30 CIRCULAR 148, U. S. DEPARTMENT OF AGRICULTURE 
Rocky Mountain States, but it is spreading in all directions. The 
adult flies of this species deposit their eggs along the edge of the lips 
and cause much annoyance to horses. The nose fly is the most dan- 
gerous of the botflies and is the one which is responsible for many 
accidents because its annoyance makes horses panicky and causes 
them to run away. ‘The bots of this species occur in the stomach and 
duodenum, and when full grown also attach in the rectum. 
Life history.—The eggs are glued to the hair of horses by the adult 
flies. The eggs of the common horse bot are hatched apparently by 
the biting and licking of the horse, and are taken into the mouth on 
the lips and tongue. The small spiny bots burrow into the mucous 
membrane of the mouth, especially the tongue, where they remain 
from 21 to 28 days before emerging and passing on to the stomach. 
FIGURE 22.—Portion of a horse’s stomach, showing bots, Gastrophilus intestinalis, 
attached to the inner lining of the stomach wall. Natural size 
The habits of the nose bot are apparently very similar to those of the 
common bot. The eggs of the chin fly hatch without friction and the 
young bots crawl to and enter the mouth. Whether the throat bot 
penetrates the tongue is not yet known. When the bots reach the 
stomach they attach themselves to its wall and remain there for a 
period of from 8 to 12 months, during which time they attain their 
complete larval development and then pass out of the body. While 
the common bot and the throat bot pass out of the body directly 
with the manure, the nose bot reattaches itself to the rectum on its 
way out, then becomes detached and reattaches itself to the anus, 
where it may remain for three or four days, and finally drops to the 
ground. Usually all fully developed bots pass out of the body by 
October and those which overwinter in the horse are the young forms. 
These bots pupate in the ground and 20 to 70 days later emerge as 
