The Two-winged Flies 355 



While the adult fleas are commonly seen, particularly in lands of soft 

 climate, like Italy and California, in immature form these insects are wholly 

 unfamiliar. The larvae (Fig. 506) are small, slender, white, footless, worm- 

 like grubs, with the body composed of thirteen segments, the first being the 

 small brown head bearing short antennae and biting mouth-parts, but no 

 eyes. The larvae seem to live on dry vegetable dust, the excreta of adult 

 fleas, and other organic detritus. The larval life varies much in duration 

 in different species, and even in the same species under varying conditions. 

 In our commonest species, the cat- and dog-flea, Pergande has found the 

 larval life to last only one or two weeks, the whole development from egg to 

 adult being completed sometimes in a fortnight. When full-grown the 

 larva spins (usually) a thin silken cocoon in the dust or litter in which it lies, 

 within which it pupates. 



The parasitic habits of fleas vary from a very temporary character to one 

 approaching permanence. In such forms as the human flea and the dog- 

 flea no stage of the immature life is passed on the body of the host (although 

 the eggs of the dog-flea are usually laid on the hairs of the host, but so loosely 

 attached that they fall off before the larvae emerge), but in the burrowing 

 kinds like the "chigoe" or "jigger," where the females become completely 

 encysted in the skin of the host, the young hatch in the tumor, and unless 

 carried out by pus probably develop there. But taken altogether the fleas 

 are to be considered as belonging to the category of "temporary external 

 parasites. " 



The species known in this country represent two families which may be 

 separated by the following key: 



Small fleas with proportionally large head; female a stationary parasite with worm- 

 like or spherical abdomen, burrowing into flesh of the host; labial palpi 

 i-segmented; no "combs" of spines on head, thorax, or abdomen. 



SARCOPSYLLIDiE. 



Larger fleas with proportionally small head; adults active temporary parasites, 

 with abdomen always compressed; labial palpi 3- to 5 -segmented; head, 

 thorax, or abdomen often with "combs" of spines 



Of the Sarcopsyllidae but two genera are known, one, Sarcopsylla, includ- 

 ing the common jigger-flea, infesting various mammals and man in the 

 tropics and probably occurring in Florida and southern Texas, and Xes- 

 topsylla, the common chicken-flea, being distinguished by having the head 

 not angularly produced. 



The jigger-flea, or chigoe, Sarcopsylla penetrans (not to be confused with 

 a minute red mite, common on lawns, which burrows into the skin and is 

 also called "jigger" or "chigger"), was described by Linnaeus in 1767 and 

 has been commonly known as a pest of man in tropical and sub-tropical 

 countries ever since. It also infests many domestic animals, as the dog, cat, 



