4 BULLETIN 248, U. S. DEPARTMENT OF AGRICULTURE. 



depend largely on finding a host at night, and they tend to keep 

 secluded during the day. 



In the case of inoculation of an animal with plague bacilli by a 

 flea it has been observed that there is a relationship between the 

 point of flea attack and the formation of the sweelings, or buboes. 

 In this connection it is interesting to note that certain species show 

 a marked tendency to infest certain portions of the host animal. 



LIFE HISTORY. 



The fleas pass through four distinct stages of development, viz, 

 the egg, the larva, the pupa or resting stage, and the adult. All of 



Fig. 1. — The dog flea (Ctenocephalus canis) : a, Egg; 6, larva in cocoon; c, pupa; d, 

 adult; e, mouth parts of same from side; f, antenna; y, labium from below, b, c, d, 

 Much enlarged; a, e, f, g, more enlarged. (From Howard.) 



the different kinds of fleas resemble one another rather closely in these 

 different stages. 



THE EGG. 



The eggs are ovoid in shape and white or creamy in color, some 

 strongly reminding one of miniature china eggs. Although rather 

 small, they are readily seen with the naked eye, especially if placed on 

 a dark piece of cloth or paper. (Fig. 1, a.) They are formed after 

 the female has been feeding on a host for a few days and are usually 

 deposited while the flea is on the host, but are not glued to the 

 hairs or feathers, as is the case with lice and some other insects. The 

 human flea probably deposits most of its eggs while free from the 

 host. The eggs usually fall from the animals in their nests; hence 



