104 THE FLEA [CH. 



dog-fleas, cat-fleas, fowl-fleas, and human fleas. The 

 proportions in which they and rat-fleas are found 

 vary greatly in different parts of the world For 

 instance, in San Francisco nine per cent, of the fleas 

 collected from rats have sometimes been found to be 

 human fleas ; whilst in Italy as many as twenty -five 

 per cent, have been identified as cat- and dog-fleas. 



It must be borne in mind that when new countries 

 are opened up by man the rats, which follow in his 

 rear, exterminate numbers of the weakly native 

 small mammals and take on their fleas A change 

 of habitat may be followed by an exchange of 

 fleas. 



Some interesting work has been done in testing 

 the appetite of different kinds of flea for human 

 blood. The oriental rat-flea (X. cheopis) has been 

 kept alive for three weeks on that diet. Other 

 species show repulsion for mankind and refuse to 

 suck. The experiments confirm the popular belief 

 that fleas have a marked preference for certain 

 individuals. When the flea has refused to bite the 

 human arm, it becomes necessary to check the ex- 

 periment by trying whether the refusal is merely 

 due to want of hunger. For this purpose a rat 

 must be at hand. It can be secured on a board 

 by two bandages fixed at each end by drawing pins. 

 The rat lies, of course, on its back with its head 

 comfortably supported by a little pillow of cotton 



