120 INSECTS AND DISEASE 
high. A similar experiment was performed with two 
monkeys placed in cages of similar pattern, one unpro- 
tected, the other surrounded by a layer of “ tanglefoot,”’ 
six-inches wide. After two nights they were removed. 
Two fleas were caught on the unprotected monkey, while 
five fleas were found stuck on the “ tanglefoot.” The 
first monkey developed bubonic plague, the other remained 
healthy. f 
(5) Guinea-pigs were let loose in houses in which cases 
of plague had occurred recently. Large numbers of Pulex 
cheopis were subsequently collected from thes. guinea-pigs, 
and they died of plague. lf, however, the guinea-pigs were 
in cages protected by flea-proof gauze, they escaped plague. 
In similar cages not protected with gauze ten per cent. of 
the guinea-pigs contracted plague, and fleas were found on 
them, though in fewer numbers than on the guinea-pigs 
that were allowed to run about. 
This evidence is conclusive. For further details, 
I must refer tc the original report. But one point must be 
mentioned. Rats can be infected by feeding on plague- 
contaminated material, for instance, the bodies of their 
dead comrades. In these cases, there are well-marked 
pathological lesions in the intestines and mesenteric buboes. 
In naturally infected rats, intestinal lesions and mesenteric 
buboes were not found in 5,000 infected animals examined. 
The Commission conclude that transmission by feeding 
is not of common occurrence in nature, and is not the method 
by which the epizootic is propagated. 
We find then that plague epizootics among rats are 
propagated by a particular flea, Pulex cheopis. This flea 
leaves the rat soon after death, with its stomach engorged 
with blood, swarming with plague bacilli. In default of 
its natural host, it will fasten on other animals, and biting 
them will infect them with plague. This is not true only 
of other rodents like the guinea-pig, but of an animal widely 
remote from the rat and akin to man, the monkey. Pulex 
cheopis will bite man freely. The inference that plague 
may be and is conveyed to man from the rat by this par- 
ticular flea is inevitable. With the exception of plague 
pneumonia, there is no reason for supposing that plague 
is naturally acquired in any other way. How well this 
