i] INTRODUCTORY 17 



the east. So we also get northern and southern forms 

 of the same species varying slightly. These variations 

 are perceptible in many forms of insects, and zoologists 

 now describe these local races as subspecies and 

 designate them with trinomials. No one, however, 

 knows enough as yet about all the various forms 

 which are assumed by fleas to attempt, except in a 

 few instances, to do so in the case of these animals. 



The study of Siphonaptera is still quite in its 

 infancy. We know little or nothing of the minute 

 geographical variation of fleas. That there is such 

 a thing can already be seen in a few species. In the 

 meantime the study of variation must be postponed 

 until collectors have amassed a more plentiful amount 

 of material; and it is best to treat all forms which 

 are to all appearance constantly different as being 

 specifically distinct until more is known about 

 variation. 



Any classification of fleas that may now be 

 attempted can only be tentative. It will be enough 

 for present purposes if the reader will remember 

 that the Order Siphonaptera can be divided into 

 three groups or families: (1) the chigoes and their 

 allies, which are the most parasitic fleas (Sarco- 

 psyUidce) ; (2) the typical fleas to which the majority 

 of species belong (Pididdce)\ and (3) the bat-fleas 

 (Ceratopsyllidce), which have certain peculiarities that 

 will be described in a later chapter. 



R. F. 2 



