i] INTRODUCTORY 9 







majority of mammals and birds have fleas. As a 

 general rule, it is true to say that certain species 

 of flea are associated with certain species of host. 

 Thus man is the true host of Pulex irritans; the 

 cat family are the true hosts of the cat-flea (Cteno- 

 cephalus felis) ; and the dog family are the true 

 hosts of the dog-flea (Ctenocephalus canis). But the 

 human flea is sometimes found on cats and dogs, and 

 cat and dog-fleas occasionally bite human beings; 

 and cat-fleas are found on dogs and dog-fleas are 

 found on cats. All fleas, so far as we know, may 

 occasionally pass from one species of host to another ; 

 but they do not, for the most part, seem to flourish 

 in unaccustomed quarters. Some fleas are more 

 catholic in their tastes than others. Some seem to 

 be very strictly confined to one host, and even when 

 starving only suck strange blood under protest. 

 There is a species of flea that has only (except by 

 accident) been found on the long-tailed field-mouse 

 and another that has only been found on the hedge- 

 hog. Other fleas are commonly found on two abso- 

 lutely distinct animals ; a good instance of this is the 

 human flea which, at all events in certain parts of 

 England, is a regular parasite of the badger. 



As distinguished from true or natural hosts one 

 must separate what may be termed casual or acci- 

 dental hosts. All animals which come in contact 

 with one another, or which live in close proximity, 



