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. 
SARCOPSYLLID.E. 
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. PuLicimE. 
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, 
