The Two-winged Flies 
33 8 
on the shoulders or legs or belly. They are licked off by the horse and 
swallowed, and the larvae hatch in the mouth or stomach and attach themselves 
to the stomach lining, living at the expense of the host. When many larvae 
thus live in the stomach (and as many as several hundred have been found 
in one animal) the horse suffers serious injury. The larvae live in the stomach 
Fig. 477.—Bot-fly of horse, male, Gastrophilus equi, abdomen of female and egg. (After 
Lugger; natural size of fly indicated by line.) 
and intestines through fall and winter, and late in the spring release their 
hold, pass through the intestine with the excretions, and burrow into the 
ground to pupate. The pupal stage lasts about a month, when the flies 
issue and the life-cycle begins again. A smaller species of bot-fly, Gastro¬ 
philus nasalis , with bright-yellow band across the abdomen, lays its eggs 
in the lips and nostrils of horses. For the rest its life-history is about like 
that of G. equi. 
The bot-flies, warble-flies, or heel-flies of cattle, whose larvae are found in 
small tumors under the skin, also have their eggs swallowed, and the young 
larvae may be found in the mouth and oesophagus. But from here they burrow 
out into the body-tissues of the host, finally coming to rest underneath the 
skin along the back. When the larva or grub is full-grown it gnaws through 
the skin, drops to the ground, pupates, and in from three to six weeks changes 
to the adult fly. The hides of cattle attacked by these flies are rendered 
nearly valueless by the holes, and are known as “grubby” hides. Osborn 
estimates that these warble-flies, of which we have two species, Hypoderma 
bovis and H. lineata , cause a loss of $50,000,000 annually in this country. 
The genus Cuterebra includes a number of species of which the rabbit 
bot-fly, C. cuniculi , is most familiar. The larvae lie in large warbles or tumors 
under the skin of the infested rabbit, and late in the summer the jack-rabbits 
and cottontails are so badly infested in some localities that hardly one can 
be found free from the pest. The adult is a large fly resembling a bumble- 
