This group of parasites, concerning which little was known until recently, has assumed a very great importance since it was learned that fleas are the carriers of bubonic plague. Now over four hundred species are known. Of these, several species commonly attack man. The most common hominoxious species are _Pulex irritans_, _Xenopsylla cheopis_, _Ctenocephalus canis_, _Ctenocephalus felis_, _Ceratophyllus fasciatus_ and _Dermatophilus penetrans_, but many others will feed readily on human blood if occasion arises. [Illustration: 89. Xenopsylla cheopis, male (×25). After Jordan and Rothschild.] We shall treat in this place of the general biology and habits of the hominoxious forms and reserve for the systematic section the discussion of the characteristics of the different genera. 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. [Illustration: 90. Dog flea (×15). After Howard.] The eggs are oval, slightly translucent or pearly white, and measure about .5 mm. in their long diameter. They are deposited 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 larvæ are elongate, legless, white, worm-like creatures. They are exceedingly 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, _Ceratophyllus fasciatus_, have so important a general bearing that we shall cite them in considerable detail. [Illustration: 91. Larva of Xenopsylla cheopis. After Bacot and Ridewood.] 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 conditions, 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 as in the case of lice, (Warburton) and varying conditions of temperature and humidity have practically no effect on the percentage of eggs which ultimately hatch.