THE BOT-FLY OF THE HORSE 



147 



Fig. 83. — Larva 

 of bot-fly. 



an earthworm moves through garden-mould, pushing out its 

 body, grasping with the spines, and then drawing up the 

 hinder segments. When it reaches the mucous lining of the 

 stomach, it takes firm hold with its mouth- 

 hooks and is then secure. Bots taken from the 

 stomach of a dead horse creep and grasp with 

 great easej they can make their way through 

 loose earth, or attach themselves to the skin 

 of the hand. When it first enters the stomach 

 the hot is very small, and much more slender 

 in proportion to its length than afterwards. 

 As it grows, it casts its skin more than once. 

 When it has attained full size, the larva is 

 25 mm. (i inch) long, somewhat conical, taper- 

 ing towards the head-end, and ending rather 

 abruptly behind. Every segment except the 

 two or three hindmost is furnished with a 

 double row of backward - pointing spines, 

 which are reddish - brown and tipped with 

 black. On the nearly circular surface which ends the body 

 is a large compound spiracle, consisting of three long curved 



slits on one side and 

 as many on the other, 

 all lying in one chitin- 

 ous plate. The slits 

 are protected by fine 

 gratings, andleadintoa 

 common air-chamber, 

 from which the main 

 air-tubes, or trachese, 

 are given off. So far 

 the description applies 

 pretty well to the 

 spiracle of any maggot 

 belonging to one of 

 the higher families of 

 Diptera, but an addi- 

 tional precaution 

 against choking is needed in a maggot which dwells in the 

 stomach of a living animal, and is liable to have its breathing- 

 hole obstructed by viscid liquids. The bot has a pair of 



Fig. 84. — (i) Eggs of bot-fly ; (2) the same magnified ; 

 (3) larvae in stomacli of horse. After Bracy Clark. 



