PARASITES OP ANIMALS. 27 



two last segments do ftot show -the spines, and the posterior 

 end of the body is rather truncate, with two fleshy lobes, 

 which cover, more or less completely, a chitinous plate, in 

 which the spiracles are situated. The mouth is at the small 

 end, between the hooks, which represent the maxillae ; the 

 mandibles, above and between these, are small and slender. 

 At the places where they attach themselves there are found 

 small pits or depressions ; but they do not appear to cause 

 ordinarily much irritation or inflammation, unless in great 

 numbers, as they sometimes are. They have been accused of 

 perforating the walls of the stomach, and this may possibly 

 be the case in very rare instances ; but the perforations of 

 the stomach, so often found in post-mortem examinations, 

 are generally caused by the digestive action of the gastric 

 juice after the death of the animal ; for after death this fluid 

 will attack the stomach as quickly as any other flesh. 



When full grown they loosen their hold, and passing 

 through the intestine, fall to the earth. But they often fix 

 themselves for a few days upon the lining of the intestine, 

 and then cause considerable irritation and pain. Probably 

 most of the acute symptoms of " bots " in horses are caused 

 in this way. They often attach themselves in and about the 

 anal orifice, and in that situation cause much suffering and 

 trouble to the horses, which under these circumstances, re- 

 fuse to travel rapidly, and even after whipping, soon relapse 

 into a slow walk, as if lazy. But the bots can be easily re- 

 moved from this location, much to the relief of the horse. 

 The bot-larvge get their growth and leave the horses from the 

 latter part of May to July, or even later. They enter the 

 surface of the soil and change to pupae within the shrunken 

 skins of the larvae. They remain in the pupa state forty or 

 fifty days, and emerge in the same way as the preceding spe- 

 cies ; consequently the flies may occur from the last of June 

 till September, or later. 



The amount of injury caused by the bots of horses has long 

 been a matter of dispute, many writers claiming that they are 

 very injurious, and evert at times, fatal ; while others deny this, 

 and consider them as harmless, or even beneficial. This can 



