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 larvae of 

 beetles and moths and butterflies. In fact, the ichneumon 

 flies do moro 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. 



One 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 



