DIPTERA. 59 



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

 point, which is always turned backwards. The head is furnished 

 ivith two hooks, which serve to fasten the larva to the internal coats 

 of the stomach. The spines with which the whole surface of the 

 body is furnished contribute to fix it more perfectly, preventing the 

 reatures, by the manner in which they are placed, from being 

 carried away by the food which has gone through the first process 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 

 til at 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 

 nitrogen, carbonic acid, sulphuretted hydrogen, and carburetted 

 hydrogen. To explain how the insect can live under such circum- 

 stances, 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, or when the 

 spiracles run the risk of being obstructed by the solid or liquid 

 substances contained in the stomach, it shuts the lips, and continues 

 to live on the air contained in its numerous tracheae." 



" Whatever niay be the value of this explanation," adds M. Joiy, 

 '' it is nevertheless very curious to see an insect pass the greater part 

 of its life in an atmosphere which would be instantly fatal to most 

 animals, and in an organ where, under the government of life, 

 chemical processes bring about the most wonderful changes of the 

 food into the substance of the animal itself But how can the insect 

 itself resist the action of these mysterious powers, and remain alone 

 intact in the midst of all these matters which are unceasingly 

 changing and decomposing ? This is another question which it is 

 difficult, or rather impossible, to explain in the present state of 

 science ; another enigma which humbles our pride, and of which He 

 who has created both man and the worm alone knows the secret." 



Arrived at a state of complete development, the larva of the 

 CEstrus imprisoned in the stomach of the horse leaves the membrane 

 to which it has been fixed, then directing the anterior part of its body 

 towards the pyloric opening of the stomach, allows itself to be 



