anatomy of insects, I shall here, therefore, only give such a slight and general sketch of the distinctions just mentioned, as will answer the end I have in view. I must here repeat what I have before observed, and what it is necessary that you should always bear in mind, namely, that at the limits of classes and of every other natural group, the characters begin to change, those peculiar to the one group beginning gradually to disappear, and those of the other to show themselves; so that it is impossible almost to draw up a set of characters so precise as exactly in every respect to suit all the members of any natural group.
Whichever way we turn our eyes on the objects of creation, above—below—athwart, analogies meet us in every direction, and it appears clear, that the Book of Nature is a Book of Symbols, in which one thing represents another in endless alternation. And not only does one animal, &c. symbolize another, but even between the parts and organs of one set of animals there is often an analogy as to their situation and use, when there is little or no affinity as to their structure—or again, the analogy is in their situation, without affinity in either structure or use. Thus certain parts in one tribe represent other certain parts of another tribe, though as to their structure there is often a striking disagreement. This is particularly observable between the vertebrate and invertebrate animals. I shall therefore, in my remarks on the general and particular structure of insects, contrast it in its most important points with that of the first-mentioned tribe.
The first thing that strikes us when we look at an insect is its outside covering, or the case that incloses its muscles and internal organs. If we examine it attentively, we find that it is not like the skin of quadrupeds and other Vertebrata, covering the whole external surface of the body; but that in the large majority it consists of several pieces or joints, in this respect resembling the skeleton of the animals just named; and that even in those in which the body appears to have no such segments, as in many of the Mites (Acarus L.), they are to be found in the limbs. This last circumstance, to have externally jointed legs, is the peculiar and most general distinction by which the Insecta of Linné, including the Crustacea, may always be known from the other invertebrate animals[52].
If we proceed further to examine the substance of this crust or covering, though varying in hardness, we shall find it in most cases, if we exclude from our consideration the shells of the Mollusca, &c., better calculated to resist pressure than that of the majority of animals that have no spine. In all the invertebrate tribes, indeed, the muscles, there being no internal skeleton, are attached to this skin or its processes, which of course is firmer than the internal substance; but in insects it is very often rigid and horny, and partially difficult to perforate, seldom exhibiting that softness and flexibility which is found in the cuticle of birds and most quadrupeds. From this conformation it has been sometimes said, that insects carry their bones on the outside of their body, or have an external skeleton. This idea, though not correct in all respects, is strictly so in this—that it affords a general point of support to the muscles, and the whole structure is erected upon it, or rather I should say within it. The difference here between Insects and the Vertebrata seems very wide; but some of the latter make an approach towards it. I allude to the Chelonian Reptiles (Testudo L.), in which the vertebral column becomes external or merges in the upper shell. The cyclostomous fishes also are not very wide of insects as to their integument. But on this subject I shall be more full hereafter.
The forms of insects are so infinitely diversified that they almost distance our powers of conception: in this respect they seem to exceed the fishes and other inhabitants of the ocean, so that endless diversity may be regarded as one of their distinctions. But on all their variations of form the Creator has set his seal of symmetry; so that, if we meet with an animal in the lower orders in which the parts are not symmetrical, we may conclude in general that it is no insect.
But it is by their parts and organs that insects may be most readily distinguished. In the vertebrate animals, the body is usually considered as divided into head, trunk, and limbs, the abdomen forming no part of the skeleton; but in the insect tribes, besides the organs of sense and motion, the body consists of three principal parts—Head, Trunk, and Abdomen—the first, as was before observed, bearing the principal organs of sense and manducation; the second most commonly those of motion; and the third those of generation—the organs of respiration being usually common to both trunk and abdomen. These three primary parts,—though in some insects the head is not separated from the trunk by any suture, as for instance in the Arachnida; and in others, head, trunk, and abdomen form only one piece, as in some mites,—still exist in all, and in the great majority they are separated by incisures more or less deeply marked: this is particularly visible in the Hymenoptera and Diptera, which, in this respect, are formed upon a common model; and in the rest, with the above exceptions, it may be distinctly traced.
