STRUCTURE 



405 



either along the ventral margin of the head or along the hind 

 edge of the pronotum (the dorsal plate covering the first segment 

 of the thorax) or in both places. The presence or absence of 

 these combs and the number of teeth in them is of considerable 

 use in identification of species. 



The legs of fleas are very long and powerful, and at first glance 

 seem to possess one more segment than do the legs of other in- 



FiG. 177. The Indian rat flea, Xenopsylla cheopis, male. X 50. (After Jordan 



and Rothschild.) 



sects. They really consist of the usual number of segments, 

 however, but are peculiar in the enormous development of the 

 first segments of the legs (coxae), which in most insects are quite 

 insignificant (Fig. 179). The shape of the sternal plate to which 

 the coxse are attached is suggestive of still another segment. 

 The great development of the coxse as well as of the other seg- 

 ments of the leg gives unusual springiness and consequently 

 enormous jumping power. The human flea, Pulex irritans, has 

 been observed by Mitzmain to jump 13 inches horizontally and 

 seven and three-fourths inches vertically. An equivalent jump 

 for a man of average height would be over 450 feet horizontally 

 and over 275 feet vertically! The jumping power must over- 

 come to some extent the disadvantage of winglessness and render 

 migration from host to host comparatively easy. All the legs 



