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.
vii. We now come to a circumstance which will detain us longer, namely, its _articulation with the trunk_, or rather with its anterior segment, the _prothorax_.--M. Cuvier makes _two_ principal kinds of articulation of the head upon the prothorax, in one of which the points of contact are solid, and the movement subordinate to the configuration of the parts; in the other, the articulation is ligamentous, the head and the thorax being united and kept together by membranes.