INTERRELATIONS OF INSECTS 



255 



the bodies of other insects; one of the few is Braula cceca, a wingless 

 dipteron found on the body of the honey bee. 



A vast number of insects, however, undergo their larval develop- 

 ment as internal parasites of other insects, and most of these parasites 

 belong to the two most specialized orders, Diptera and Hymenoptera. 



The larvae of Bombyliidae feed upon the eggs of Orthoptera and upon 

 larvae of Lepidoptera and Hymenoptera. Tachinidae are the most im- 

 portant dipterous parasites of other insects and lay their eggs most 

 frequently upon caterpillars; the larvae bore into their victim, develop 

 within its body, and at length emerge as winged insects. These parasites 

 often render an important 

 service to man in checking 

 the increase of noxious 

 Lepidoptera. 



The great majority of 

 insect parasites many 

 thousand species belong 

 to the order Hymenoptera, 

 constituting one of the 

 primary divisions of the 

 order. They are immense- 

 ly important from an eco- 

 nomic standpoint, partic- 

 ularly the Ichneumonidae, 

 of which more than ten 

 thousand species are 



already known. Our most conspicuous ichneumonids are the two species 

 of Thalessa, T. atrata and T. lunator (Fig. 275), with their long ovipositors 

 (three inches long in lunator, and four to four and three-quarters inches 

 in atrata) . Thalessa bores into the trunks of trees in order to reach the 

 burrows of another large hymenopteron, Tremex columba (Fig. 31), upon 

 whose larvae the larva of Thalessa feeds. 



The enormous family Braconidae, closely related to Ichneumonidae, 

 is illustrated by the common Apanteles congregatus, which lays its eggs 

 in the caterpillars of various Sphingidae. The parasitic larvae feed upon 

 the blood and possibly also the fat-body of their host, and at length 

 emerge and spin their cocoons upon the exterior of the caterpillar (Fig. 

 276), sometimes to the number of several hundred. Species of Aphidius 

 transform within the bodies of plant lice, one to each host, and the imago 

 cuts its way out through a circular opening with a correspondingly 



FIG. 275. Oviposition of Thalessa lunator. 

 After RILEY. 



Natural size. 



