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.