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. [Illustration: 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 {91}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.