A brief account of the structure of the integument will be found in the chapter dealing with the post-embryonic development.
The three great regions of the Insect body are functionally as well as anatomically distinct. The head bears the most important of the sense organs, viz. the antennae and ocular organs; it includes the greater of the nerve-centres, and carries the mouth as well as the appendages, the trophi, connected therewith. The thorax is chiefly devoted to the organs of locomotion, bearing externally the wings and legs, and including considerable masses of muscles, as well as the nerve centres by which they are innervated; through the thorax there pass, however, in the longitudinal direction, those structures by which the unity of the organisation is completed, viz. the alimentary canal, the dorsal vessel or "heart" for distributing the nutritive fluid, and also the nerve cords. The abdomen includes the greater part of the organs for carrying on the life of the individual and of the species; it also frequently bears externally, at or near its termination, appendages that are doubtless usually organs of sense of a tactile nature.
In the lower forms of Insect life there is little or no actual internal triple division of the body; but in the higher forms such separation becomes wonderfully complete, so that the head may communicate with the thorax only by a narrow isthmus, and the thorax with the abdomen only by a very slender link. This arrangement is carried to its greatest extreme in the Hymenoptera Aculeata. It may be looked on as possibly a means for separating the nutrition of the parts included in the three great body divisions.
Along each side of the body extends a series of orifices for the admission of air, the stigmata or spiracles; there are none of these on the head, but on each side of most of the other segments there is one of these spiracles. This, however, is a rule subject to many exceptions, and it is doubtful whether there is ever a spiracle on the last abdominal segment. Even in the young stage of the Insect the number of these stigmata is variable; while in the perfect Insect the positions of some of the stigmata may be much modified correlatively with the unequal development or consolidation of parts, especially of the thorax when it is highly modified for bearing the wings.
The segments of the Insect are not separate parts connected with one another by joints and ligaments; the condition of the Insect crust is in fact that of a continuous long sac, in which there are slight constrictions giving rise to the segments, the interior of the sac being always traversed from end to end by a tube, or rather by the invaginated ends of the sac itself which connect with an included second sac, the stomach. The more prominent or exposed parts of the external sac are more or less hard, while the constricted parts remain delicate, and thus the continuous bag comes to consist of a series of more or less hard rings connected by more delicate membranes. This condition is readily seen in distended larvae, and is shown by our figure 48 which is taken from the same specimen, whose portrait, drawn during life, will be given when we come to the Coleoptera, family Cleridae. The nature of the concealed connexions between the apparently separate segments of Insects is shown at m, Fig. 47, p. [88].
Fig. 48—Tillus elongatus, fully distended larva.
As the number of segments in the adult Insect corresponds—except in the head—with the number of divisions that appear very early in the embryo, we conclude that the segmentation of the adult is, even in Insects which change their form very greatly during growth, due to the condition that existed in the embryo; but it must not be forgotten that important secondary changes occur in the somites during the growth and development of the individual. Hence in some cases there appear to be more than the usual number of segments, e.g. Cardiophorus larva, and in others the number of somites is diminished by amalgamation, or by the extreme reduction in size of some of the parts.
Besides the division of the body into consecutive segments, another feature is usually conspicuous; the upper part, in many segments, being differentiated from the lower and the two being connected together by intervening parts in somewhat the same sort of way as the segments themselves are connected. Such a differentiation is never visible on the head, but may frequently be seen in the thorax, and almost always in the abdomen. A dorsal and a ventral aspect are thus separated, while the connecting bond on either side forms a pleuron. By this differentiation a second form of symmetry is introduced, for whereas there is but one upper and one lower aspect, and the two do not correspond, there are two lateral and similar areas. This bilateral symmetry is conspicuous in nearly all the external parts of the body, and extends to most of the internal organs. The pleura, or lateral regions of the sac, frequently remain membranous when the dorsal and ventral aspects are hard. The dorsal parts of the Insect's rings are also called by writers terga, or nota, and the ventral parts sterna.
The appendages of the body are:—(1) a pair of antennae; (2) the trophi, constituted by three pairs of mouth-parts; (3) three pairs of legs; (4) the wings[[19]]; (5) abdominal appendages of various kinds, but usually jointed. Before considering these in detail we shall do well to make ourselves more fully acquainted with the elementary details of the structure of the trunk.