[Illustration: FIG. 34.--Development of Carcinus moenas. A, zoæa stage; B, megalopa stage; C, final state (after Couch).]
Insects are the highest form of articulates. In these, therefore, we find the modification is still greater than in crustaceans, though even here the ring-and-appendage structure is plain enough in most cases.
One of the best evidences of high grade among animals is the gathering of the segments into distinct groups, and especially the distinctness of the _head_ as one of these groups. In worms and lower crustaceans there is no grouping at all, the skeleton being a continuous series of joints, only slightly modified at the anterior and posterior extremities. In the higher crustacea, and in spiders and scorpions, they are grouped into two regions, viz., cephalo-thorax and abdomen. In insects they are grouped into three very distinct regions--head, thorax, and abdomen. In insects, therefore, we find for the first time the head distinctly separated from the rest of the body. This is an evidence of high grade, because it shows the dominance of head-functions.
FIG. 35.--External anatomy of Caloptenus spretus, the head and thorax disjointed; up, uropatagium; _f_, furcula; _c_, cercus (drawn by J. T. Kingsley). ]
The insect, such, for example, as a grasshopper, consists of seventeen or eighteen segments (Fig. 35). Of these, four belong to the head, three to the thorax, and about ten to the abdomen. Those of the abdomen are all separated and movable; those of the thorax and head are more or less consolidated. The appendages of the head-segments become antennæ and jaw-parts, i. e., mandibles--maxillæ and labium; the appendages of the thorac-segments become legs (the wings are not homologous with appendages), while those of the abdomen are aborted. The steps of the gradual consolidation on the one hand, and the abortion on the other, may be traced in the embryo or larva--i. e., in the caterpillar or the grub of a bee or a beetle. In the caterpillar, for example, there is no grouping into three regions, there is no consolidation, and all the segments have appendages. Again, the almost infinite variety in the mouth-parts among insects, brought about by adaptive modifications for biting, for piercing, and for sucking, and yet the essential identity of all to the more simple and generalized structure of the grasshopper, is an admirable illustration of the same principle. But to dwell upon these minor points would carry us too far.
