120 



Parasitic Arthropoda 



90. Dog flea (x!5). 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 sermticeps. 

 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 

 cast 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 