The head of insects is clearly analogous to that of vertebrate animals, except in one respect, that they do not breathe by it. It is the seat probably of the same senses as seeing, hearing, smelling, tasting—and more peculiarly perhaps of that of touch. The eyes of insects, though allowed on all hands to be organs of sight, are differently circumstanced in many particulars from those of the animals last mentioned; they are fixed, have neither iris nor pupil, are often compound, and are without eyelids to cover them during sleep or repose; there are usually two compound ones composed of hexagonal facets, but in some instances there are four; and from one to three simple in particular orders. The antennæ of insects in some respects correspond with the ears of the animals we are comparing with them; but whether they convey the vibrations of sound has not been ascertained: that they receive pulses of some kind from the atmosphere I shall prove to you hereafter—so that if insects do not hear with them in one sense, they may, by communicating information, and by aëroscopy, to use Lehman's term, not directly in his sense[53], supply the place of ears, which would render them properly analogous to those organs. That in numbers these remarkable organs are tactors is generally agreed, but this is not their universal use. That insects smell has been often proved; but the organ of this sense has not been ascertained. What has improperly been called the clypeus, or the part terminating the face above the upper lip (labrum), is in the situation of the nose of the Vertebrata, and therefore so far analogous to it, and in some cases even in form: I therefore call it the nose. Whether this part represents the nose by being furnished with what answer the purpose of nostrils, residing somewhere at or above the suture that joins it to the upper lip, I cannot positively affirm; but from the observations of M. P. Huber, with regard to the hive-bee, it appears that at least these insects have the organ of the sense in question somewhere in the vicinity of the mouth, and above the tongue[54]: analogy, therefore, would lead us to look for its site somewhere between the apex of the nose and the upper lip; and in some other cases, which I shall hereafter advert to, there is further reason for thinking that it actually resides at the apex of the nose. The organ of taste in insects, though some have advanced their palpi to that honour, is doubtless in some part within the mouth analogous in a degree to the tongue and palate of the higher animals. The organs of manducation, in what may be deemed the most perfect description of mouth, consist of an upper lip closing the mouth above, a pair of mandibles moving horizontally that close its upper sides, and a lower lip with a pair of maxillæ attached to it, which close the mouth below and on the under sides, both labium and maxillæ being furnished with jointed moveable organs peculiar to annulose pedate animals, called palpi. In some tribes these organs assume a different form, that they may serve for suction; but though in many cases some receive an increment at the expense of others, and a variation in form takes place, none, as M. Savigny has elaborately proved, are totally obliterated or without some representative[55]. The organs now described, except the upper lip, are formed after a quite different type from those of Vertebrata, with which they agree only in their oral situation and use.
The second portion of the body is the Trunk, which is interposed between the head and abdomen, and in most insects consists of three principal segments, subdivided into several pieces, which I shall afterwards explain to you. I shall only observe, that some slight analogy may perhaps be traced between these pieces and the vertebræ and ribs of vertebrate animals, particularly the Chelonian reptiles. This is most observable in Gryllus L. and Libellula L., in which the lateral pieces of the trunk are parallel to each other[56]. In the Diptera and many of the Aptera most of these pieces are not separated by sutures. Each of the segments into which the trunk is resolvable bears a pair of jointed legs, the first pair pointing to the head, and the two last to the anus. These legs in their composition bear a considerable analogy to those of quadrupeds, &c., consisting of hip, thigh, leg, and foot; but the last of these, the foot or Tarsus, is almost universally monodactyle, unless we regard the Calcaria that arm the end of the tibia, as representing fingers or toes, an idea which their use seems to justify. Acheta monstrosa and Tridactylus paradoxus, however[57], exhibit some appearance of a phalanx of these organs. They differ from them first in number, the thoracic legs being invariably six in all insects, with the exception of the Octopods or most of the Trachean Arachnida, which have usually eight. In the Myriapods, though there are hundreds of abdominal legs, only six are affixed to the trunk. Next they differ with regard to the situation of their legs; for though the anterior pair or arms are analogous in that respect, the posterior pair are not, since in quadrupeds these legs are placed behind the abdomen, but in insects before it—in fact, in the former the legs may be considered as placed at each end of the body, excluding only the head and tail, but in the latter in the middle. Though they correspond with those of quadrupeds in being in pairs or opposite to each other, yet their direction with respect to the body is different, the legs of quadrupeds, &c. being nearly straight, whereas in insects they are bent or form an angle, often very obtuse at the principal articulations, which occasions them to extend far beyond the body, and when long to inclose a proportionally greater space. The wings are the organs of motion with which the upper side of the trunk is furnished; and these, though they are the instruments of flight, are in no other respect analogous to those of birds, which replace the anterior legs of quadrupeds, but approach nearer, both in substance and situation, to the fins of some fishes, and perhaps in some respects even to the leaves of plants. M. Latreille is of opinion, That the four wings or their representatives replace the four thoracic legs of the decapod Crustacea[58]. Upon this opinion, which shows great depth of research and practical acumen, I shall have occasion to express my sentiments when I come to treat more at large on the anatomy of the trunk and its members; at any rate they do not replace the two anterior pair of legs of the hexapod Aptera. When merely used as wings, they commonly consist of a fine transparent double membrane, strengthened by various longitudinal and transverse nervures, or bones as some regard them, accompanied by air-vessels, of which more hereafter, as well as of their kind and characters. I shall only observe, that insects are known from all other winged animals, by having four wings, or what represent them, and this even generally in those that are supposed to have only a pair. Another peculiarity distinguishes the trunk of insects that you will in vain look for in the vertebrate animals—these are one or two pair of lateral spiracles or breathing pores. Though the respiratory sacs, &c. of birds are almost as widely dispersed as the tracheæ and bronchiæ of insects[59], yet their respiration is perfectly pulmonary, and nothing like these pores is to be discovered in them.
The principal peculiarity of the third part of the body, the abdomen, is its situation behind the posterior pair of thoracic legs, and its rank as forming a distinct portion of what represents the skeleton. In most insects it is so closely affixed to the posterior part of the trunk as to appear like a continuation of it, but in the majority of the Hymenoptera and Diptera, and in the Araneidan Arachnida, or spiders, it is separated by a deep incisure; and in the first-mentioned tribe is mostly suspended to the trunk by a footstalk, sometimes of wonderful length and tenuity. In the Mammalia the male genital organs are partly external; but in insects as well as in many of the vertebrate animals, except when employed, they are retracted within the body. This part is the principal seat of the respiratory pores or spiracles, many having eight in each side, while others have only one.