118 INSECTS AND DISEASE 
Europe, but common in warm climates, which was till 
recently undescribed, but is now known as Pulex cheopis, 
Rothschild. Several species of flea are found on rats. 
The human flea, Pulex irritans, and the dog-flea, Pulex 
serraticeps, are occasionally found, but merely as stragglers. 
There are three species specially attached to the rat, known 
as Ceratophyllus fasciatus, Ctenopsyllus muscult, and Pulex 
cheopis. Much experimental work has been vitiated by 
neglecting to identify the species of flea concerned. 
Pulex cheopis is nearly allied to the human flea. That 
it will readily bite man has been ascertained repeatedly. 
The statement that rat-fleas will not bite man are derived 
from experiments with the other species. It can indeed 
be kept alive for weeks by being allowed to suck human 
blood. Furthermore, it has been found on man. For 
instance, Liston writes, “‘ About the 6th or 7th of April, rats 
began to die in large numbers in a chaw], or block of tene- 
ment houses. Suddenly the deaths among rats ceased, 
and on April 11th, the people became tioubled with fleas. 
The fleas became so nume.:ous that they had to quit their 
rooms and sleep out on the verandah. While living on the 
verandah on April 17th, one of the inhabitants of the 
particular room in which the fleas were taken became 
infected with plague. Another case occurred on the 
same day in a room adjoining. The people who inhabited 
the room where the above case occurred, were induced to 
collect some of the fleas. An examination of this collection 
was most instructive. Now I must tell you that on previous 
occasions, of 246 fleas that were caught on man under 
normal conditions, 1 had found only one rat flea, Pulea 
cheopis. But of the collection of 30 fleas caught on man 
under the circumstances above recorded, no less than 
14 were rat fleas.” 
Though the geographical distributicn of the rat-flea 
has not yet been worked out, there are indications that 
the freedom of certain ports from plague infection are du>, 
not to any unusual vigilance in their port authorities, nor 
to any superior excellence in their sanitarv conditions. 
but merely tothe scarcity or absence of this particular flea 
on the local rats. 
Let me now give a summary of the experimental 
