PARASITISM AND DEGENERATION 



359 



FIG. 216. The fish louse, 

 Lernoecera: a, Adult; b, 

 larva. 



Among the insects there are many kinds that live para- 

 sitically for part of their life, and not a few that live as 

 parasites for their whole life. The true 

 sucking lice and the bird lice live for 

 their whole lives as external parasites 

 on the bodies of their host, but they 

 are not fixed that is, they retain their 

 legs and power of locomotion, although 

 they have lost their wings through de- 

 generation. The eggs of the lice are 

 deposited on the hair of the mammal 

 or bird that serves as host; the young 

 hatch and immediately begin to live as 

 parasites, either sucking the blood or 

 feeding on the hair or feathers of the 

 host. In the order Hymenoptera there 

 are several families, all of whose mem- 

 bers live during their larval stage as parasites. We may 

 call all these hymenopterous parasites ichneumon flies. The 

 ichneumon flies are parasites of other insects, especially of the 



larvae of beetles and moths and butter- 

 flies. In fact, the ichneumon flies do 

 more to keep in check the increase of in- 

 jurious and destructive caterpillars than 

 do all our artificial remedies for these in- 

 sect pests. The adult ichneumon fly is 

 four-winged and lives an active, indepen- 

 dent life. It lays its eggs either in or on 

 or near some caterpillar or beetle grub, 

 and the young ichneumon, when hatched, 

 burrows into the body of its host, feed- 

 ing on its tissues, but not attacking such 

 organs as the heart or nervous ganglia, 

 whose injury might mean immediate death 

 to the host. The caterpillar lives with the 

 ichneumon grub within it, usually until 

 nearly time for its pupation. In many 

 instances, indeed, it pupates 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. 218) or in a tiny silken cocoon outside 



FIG. 217. The ichneu- 

 mon fly, Pimpla con- 

 quisitor, laying eggs 

 in the cocoon of the 

 American tent cater- 

 pillar moth. (After 

 Fiske; about natural 

 size.) 



