LH. XI. J THE GAD-FLY. 205 



which affords it shelter, food, and an equal tempera 

 ture in all seasons. 



The egg, having been introduced, gives rise to a 

 tumour, which gradually enlarges with the growth 

 of its inhabitant ; during this period, the hole perfo- 

 rated by the insect to deposite its egg, so far from 

 closing, becomes wider. The bot, or worm, requires 

 a constant supply of fresh air ; and its stigmata, si- 

 tuated at its posterior extremity, are constantly ap- 

 plied to the orifice. In this it acts like a skilful sur- 

 geon, who desires to keep an issue open ; for the ap- 

 plication of its tail to the orifice keeps up the irrita- 

 tion and prevents its closing. Not only must there 

 be an opening for the admission of the air, but, as 

 the bot subsists on the pus which arises from the in- 

 terior of the tumour, if there were no outlet for the 

 excess, its too great accumulation would convert the 

 nest into an abscess, in which the animal, instead of 

 being nourished, would be destroyed. 



When it has acquired its full growth, its old habi- 

 tation becomes unfit for its approaching metamor- 

 phosis ; accordingly, it dilates the orifice of the tu- 

 mour by using its own body in the same manner as 

 a surgeon applies a sponge-tent. It thrusts a ring or 

 two through the hole, and then withdraws them, and 

 so on, until it crawls out backwards. This opera- 

 tion, according to Reaumur, is usually completed in 

 the cool of the morning. The worm, having es- 

 caped from its den, rolls off the animal on which it 

 has fed, and falling on the ground, seeks the shelter 

 of some fissure, or stone, and there its own skin har- 

 dens into a black cocoon, within which it under- 

 goes its last change, when it comes out a perfect 

 insect. 



I.— S 



