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; c, 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 



