350 



THE INSECTS 



2. Eyes absent. 



(a) Collar of combs on prothorax and four short ones along 



inferior border of head. Ctenopsylla musculi. 

 B. Without combs. 



(a) Ocular bristle arises near upper anterior margin of eye. A 

 line between this and the oral bristle approximately ver- 

 tical. Two bristles posterior to antennae. Xenopsylla 

 cheopis. Loemopsylla cheopis. Formerly Pulex cheopis. 



-ZArery- 



FIG. 84. i and 2, male and female Xenopsylla cheopis. 3, Head of Ceratophyllus. 

 4 and 5, male and egg distended female of Sarcopsylla penetrans. 



(b) Ocular bristle arises near lower anterior margin of eye. A 

 line between this and the oral bristle approximately hori- 

 zontal. One bristle posterior to antennae. Pulex irritans. 



The common human flea of Europe is the Pulex irritans; that of the United States 

 the Ctenocephalus serraticeps or dog flea. The flea that is prominently implicated 

 with plague is the Xenopsylla cheopis, on account of its being the common rat flea 

 of India, where it has been much studied. It resembles P. irritans t but is more yellow 

 than brown in color. It also has a greater number of bristles on the head. The 

 ocular bristle runs above and in front of the eye; that of P. irritans below. It is 

 principally the flea of Mus decumanus (M. norvegicus), the sewer rat; but the house 

 rat, M . rattus, becomes infected from coming in contact with the sewer rat in the 



