FROM EGG TO INSECT SNODGRASS 395 



segmental lines. There are never more than 21 segments formed 

 in all ; the number is usually fewer, and it is possible that the last 

 division is not to be regarded as a true segment. Segmentation of 

 the body is one of the fundamental characters of insects, but it is 

 characteristic also of one large group of worms, of the crabs, lob- 

 sters, and shrimps and all their relations, of the centipedes, and of 

 the spiders, as well as of other less familiar animals related to these 

 several gro'ups. Evolutionists take this to mean that all these 

 creatures have had a common ancestry, and that the primitive 

 ancestral forms were something similar to the segmented worms, 

 since the latter are the simplest representatives of segmented animals 

 living to-day. The segmented worms belong to the zoological group 

 known as the Amielida, of which the common earthworm is a mem- 

 ber; the other segmented animals mentioned above constitute the 

 Arthropoda, of which the insects form one class, the spiders an- 

 other, the centipedes another, and the shrimps, lobsters, crabs, and 

 their relations another called the Crustacea. 



In the insect embryo, the first three segments are usually but in- 

 distinctly marked, since they unite at an early stage to form a large 

 bilobed swelling at the front end of the body (fig. 14 A, Pre). This 

 swelling is the first embryonic head, named the procephalon 

 ("kephale" being Greek for head), and the embr3^o at this stage 

 most likely represents an early ancestral stage in the evolution of 

 insects, when an insect consisted of a three-segmented head and a 

 long segmented body. As the embryo develops, however, it adds the 

 first three primitive body segments to the procephalon and comes 

 to have a head formed of six segments (fig. 14 B, H). This is the 

 structure of the head of a modern adult insect, in which, though it 

 eventually becomes a solid cranium-like piece, there is always to be 

 discovered traces of a six-segment origin. The next three body seg- 

 ments become grouped to form the middle division, or thorax (fig. 

 14 B, Th), of the adult -insect. The remaining segments constitute 

 the abdomen {Ah), the number of segments in which varies in the 

 adult in different insects, since some of the terminal ones may be 

 lost during development, or have been lost in the course of evolution. 



It is clear that insects in their evolution have tended toward a 

 grouping of their segments to form a three-part body, and this body 

 division becomes most accentuated in the higher members of the 

 class (compare fig. 21, a grasshopper, with figure 15 D, a fly). 

 Now, the members of the other classes of the Arthropoda have fol- 

 lowed other lines of specialization in the grouping of the body 

 segments and in the uses to which they are put. The crustaceans 

 have aimed at a two-part division of the body, Avith the principal 

 viscera in the first, and the second specialized as a swimming organ, 



