PARASITIC INSECTS 53 



lap it had been held for a short time. The eggs hatch in about 

 ten days; the larvae are full-grown in twelve days, and the adult 

 emerges two weeks later. 



The house flea is very much like the cat and dog flea but lacks 

 the spines on the side of the head. House fleas usually conceal 

 themselves in bedding and clothing, venturing out, particularly 

 at night, to feed upon the blood of their victims. Their eggs 

 are laid in dusty crevices or under carpets. The careful removal 

 of dust will decrease their numbers and a thorough dusting of 



FIG. 32. Cat and dog flea : A, adult ; B, Egg ; C, larva in cocoon. (After 



Howard.) 



their breeding places with insect powder (pyre thrum) will 

 destroy them, but no amount of cleanliness will protect a human 

 being who enters an infested building. Recently fleas have 

 become of special world-wide importance because of their rela- 

 tion to the transmission of bubonic plague. (See Chapter X, 

 page 98.) 



Ticks. Many more or less degenerate insects are called 

 " ticks," although this name really belongs to certain small 

 relatives of the spider possessing four pairs of legs. The 

 ticks are parasitic on certain birds, sheep, and horses. Of these 

 the sheep tick (Fig. 33) is especially important. This insect is 

 about one fourth of an inch long and spider-like in appearance, 

 with strong sucking mouth parts but no wings. It moves about 



