VII 

 SOME HUMAN PARASITES 



FLEAS 



ONE of the most persistent of what may be termed the truly 

 domestic insects is the human flea, Pulex irritans (fig. 76). 

 Go where one will, and be one ever so clean in one's clothing 

 and person, sooner or later, one is certainly destined to 

 receive the attentions of these unpleasant little creatures. 

 The order Siphonaptera, to which the human flea belongs, 

 is, by some, approximated to the Diptera or two-winged 

 flies, by others to the Coleoptera or beetles, in short, its 

 exact systematic position is uncertain ; it is comprised of 

 over four hundred species. All fleas are parasitic in habit, 

 but, unlike the majority of parasites, they do not spend all 

 their time on the host; they have been aptly termed 

 temporary parasites. Many parasites, again, are found on 

 only one host, in fact we often find parasites possessing 

 special modifications, fitting them for life on some particular 

 host; but although certain species of flea are associated 

 with certain species of host, they will at times pass from 

 one host species to another. Practically all mammals and 

 many birds have fleas, and sometimes the same species of 

 flea is found on strangely different hosts: for example, 

 the human flea is also parasitic on the badger ; dog fleas 

 (fig. 77), too, will bite human beings ; again, certain species 

 of flea have never been found on more than one host. 



What may be termed an exchange of fleas is so common 

 as to be exceedingly puzzling to those who are engaged in 

 trying to learn their habits, and the subject is not rendered 



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