9 FARMERS’ BULLETIN 683. 
As examples may be cited the dog flea (fig. 1) ,1 which normally feeds 
on dogs and cats, but which when excessively numerous may prove a 
troublesome pest to man. The human flea normally attacks man, 
but may be found on a number of other animals. Rat fleas, in the 
absence of their usual hosts, will bite man, and it is in this instance 
that fleas are responsible for the inoculation of man with bubonic 
plague. 
There is a marked variation in the habits of fleas in regard to the 
intimacy with which they are associated with their hosts. Some 
kinds remain upon host animals practically all of the time. In fact, 
the chigoe flea normally buries itself in the skin of the host and 
there develops its eggs and dies. The sticktight flea, or chicken 
flea, has this habit of intimate association with the host, but does not 
bury itself in the flesh of the animal. Dog fleas ordinarily remain 
upon the domestic animals almost continuously throughout their 
existence, but are not attached, feeding only at intervals. The 
human flea has adapted itself to its host so closely that it remains 
upon man but little, being free the greater portion of the time. 
There are four different stages in the life of fleas, as is the case 
with many other insects; these are the egg, larva, pupa, and adult. 
A number of eggs are deposited by each adult flea. The egg lay- 
ing, alternated with feeding, extends over a considerable period of 
time. In most cases the ova are deposited by the fleas while the 
latter are on the host, but as they usually are not cemented to the 
hair or feathers they fall out in the nest or resting place of the 
animal. The eggs are white or cream in color and ovoid in shape. 
Large numbers of them may often be seen on mats or cushions upon 
which infested dogs or cats sleep. Especially are they easily ob- 
served when on dark-colored cloths. The egg of the common dog 
flea is illustrated in figure 1 at a. Hatching usually takes place in 
from 2 to 12 days. 
The larva when first hatched is very minute, of whitish color, and 
quite active. (See fig. 2, larva of European rat flea.) In this stage 
none of the fleas is parasitic. They depend upon various animal and 
vegetable débris, including the excrement of the adult fleas, for food. 
During their growth the skin is shed two or three times, and between 
four days and several months after hatching a silken cocoon is spun, 
and in this the larva transforms to the pupal or resting stage. (See 
fig. 1, D and ¢, pupa of dog flea.) 
1The fleas mentioned in this bulletin are known scientifically as follows: Dog flea, 
Ctenocephalus canis Curtis; cat flea, Ctenocephalus felis Bouch.; human flea, Pulex 
irritans L.3; rat fleas, Xenopsylla cheopis Roth. (the Indian rat fiea), Ceratophyllus 
fasciatus Bose. (the European rat flea), and others; chigoe, Dermatophilus penetrans L. ; 
sticktight or chicken pox flea, Hchidnophaga gallinacea Westw. 
