300 
Fleas on Rodents 
the animals have been chloroformed and the parasites removed. In the 
case of C. fasciatus and L. cheopis the fleas were generally placed on the 
trunk or on the back of the neck; in the case of Ct. musculi they were 
placed on the hind part of the body. In other words, we have placed 
the fleas in such positions upon the animal that, judging from our experi¬ 
ence with rats, it would be necessary for the fleas to move about in order 
to reach the positions of election for each species. 
We have used special care in the guinea-pig experiments as we 
wished the results to be comparable as far as possible with those of 
the English workers in India. When we wished to remove the fleas 
the guinea-pigs were wrapped in a towel saturated with ether, thus 
stupefying the parasites and preventing any alteration of their position. 
In the case of rats the determinations have been made for the most 
part with animals brought to the laboratory alive, and the fleas removed 
by one of the procedures to be mentioned later. In a few cases we have 
put fleas on a rat previously freed of fleas and confined in a mouse jar. On 
the following day the animal was combed and a note made of the region 
from which the parasites came. In other cases we have transferred fleas 
from rats to squirrels and vice versa, and have noted the distribution of 
the parasites on the new host. 
In some cases the rat was injected intraperitoneally with a solution 
of potassium cyanide. This invariably killed the animal within a 
minute, and the search for parasites was made at once. This method 
was intended to prevent any change of position of the fleas due to such 
struggling as occurred when the animal was coming under the influence 
of the anaesthetic. Occasionally the animal, as soon as it was dead, was 
wrapped in filter paper saturated with chloroform ; this effectually pre¬ 
vented any change of position on the part of the parasites. The results 
obtained in this way did not differ materially from those obtained by the 
simpler process of chloroforming the rat and the fleas simultaneously, and 
as the latter method was more convenient we have employed it in the 
majority of the determinations. 
The results obtained by the various methods with each of the species 
of mammals and of fleas were essentially the same: i.e. the method em¬ 
ployed had no influence on the result of the enumeration. Some of 
these regional determinations were made in Oakland, California, through 
the courtesy of Passed Assistant Surgeon Carroll Fox. 
The average number of fleas on a single rodent was generally so 
small that, for the purpose of eliminating errors, we have grouped 
together a number of rodents, including in one group all combed on 
the same day. 
