FLEAS FOUND ON RATS 463 



does not as a rule take much notice of the observer, and one notes 

 that the pain of the bite is not felt during the time the mouth parts 

 are being driven in, but after a short interval, when the blood has 

 begun to flow ; from this it may be inferred that it is the saliva 

 which causes the irritation, and not the puncturing stylets. 



The common observation that some individuals suffer more from 

 the bite of the flea than others was borne out in a curious manner by 

 the experiments of Chick and Martin. In these eight human hosts 

 were tried ; in seven little or no irritation was produced, while in one 

 quite. severe inflammation was set up around each bite. The bite of 

 Cr. fa set at us and of Ct. felts is not nearly so irritating to most people 

 as that of Pulex irritans. 



Whether fleas spend the whole or nearly the whole of their lives 

 on the skin of the host, or whether they habitually leave it for diges- 

 tion, has not been determined with accuracy. In the dissection of 

 fleas (e.g. Ct. felis) one finds them at all stages of digestion and with 

 ovaries containing ova at all stages of maturation, even when dissected 

 immediately after removal from the body of the host. In the examina- 

 tion of nests both of birds and mammals one finds as a rule larvae 

 and pupae only, and very few imagines. On the other hand, the human 

 flea appears to leave its host when it has fed. The females must, 

 of course, in all cases leave the body to deposit their eggs. 



Like all other blood-sucking insects, fleas leave the body of the host 

 immediately on the death of the latter, and seek out a fresh host. Should 

 one of the proper species not be available, as may be the case when 

 an epizootic of rat plague .has occurred, the fleas will attack an abnormal 

 host, which, in this case, may be man. 



From what has already been said with regard to the choice of host, it 

 will be evident that one may expect to find many species of fleas on the 

 rat in addition to those which are normally its parasites. 

 Pulex irritans, for instance, is occasionally, though 

 rarely, found on the rat, and Echidnophaga gallinaceus 

 on rats which have been captured in hen houses. The cat and dog fleas 

 are found on rats which have lived in places where these animals are 

 kept. 



Long lists have been compiled of the species which have been taken 

 on rats, but many of these have been recorded only once or twice, and are 

 of no practical importance. The following are the species commonly 

 met with. 



Xenopsylla clieopis. The common rat flea of the.;tropics. . ;_ -_~ 



