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. — Wing's of Exetastes fascrpennis. 
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, 
