90 INTRODUCTION. 



use of this instrument is to pierce the skin of the 

 caterpillar, and to form a conduit for conveying the . 

 eggs into the hole thus prepared for their reception. 

 When the fly has selected a caterpillar fitted for her 

 purpose, she alights upon its back, and plunges her 

 weapon into its body, chiefly at the incisures of the 

 segments, depositing an egg at every insertion, This 

 operation is repeated till no fewer than thirty or forty 

 eggs are sometimes laid in the body of a single ca- 

 terpillar. These are soon hatched in their singular 

 nidus, and the grubs which they produce imme- 

 diately begin to feed on the substance of the living 

 animal. They do not, however, devour every part 

 indiscriminately, but are taught by a wonderful in- 

 stinct to abstain from injuring any vital organ, as if 

 aware that their own existence depended upon that 

 of their unwilling foster-parent. In consequence of 

 this, the caterpillars survive for a considerable time, 

 and sometimes retain sufficient strength to assume 

 the pupa state, in which, however, they invariably 

 perish. But most frequently the grubs arrive at 

 maturity before that change takes place, and in that 

 case they escape from the body of the caterpillar by 

 gnawing a passage through its sides. Having in 

 this way effected their liberation, they arrange them- 

 selves round the sides of the caterpillar, which is 

 now so exhausted that it soon dies, and spin cocoons 

 of a fine yellow colour, in which they are transformed 

 into pupae. When the perfect fly is ready to emerge, 

 it pushes open a small lid at one end of the cocoon, 



