v] HUMAN FLEA AND OTHER SPECIES 63 



respects it is the most specialized of all the Pulicidce. 

 The chigoes (Sarcopsyllidce) resemble it and are 

 doubtless derived from the Pulicidce. The chief 

 structural character of this interesting insect is the 

 greatly reduced thorax. But it can be distinguished 

 from any other known flea by the fact that the 

 upper segment of the hind leg (hind coxa) bears a 

 number of hairs on the inner surface of the posterior 

 portion. A more noteworthy feature in this flea 

 is the presence, in a large proportion of specimens 

 of both sexes, of a small tooth at the edge of the 

 head. This small tooth is sometimes absent ; but, when 

 present, both its position and its structure indicate 

 that it corresponds to the fifth tooth in the head 

 comb of the dog-flea (Ctenocephalus canis) (Fig. 7). In 

 the hedgehog-flea (Ct. erinacei) the teeth of the combs 

 both on the head and on the thorax are small in size 

 and few in number. Occasionally they almost dis- 

 appear. The conclusion seems justified that the 

 human flea is descended from an ancestral form with 

 combs. To discuss whether the combs became useless 

 and were lost when the host lost the hairy covering 

 of its body would lead into regions of vague specula- 

 tion and occupy time unprofitably. 



The nearest allies of the human flea, which are 

 found on various animals, are all inhabitants of the 

 Old World. The indigenous fleas of America are 

 only distant relatives of Pulex irritans. Our know- 



