ON THE SUBJECT OF PLAGUE 
869 
dislike wetness. The breeding-places differ with the habits of 
the host. Thus, in the case of Mus decumanus, they are found 
in the burrows, while in the case of M. rattus in all sorts of 
situations which afford a shelter to these animals. Thus, on 
grain or gunny-bags. This, as a source of distribution of the 
flea, should be kept in mind. 
Temperature has a decided effect on the breeding of fleas. 
A high mean temperature of about 90° F. not only seems to 
restrain the imago from depositing eggs, but is deleterious to 
the development of the eggs into larvae. There seems to be 
an optimum temperature at which breeding takes place more 
vigorously than at other temperatures. In Bombay this 
appeared to be from 75° to 80° F. 
A French worker in Annam states that P. cheopis eggs do 
not hatch there under 55° F., and at 90° F. 75 per cent, of them 
remain sterile. He states that for the active larvae a certain 
amount of moisture and a still atmosphere are necessary. 
Badly ventilated houses, therefore, favour their develop- 
ment. He also states that the adult flea can live thirty-eight 
days without food, and that it lives for preference on the body 
of its host rather than in its bed, which makes it a far more 
dangerous agent in plague transaction. 
Fleas have Particular Hosts. — Different kinds of fleas have 
their own hosts in particular. Thus P. irritans is natural to 
man as P. cheopis is to the rat. This does not mean that 
fleas will not attack other animals than their normal hosts : 
for instance, P. felis has been found on the dog, cat, tiger, 
panther, goat, horse, rat, hedgehog, kangaroo, deer, guinea- 
pig, rabbit, monkey, and on man. But, nevertheless, they 
prefer their natural hosts. 
P. irritans, the human flea, does not thrive on the rat, 
and, when artificially placed on such a host, rapidly dies off. 
Yet it can be infective for at least four days after feeding on 
a plague rat. In some experiments, however, which were 
carried out in Bombay, it was found that the guinea-pig is 
as readily chosen by P. cheopis as its true host the rat. When 
many rat fleas are present, some of them will attack man, even 
when a rat is available for their food supply. Conversely, 
when the number is small, they will not desert their true host 
