THE GREAT WHITE BUTTERFLY. 155 



The unremitting and watchful researches of na- 

 turalists have, however, fully explained the seeming 

 mystery, and in its explanation brought to light 

 another wonderful provision in the beautiful economy 

 of Nature, by means of which the balance of num- 

 bers in many classes of animal life is preserved or 

 restored. Mr. Haworth observes that in dry seasons 

 the larvae of the White Butterfly multiply in such a 

 manner as to commit great havoc, but that the same 

 causes produce also an unusual number of the 

 small Ichneumon fly, Microgaster Glommeratus, 

 whose instinct it is to pierce the skin of these Cater- 

 pillars in several places, depositing an egg in each 

 incision. These eggs, rapidly hatched by the in- 

 ternal heat of the body of the victim, produce small 

 footless grubs which live upon the fatty portions 

 only of the Caterpillar's body, who continues to feed 

 more voraciously than ever for their supply, as 

 though unconscious of the presence of the destructive 

 intruders. When, however, the time for change 

 arrives, and the Caterpillar slings itself up to pass 

 to the next stage of its existence, the larvae of the 

 Ichneumon rapidly consume all the remaining in- 

 terior, leaving nothing but the external skin, which, 

 when they have perfected their cocoons for under- 

 going their own change, is burst by their increased 



