622 



THE STUDY OF INSECTS. 



destruction of insect pests ; but in most cases these other 

 insects are simply predaceous, pouncing upon and destroy, 

 ing such insects as they can overcome. But the true para- 

 sites act in a very different way. Although some species 

 are external parasites, most of them live within the bodies 

 of their victims, within which they pass their entire larval 

 existence. Their presence in this strange situation is due 

 to the fact that the parent lays her eggs within or upon the 

 body of the insect to be destroyed. When the egg is laid 

 upon the body of the victim, the larva as soon as it hatches 

 bores its way into the body. So in either case the young 

 parasite is in the midst of suitable food. It is probable that 



Fig. 748. — Wings of Exetastes fascipennis, 



the parasite feeds only on the blood of its host ; hence the 

 parasitized insect is not destroyed at once, but lives on with 

 the parasite within it, which gradually attains its growth. 

 Finally, the parasitized insect perishes ; and from the larva 

 that has been nourished in its body there is developed a 

 winged creature, which in turn lays its eggs in other victims. 

 Frequently a parasitic insect lays several eggs within a single 

 victim, so that a number of parasites may be developed 

 within the body of a single insect. Each species of these 

 parasites infests only certain insects, each insect having, to 

 a great extent, its peculiar parasites. 



Although the Ichneumonidae include some minute forms, 



