32 THE FLEA [CH. 



wing but a portion of the thorax present in all 

 insects. It is of no special service to the flea except 

 as a portion of the thoracic armature which covers 

 the body. 



The larva of a flea has no legs ; the adult insect 

 has six. A study of other embryo insects shows that 

 the ancestors of insects had many legs. It is an 

 interesting problem why insects lost the legs on 

 their abdomens, why legs should now invariably be 

 restricted to the thorax, and why there should never 

 be more than three pairs. In the earliest known 

 insects which lived on the earth, before winged forms 

 were evolved, the number of legs was already six. 

 But our knowledge of fleas is too small to attempt, at 

 present, to trace their exact line of ancestral descent. 



The abdomen of a flea consists of ten segments. 

 The horny plates which cover the dorsal side are 

 called tergites; those on the ventral side sternites. 

 In fleas, as in all holometabolous insects, that is 

 those which pass through a complete metamorphosis, 

 the sternite of the first abdominal segment is sup- 

 pressed and has completely disappeared. The tergite 

 which covers the dorsal part of the first abdominal 

 segment nearest to the thorax is, however, always 

 present. 



The ultimate segments of the male and female 

 flea are modified for reproductive purposes and of 

 these segments more must be said later. 



