The-Two-winged Flies 337 



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 ((Estridse) 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, FIG. 476. Larva 

 dogs, cats, and even man. The larvae of still other species Ir^umc'uU^irom 

 burrow in the nasal passages of the sheep, the antelope, wood-rat, Neoto- 

 the horse, the camel, the buffalo, and various deer species. ^ S P - ( Natural 

 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 larva? 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 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 



