400 NORTH AMERICAN DIPTERA 



Family Gasterophilidae — The Horse Bot Flies 



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Gasterophilus intestinalis, wing and part of head showing antennae. 



Flies of moderate size, without bristles, the thorax pilose; mouth 

 parts very small; apical crossvein absent. 



Head short and deep, pilose; front broad; antenna^ small, situated 

 in deep pits separated by a carina; second segment without groove; 

 arista bare; mouth parts very small, the x^alpi swollen, though short, 

 larger than the proboscis; ocelli present. Scutellum large; postscutellum 

 absent. Legs of moderate length. Apical crossvein absent, the vein clos- 

 ing the discal cell also absent and the fourth and fifth veins evanescent 

 apically. Squama small. Abdomen elongate oval in the male, slightly 

 tapering, in the female tapering apically, the ovipositor large and pro- 

 tuberant. 



The bot flies are frequently serious pests of horses and may be 

 collected in the neighborhood of these animals, although they are re- 

 markably good fliers. The larva? live in the stomach, throat and nasal 

 passages of horses and related animals and reports of their occurrence 

 in other animals are very rare. The eggs are laid by G. intestinalis on 

 the legs or shoulders of the victim and are taken into the mouth when 

 the animal licks these parts; G. veterinus lays its eggs on the underside 

 of the head and it is believed that the larvae make their way through 

 the skin and into the mouth and thence to the phar;^aix where they at- 

 tach themselves. G. hamorrhoidalis lays its eggs about the lips of the 

 horse and the young larva? pass to the rectum where they fasten them- 

 selves to the walls. Horses apparently recognize the flies, at least those 

 of veterinus, and it is not an uncommon sight to see horses with tlicir 

 heads resting on each other's backs as a protection against egg laying. 



Gasterophilus has usually been placed in the OEstridae but Girsch- 

 ner long since pointed out that it was more closely related to the 

 Museidffi (Anthomyidge) and it has been included in tliat family. Other 



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