62 THE INSECT WORLD. 
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 im its numerous 
trachez.” 
‘‘ Whatever may be the value of this explanation,” adds M. 
Joly, “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 govern- 
ment 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 
(istrus imprisoned in the stomach of the horse leaves the mem- 
brane 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 carried away with the excrementitious matter. It 
traverses, mixed with the excrementary bolus, the whole length of 
the intestinal canal, leaves it by the anal orifice, and on touching 
the ground at once seeks a suitable place to go through the last 
but one of its metamorphoses. : 
The skin then gets thick, hardens, and becomes black ‘acide 
All the organs of the animal are composed of a whitish amorphous 
pulp, which soon assumes its destined form, and the insect becomes 
perfect. It then lifts a lid at the anterior part of its cocoon, 
emerges, dries its wings, and flies off. 
The Bot-fly (Gistrus bovis, Fig. 45) has a very hairy body, 
large head, the face and forehead covered with light yellow hair, 
the eyes brown, and the antennz black. The thorax is yellow, 
barred with black; the abdomen of a greyish white at the base, 
covered with black hair on the third segment, and the remainder 
of an orange yellow; the wings are smoky brown. 
As soon as the cattle are attacked, they may be seen, their 
