DIPTERA. 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 larvee 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. 

ee 
through the first pro- SSS 
i a Fig. 44.—Portion of the stomach of the horse, and larvee 
cess of digestion. of Gstrus (gasterophilus) equi. 
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 nitrogen, carbonic 
acid, sulphuretted hydrogen, and carburetted hydrogen. ‘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, 
