v] HUMAN FLEA AND OTHER SPECIES 67 



1 bird-flea were caught. In a third, 150 P. irritans 

 and 4 cat-fleas * were captured in a short time. 



The piercing organs of the human flea are strong 

 and well developed. This is rare in a flea which, far 

 from having adopted stationary habits, is a very active 

 insect. It has been suggested, with some show of 

 probability, that the wide and strongly serrated 

 mandibles were acquired after man became the 

 host. The naked skin and rough garment of man- 

 kind would render the claws and legs of the flea 

 insufficient to keep the insect in a steady position 

 when feeding. Natural selection would, in due course, 

 strengthen the mouth organs. 



The division of mankind into different races, many 

 of which are quite as distinct as the various species 

 of some genus among other animals, leads one to 

 expect various races among the fleas which are 

 parasitic on them. If the sand-martin and the 

 house-martin, the rat and the mouse have dis- 

 tinguishable fleas, one might suppose that the 

 Caucasian and the Hottentot, the Australian native 

 and the Red Indian would follow suit. It may be 

 that further study will show that the human flea now 

 consists of a number of different races. In only one 

 case, however, does a development of this kind in 

 fact appear. Fleas taken off Mexican Indians show 

 slight but fairly constant differences from the true 

 Pulex irritans. The specimens are smaller in size, 



52 



