The Siphonaptera or Fleas
The Siphonaptera, or fleas ([fig. 89]) are wingless insects, with highly chitinized and laterally compressed bodies. The mouth-parts are formed for piercing and sucking. Compound eyes are lacking but some species possess ocelli. The metamorphosis is complete.
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.
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].
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]).