337 
The-Two-winged Flies 
bumble-bees, and locusts. “The eggs,” according to Williston, “are laid 
directly upon the bodies of the bees or wasps during flight. The young 
larvae burrow within the abdominal cavity of their host and there remain, 
the posterior end directed toward the base of the abdomen, feeding upon 
the non-vital portions, until ready to transform into the mature fly, when they 
escape from between the abdominal wings of the insect.” The quiescent 
pupal stage is then passed within the body of the host, a rather unusual 
phenomenon in insect life. 
In the genera Conops and Physocephala (Fig. 475) the abdomen is distinctly 
peduncled as in the thread-waisted wasps, while in Myopa, Zodion, Oncomyia, 
and others the abdomen is sessile or constricted only at the very base. 
Under the name bot-flies (CEstridae) some of the most interesting members 
of the order Diptera are widely, but superficially, known. The flies themselves 
are much less familiar than their eggs and larvae, the glistening white eggs 
of some species being often seen attached to the flanks, legs, 
or feet of a horse or cow, and the stomach-inhabiting larvae 
being well known to stockmen as the cause of much suffer¬ 
ing and injury to their animals. In addition to the “bots” 
which live in the stomach and intestines of horses and 
cattle, several other species live under the skin of the same 
animals, as well as of goats, sheep, antelope, rabbits, rats, 
dogs, cats, and even man. • The larvae of still other species 
burrow in the nasal passages of the sheep, the antelope, 
the horse, the camel, the buffalo, and various deer species. 
The flies are heavy-bodied, often densely hairy, banded in¬ 
sects, looking rather like small bumble-bees whose mouth-parts are so atrophied 
that they can probably take no food at all. They lay their eggs on the hairs 
or skin of their special host animal, and the larvae on hatching bore directly 
through the skin and into the tissues of the host, or, as in the case of the 
familiar bot-fly of the horse and the heel-fly or warble of cattle, the eggs are 
taken into the mouth of the host by licking, swallowed, and thus introduced 
directly into the stomach, to whose walls the larvae either attach themselves or 
through which they burrow into the true body-cavity of the host. 
Less than 100 species of bot-flies are known in the whole world, 
but the parasitic habits and resulting economic importance of these flies 
have resulted in making the family well known. The most widely dis¬ 
tributed and best known species is probably the horse bot-fly, Gastrophilus 
equi (Fig. 477). This fly, which may be seen in open sunny places along 
the roadways, is about J inch long, brownish yellow, with some darker 
markings, but much resembling a honey-bee in appearance. The female 
has the abdomen elongate and bent forward underneath the body. The 
light-yellow eggs are attached by a sticky fluid to the hair of the horse 
Fig. 476. —Larva 
of bot-fly, Cutere- 
bra cuniculi , from 
wood-rat, Neoto- 
ma sp. (Natural 
size.) 
