148 INJURIOUS AND USEFUL INSECTS 



fleshy lobes at its hinder end, which can meet and completely 

 shut off all access to the spiracle ; in the living larva these 

 lobes open and close continually. 



The bots cluster together, and fifty are sometimes found near 

 together in one stomach. They are more commonly attached 

 at the lower end than elsewhere. Now and then one will be 

 met with in the intestine, or even in the oesophagus. They bury 

 themselves in the raucous lining, and feed upon the products 

 of the inflammation which they themselves set up. When the 

 bots are removed holes are left where they lay. Bracy Clark 

 tries hard to believe that bots, like the blisters and issues of 

 the surgeon, may be beneficial to the horse. But it is not 

 easy to persuade ourselves that so violent a remedy can be 

 wholesome, especially when we remark that it is administered 

 whether there is disease or not. Even if this explanation is 

 allowed to pass, the position cannot be saved, for parasites 

 are known which regularly cause their hosts to die a painful 

 death — a result which no ingenuity can represent as beneficial 

 to both parties ! Horses do not appear, however, to suffer so 

 much from bots as one would have expected. Bracy Clark 

 tells us that few of those which are pastured on commons 

 escape, and all horses turned out to graze in autumn run a 

 great risk of infection, more in some districts than in others. 

 Horses which work in cities and towns are generally exempt. 



When the bot is full-fed, that is, about June, it looses its 

 hold, and passes down the intestine. Even in this latest stage 

 it sometimes causes the horse much annoyance by attaching 

 itself for a time to the lower end of the rectum. Then it drops 

 off, or is passed out with the dung. Those bots which are 

 lucky enough to light upon soft ground, enter it and pupate. 

 As in the blow-fly, the larval skin is not cast, but retained as 

 an outer protective envelope. At the head-end a pair of small 

 projections protrude through holes in the larval skin. These 

 are the breathing-holes of the pupa. After several weeks spent 

 underground as a pupa, the fly becomes free by bursting open 

 a defined part of the larval skin, which is raised as a circular 

 lid. The raising of the lid is effected by the protrusion from 

 the forehead of a bladder-like swelling, and this singular con- 

 trivance is made use of by the blow-fly, and many other 

 Diptera, as well as by the bot-fly. Through the opening the 

 fly creeps out, leaving both larval and pupal skins behind. 



