No. 637] 



PARASITISM 



137 



teristic of this type of insect parasitism is its restriction 

 to the larval stages, although extending over the entire 

 growth period. This is by no means unique among ani- 

 mals, but it is one of the distinguishing characteristics 

 between this type of parasitism and the one next to be 

 considered. In its perfected state this relation between 

 host and parasite is a marvelously balanced association 

 and one which we might expect to furnish valuable data 

 on the correlation between taxonomy and habits. 



A second type of parasitism encountered in the Hymen- 

 optera is that exemplified by most of the parasitic bees 

 and wasps. This has recently been discussed by Wheeler 

 ('19). Here the parasitic larva is really at first preda- 

 tory so far as food-habits are concerned, devouring the 

 host larva shortly after hatching. The sequence of events 

 is initiated by the preparation of the larval food-supply 

 of the host by the mother bee or wasp. Most solitary 

 wasps store up, in a nest which they have prepared, one 

 or more insects which they have paralyzed by means of 

 the sting, and attach one of their eggs to the body of the 

 prey. Under normal conditions the larva hatching from 

 this egg consumes the prey, attaining full growth, and 

 later completing its metamorphosis. Bees behave in 

 much the same manner, but the store of food in the nest 

 consists of honey and pollen. When parasitism inter- 

 venes, the egg of the parasite is also placed upon the food 

 supply, and on hatching, the larva of the host and para- 

 site find themselves in proximity, each ready to appro- 

 priate the contents of the nest. In numerous cases that 

 have been studied (Graenicher, '05), the larva of the 

 parasitic form has more powerful jaws than its rival, and 

 encounters little difiiculty in destroying it. It now pro- 

 ceeds to consume the food-supply exactly as the host 

 larva would have done, casting off its enlarged mandibles 

 at the first molt. Thus the actions of the larva savor 

 not at all of parasitism, but it is in the fixed habits and 

 instincts of the adult, which require the nests of particu- 

 lar wasps and bees, that the parasitic relation holds. 



