DIPTEBA. 



61 



Fig. 44, taken from a drawing which accompanies M. Joly's 

 memoirs, represents the state of a horse's stomach attacked by 

 the Gad-fly. 



The larvae are of a reddish yellow, and each of their segments 

 is armed at the posterior edge with a double row of triangular 

 spines, large and small alternately, yellow at the base, and black 

 at the point, which is always turned backwards. The head is 

 furnished with two hooks, which serve to fasten the larva 

 to the interior coats 

 of the stomach. The 

 spines with which the 

 whole surface of the 

 body is furnished con- 

 tribute to fix it more 

 solidly, preventing the 

 creatures, by the man- 

 ner in which they are 

 placed, from being 

 carried away by the 

 food which has gone 

 through the first pro- 

 cess of digestion. 



It is probable that this larva, so singularly deposited, is 

 nourished by the mucus secreted by the mucous membrane of 

 the stomach, and that it breathes the air which the horse swallows 

 with its food during the process of deglutition. 



It must be acknowledged, however, that it is in the midst of a 

 gaseous atmosphere which is very unhealthy, for nearly all the 

 gases generated in the stomach of the horse are fatal to man and 

 to the generality of animals, as they consist of azotic, carbonic, 

 sulphuretted hydrogen, and hydro- carbonic acids. To explain 

 how the insect can live under such circumstances, M. Joly has 

 suggested the following ingenious hypothesis : 



"When the stomach which the larva inhabits," says this 

 learned naturalist, " contains only oxygen or air that is nearly 

 pure, the insect opens the two lips of the cavity which contains 

 the spiracles, and breathes at its ease. When the digestion of the 

 alimentary substance generates gas which is unfit for respiration, 



Fig. 44. Portion of the stomach of a Horse, and larvse of 

 (Estrus (gastrus) equi. 



