338 



The Two-winged Flies 



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. 

 Lugger; natural size of fly indicated by line.) 



(After 



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 nasahs, 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 larva? 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- 



