THE FOUR-WINGED PARASITES 183 



The mother insect deposits one or more eggs in a caterpil- 

 lar. The eggs hatch into tiny maggots. The maggots 

 grow inside the body of the host, absorbing its substance. 

 They finally change to pupae, generally within the body of 

 the dead or dying host, to emerge later as fully developed 

 Chalcid Flies. 



Many of the Chalcids are parasites of borers, gallflies, 

 and leaf miners. As a rule, the larvae in such cases develop 

 as external parasites upon their Victims. You may often 

 find examples of these if you will examine a number of 

 leaf-miner cavities. Many other Chalcids are parasites 

 upon the larvae of the ichneumon flies — that is, they are 

 secondary parasites. In such cases, also, the Chalcid larvae 

 generally develop as e 'ternal feeders upon the ichneumon- 

 fly larvae. 



A considerable number of the Chalcid Flies, however, 

 have lives that cannot be told in the few words that will 

 give the life history of the others. They go through so 

 wonderful a course of development that it has required the 

 most careful studies of many scientists for years to trace it 

 out. Even now we know but little, and that only in re- 

 lation to a very few species. But what we do know enables 

 us to explain certain facts which before were very puzzhng. 



If you will bring in a lot of nearly full-grown larvae of 

 the Cabbage Plusia and keep them in a vivarium, a good 

 many of them will be likely to die because they are in- 

 fested with parasites. If you will isolate those that die, 

 keeping them in small bottles or boxes so you can see how 

 many parasites come from each caterpillar, you will prob- 

 ably find that from some there emerge hundreds if not 

 thousands of tiny four-winged Chalcid Flies. In one case 

 three thousand such flies were counted from one cater- 

 pillar. 



