296 THE INSECTS 



the flea is flattened laterally. They may or may not have eyes, and certain con- 

 spicuous structures called combs are of importance in classification. In the meta- 

 morphosis of the flea the eggs are hatched out in dust of crevices, etc., into bristled 

 larvae in about one week. The larva forms a cocoon and develops into a nymph 

 which has three pairs of legs. The nymphs emerge from the cocoon as adult fleas 

 in about three weeks after the larva forms it. 



KEY TO THE FLEAS. 



A. With combs. 



1. Eyes present. 



a. Combs along inferior border of head and on prothorax. 

 Ctenocephalus serraticeps. 



b. Combs only on prothorax. Ceratophyllus fasciatus. 



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 vertical. 

 Two bristles posterior to antennae. Xenopsylla cheopis. 

 Lxmopsylla cheopis. Formerly Pulex cheopis. 



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

 between this and the oral bristle approximately horizontal. 

 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 implicated with plague 

 is the Xenopsylla cheopis. It resembles P. irritans, 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, the sewer rat; but the house rat, M. rattus, becomes infected 

 from coming in contact with the sewer rat in the basement. 



Ceratophyllus fasciatus is the common rat flea of Europe and the U. S. In the 

 tropics X. cheopis is the common rat flea (98% in India). Ctenocephalus serrati- 

 ceps, Ctenopsylla musculi and Pulex irritans have also been frequently found on 

 both Mus norvegicus and M. rattus. To distinguish M. norvegicus from M. rattus 

 we have in the former (i) ears which barely reach the eyes when laid forward and 

 (2) tail rather shorter that length of head and body together (only 89% of length 

 of head and body together). With M. rattus the tail is longer than the head and 

 body together (25% longer) and the extended ear covers or reaches beyond the 

 middle of the eye. M. rattus has a sharper nose, longer and more delicate tail 

 and thinner ears than M. norvegicus (formerly M. decumanus). 



M. alexandrinus is a variety of M. rattus. Rats and mice belong to the family 



