68 THE WHENCE AND THE WHITHER OF MAN 



wonderful transformations of these modified legs we 

 cannot enter. 



The muscles are no longer arranged to form a sack 

 as in annelids. Transverse muscles, running parallel 

 to the unyielding plates of chitin or horn could ac- 

 complish nothing. They have largely disappeared. 

 The work of locomotion has been transferred from the 

 trunk to the legs. 



The abdomen of the insect is as clearly composed of 

 distiact segments as the body of the annelid. Of these 

 there are perhaps typically eleven. The thorax is com- 

 posed of three segments, distinct in the lowest forms, 

 fused in the highest. This fusion of segments in the 

 thorax of the highest forms furnishes a very firm 

 framework for the attachment of wings and muscles. 

 These wings are a new development, and how they 

 arose is still a question. But they give the insect the 

 capability of exceedingly rapid locomotion. 



The three pairs of jaws, modified legs, in the rear 

 half of the head show that this portion is composed of 

 three segments. For only one pair of legs is ever de- 

 veloped on a single segment. Embryology has shown 

 that the portion of the head in front of the mouth is 

 also composed of three segments. Possibly between 

 the prse- and post-oral portions still another segment 

 should be included, making a total of seven in the 

 head. The head has thus been formed by drawing for- 

 ward segments from the trunk, and fusing them suc- 

 cessively with the first or primitive head segment. This 

 is difiicult to conceive of in the fully developed insect, 

 where the boundary between head and thorax is very 

 sharp. But the ancestors of insects looked more like 

 thousand-legs or centipedes, and here head and thorax 



