PLAGUE, RATS AND FLEAS. 285 



come now to rat fleas. The common flea found on Mils rattus is Pulex 

 theopis. It is therefore the common rat flea of India. It is a small 

 light-coloured flea, which is particularily sensitive to light, loving 

 the dark ; and it is, therefore, more or less nocturnal in habit. It is sel- 

 dom found apart from the rat in this country* and lives chiefly in the 

 haunts of the black rat, among grain bags and in the roofs of houses, 

 etc. At first sight it is very like the human flea. The flea commonly 

 found on Mus decumanus is called Ceratophyllus fasciatus. In so far as 

 Mus decumanus is rare in this country, this ilea is also seldom found in 

 India ; but it is the common flea found on the rat in Europe. 



These fleas can be distinguished from one another in both sexes by 

 noting if there is a comb of bristles behind the head. Pulex felis and 

 Ceratophyllus fasciatus have both got combs in this situation. Pulex 

 felis has in addition a set of teeth-like bristles surrounding its mouth ; 

 Ceratophyllus fasciatus has not got these bristles. Pulex irritans and 

 Pulex cheopis have no comb of bristles behind their head. They are 

 readily distinguished by the length of the anti-pygideal bristles, which 

 are short in the human flea and long in the rat flea. The claws of the 

 human flea, too, are very large and scythe-like; while the rat flea has 

 more elegant scycle-shaped prehensile organs. The males of all four 

 species are at once distinguished by the characteristic shape of the 

 claspers. 



Now you will notice that most of these fleas have a particular host. 

 The cat flea, however, is more or less an exception to this rule, for it is 

 often found on other hosts than cat or dog. If this is the case, how is it 

 possible to explain the infection of man from the rat, when, under ordin- 

 ary circumstances, the human flea is only found on man, and the rat 

 flea on the rat ? This fact, more than any other, I think, has prevented 

 men from accepting the flea theory of the spread of plague, for not a 

 few instances have now been reported in which plague has been directly 

 communicated from the diseased rat to the healthy rat by means of flea& 

 But, it is objected, how can men become infected if the rat flea is never 

 found on man ? I have had the good fortune to discover an explanation 

 of how the rat flea can communicate the disease to man, although nor- 

 mally the rat flea, Pulex cheopis, is scarcely ever found on man. 



In March 1903 some guinea-pigs, which had died in the Victoria 

 Gardens, were sent to me for examination. I found that they had died 

 of plague. I immediately visited the gardens to see if I could find any 



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