HYMENOPTERA 



249 



upon or within a larva or an egg, the larger forms laying but a 

 single egg on a larva, while the smaller species may deposit a con- 

 siderable number within a large caterpillar. The young larva at 

 once enters the body of the host and feeds upon its blood, not 

 interfering with the principal tis- 

 sues and organs, so that the host 

 goes on growing and furnishing 

 food to the parasite. Finally, 

 however, the parasite so depletes 

 the vitality of the host that it 

 dies, though often not until it 

 has transformed to a pupa. The 

 parasitic larva then spins its co- 

 coon, usually either within or upon 

 the dead host, and in due time 

 the adult parasite emerges and 

 continues the good work. To a 



FIG. 394. Long-tailed ichneumon-fly (Thalessa lunator). (Natural size) 



The parts of the long ovipositor normally lie together as a single organ ; the figure at the left 

 shows the manner of inserting the ovipositor in wood. (After Comstock) 



certain extent many parasites are peculiar to certain host insects, 

 though many of them attack various larvae or caterpillars having 

 similar habits. Frequently many of our worst insect pests are 

 brought under control by the beneficent work of these little parasites, 



