PREDACEOUS AND PARASITIC INSECTS 47 
the dragon fly, poised in air and 
waiting to pounce on some unwary 
enat or fly, is predaceous. 
A parasitic insect, on the other 
hand, usually is highly specialized 
for existence onsome particular species 
of host, and has reached such depend- 
ence on its host that if the latter 
dies before the parasite has completed 
Fic. 48.— Larva, showing 
exit holes of parasites. Fic. 49.— Eggs of a parasite on a cut- 
Original. worm. Original. 
its life round, the parasite perishes. Many parasitic insects live 
within the bodies of their hosts. A familiar example is found in the 
species that lays its eggs in the body of the tomato worm, the 
parasitic grubs finally gnawing their way to the surface, where they 
spin tiny, white cocoons on the body of their host. 
For most of us, the 
tremendous work of para- 
sitic and predaceous spe- 
cies in destroying insect 
pests passes unnoticed. 
It is brought to mind 
when we see or hear of 
a bs break 80 : . ; 
. bad eu break of Some Fic. 50.— Cocoons of parasites on a larva. 
injurious Insect, and later Original. 
observe that the threaten- 
ing species has suddenly grown scarce — sometimes seemingly disap- 
peared from the face of the earth in the very localities where it 
had been abundant. If we were to follow up such cases carefully, 
we should find, as a rule, that as soon as the threatening species 
