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.