l62 



INSECTS 



The great order Hymenoptera, in which insect para- 

 sitism is developed to a remarkable extent, contains 

 no species that live on vertebrate animals. 



The Diptera, or flies, on the other hand, in which 

 specialization has been almost as extreme as in the 

 Hymenoptera, have developed a considerable number 

 of forms that depend for their living entirely upon the 

 higher animals. 



Fig. 69. — A rabbit flea. 



The little family of fleas, which are very highly 

 specialized flies, although now usually classed in an 

 order by themselves, are all parasites on warm-blooded 

 animals covered with hair or feathers. They are small, 

 brown, transversely flattened, set with spines or stiff 

 hair directed backward, and the legs are powerful, 

 fitted for jumping. This characteristic form makes it 

 very easy for them to move about among the hair and 

 feathers, and this they do in a sort of jerky way as if 

 they were making short jumps, each of which carries 

 them a shorter or longer distance and enables them 



