PARASITISM AND DEGENERATION 189 
hair or feathers of the host. In the order Hymenoptera 
there are several families, all of whose members live during 
their larval stage as parasites. We may call all these hy- 
menopterous parasites ichneumon flies. The ichneumon 
flies are parasites of other insects, especially of the larve of 
beetles and moths and butterflies. In fact, the ichneumon 
flies do more to keep in check the increase of injurious and 
destructive caterpillars than do all our artificial remedies 
for these insect pests. The adult ichneumon fly is four- 
winged and lives an active, independent life. It lays its 
eggs either in or on or near some caterpillar or beetle grub, 
and the young ichneumon, when hatched, burrows about in 
the body of its host, feeding on its tissues, but not attacking 
such organs as the heart or nervous ganglia, whose injury 
would mean immediate death to the host. The caterpillar 
lives with the ichneumon grub within it, usually until nearly 
Fig. 115.—Parasitized caterpillar from which the ichneumon fly parasites have 
issued, showing the circular holes of exit in the skin. 
time for its pupation. In many instances, indeed, it pu- 
pates, with the parasite still feeding within its body, but it 
never comes to maturity. The larval ichneumon fly pupates 
either within the body of its host (Fig. 115) or in a tiny 
silken cocoon outside of its body (Fig. 116). From the 
cocoons the adult winged ichneumon flies emerge, and 
after mating find another host on whose body to lay their 
eggs. 
Sr Giie of the most interesting ichneumon flies is Thalessa 
(Fig. 119), which has a remarkably long, slender, flexible 
ovipositor, or egg-laying organ. An insect known as the 
