182 ELEMENTARY ZOOLOGY 



arise the two pairs of wings which are the most striking 

 structural features of insects. Not all insects are winged, 

 (fig. 45), and of those which are a few have only one pair 

 of wings, but the great majority of them have two pairs of 

 well-developed wings (fig. 46), which give them, as com- 

 pared with the other animals we have studied, a new and 

 most effective means of locomotion. The great numbers 



FIG. 46. A four- winged insect; a stone fly, Per la sp., common about 

 brooks. (From Jenkins and Kellogg.) 



of insects and their preponderance among living animals 

 is undoubtedly largely due to the advantage derived from 

 their power of flight. The hindmost part of the body, 

 the abdomen, is composed of from seven to eleven seg- 

 ments, only the last one or two of which are ever provided 

 with appendages. When such posterior abdominal 

 appendages are present they form egg-laying or stinging 

 or clasping organs. 



