120 
Parasitic Arthropoda 
Dog flea (xl5). After Howard. 
The most common fleas infesting houses in the Eastern United 
States are the cosmopolitan dog and cat fleas, Ctenocephalus canis 
(fig. 90) and C. felis. Their life 
cycles will serve as typical. 
These two species have until 
recently been considered as one, 
under the name Pulex serraticeps. 
See figure 92. 
The eggs are oval, slightly 
translucent or pearly white, and 
measure about .5 mm. in their 
long diameter. They are de¬ 
posited loosely in the hairs of 
the host and readily drop off as the animal moves around. Howard 
found that these eggs hatch in one to two days. The larvae are 
elongate, legless, white, worm-like creatures. They are .exceed¬ 
ingly active, and avoid the light in every way possible. They 
east their first skin in from three to seven days and their second 
in from three to four days. They commenced spinning in from 
seven to fourteen days after hatching and the imago appeared 
five days later. Thus in summer, at Washington, the entire life 
cycle may be completed in about two weeks, (cf. fig. 91, 92). 
Strickland’s (1914) studies on the biology of the rat flea, Cerato- 
phyllus fasciatus, have so important a general bearing that we shall 
cite them in considerable detail. 
He found, to begin with, that there is a marked inherent range 
in the rate of development. Thus, of a batch of seventy-three eggs, 
all laid in the same day and kept together under the same condi- 
91. Larva of Xenopsylla cheopis. After Bacot and Ridewood. 
tions, one hatched in ten days; four in eleven days; twenty-five in 
twelve days; thirty-one in thirteen days; ten in fourteen days; one 
in fifteen days; and one in sixteen days. Within these limits the 
duration of the egg period seems to depend mainly on the degree 
of humidity. The incubation period is never abnormally prolonged 
