HUMAN FLE.\ 415 



cosmopolitan in distribution, probal)ly having originated in 

 Europe, wlience it was introduced with Europeans to all parts 

 of the world. This flea is the species which has made California 

 as famous for its fleas as is New Jersey for its mosquitoes. The 

 relatively cool humid summer climate combined with a mild 

 wet winter make the Pacific Coast of the United States an ideal 

 place for this pest. Though more or less of a nuisance through- 

 out the year in mild climates, this flea is less troublesome in 

 winter, due to relative inactivity, to slower reproduction, and 

 to the fact that small mammals are more commonly attacked at 

 this time of year. 



The human flea is readily distinguished from most common 

 species in temperate climates by the absence of any combs, 

 either on the head or thorax. From the Indian rat flea, Xeno- 

 psylla cheopis (Fig. 177), it is difficult to distinguish, the essential 

 difference being the presence in the rat flea and absence in the 

 human flea of a vertical chitinous thickening of the mesoster- 

 num, i.e., the plate to which the middle leg is artisulated on 

 either side. The antennae of the human flea are shorter and 

 more knoblike than are those of Xenopsylla. 



The human flea secretes itself in crevices and cracks of houses, 

 in floors, rugs, bedding, etc., coming forth chiefly at night to 

 pierce the flesh and suck the blood of its hosts. The suscep- 

 tibility of different individuals to flea bites is variable. The 

 irritation that is normally produced, probably chiefly as a result 

 of the injection of the insect's salivary secretions into the wound, 

 causes the formation of a reddish pimple with more or less swell- 

 ing. Some people, however, are apparently entirely immune to 

 flea bites and feel no pain from them. The writer is one of these 

 fortunate individuals. On his first visit to California he had 

 been fully warned concerning the ravages of the fleas but found to 

 his pleasant surprise that the onl}^ discomfort felt from fleas was 

 the tickling occasionally caused by their movements beneath 

 his clothing. A college roommate, however, was attacked to 

 such an extent as to be unable to sleep, and spent a considerable 

 part of many nights in pursuit of the wily fleas and in violent 

 massaging of painful wounds. 



As has been noted, the human flea may act as a transmitter 

 of plague, infantile kala-azar and tapeworm {Dipylidium) in- 

 fection, though it is not the chief villain in the spread of any one 

 of these diseases. 



