RELATION TO OTHER ANIMALS 



179 



the anus. When it reaches the ground it burrows into 

 it at once, changes to a pupa, and not until a month 

 thereafter does it transform into an adult fly. A full 

 year is thus required for the development of the species, 

 and there is only one danger season for infection the 

 period during July and August when the flies are on 

 the wing. 



FIG. 80. The horse bot, Gastrophilus equi: a, egg, attached to hair; b, young; 

 c, full-grown larva; d, adult fly; e, hooks of larva. 



A few bots, while not of course an advantage, will 

 not hurt a horse. More than a few will cause trouble, 

 in proportion to the amount of infestation. There is, 

 naturally, the irritation to the membrane to which the 

 insects are attached, causing digestive derangement; 

 there is also the positive drain upon the system by the 

 feeding maggot which is apt to weaken the animal and 

 to intensify the effect of the digestive trouble; and 

 finally, when there are many bots, they tend to form a 

 mechanical obstruction to the passage of food from 



