62 THE INSECT WOELD. 



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 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 

 (Estrus 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 

 intestine channel, 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 inside. 

 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 ((Estrus bovis, Fig. 45) has a very hairy body, 

 large head, the face and forehead covered with light yellow hair, 

 the eyes brown, and the antennae 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 



