522 HYMENOPTERA 



species attacks a primary parasite, are extremely numerous. It 

 is also pretty certain that tertiary parasitism occurs, and Riley 

 is of opinion that even quaternary destruction is not outside the 

 range of probability. 



The physiological problems connected with Insect parasitism 

 are of great interest to the entomologist ; the modes of nutrition 

 and respiration of these encaged creatures could not fail to be 

 most instructive were we fully acquainted with them. It is 

 obvious that when an Insect-egg is laid inside another Insect's 

 egg, and the parasite has to undergo the whole of its growth 

 therein, it is in the strangest condition as regards nutrition. It 

 is unnecessary for the intruded egg to have yolk of its own ; 

 moreover, the embryonic mode of nutrition may be continued 

 during what would, with other Insects, be the larval period. 

 And it seems to be the case that both these conditions are 

 actually met with in the lives of egg-parasites. The embryology 

 and post-embryonic development of parasitic Hymenoptera have 

 already been ascertained to be of the most extraordinary nature. 

 Great variety, however, will no doubt be found to exist, as will be 

 readily understood if we tabulate the conditions of the early life 

 of various parasitic Hymenoptera. 



1. The egg may be laid outside a larva, and the embryonic 

 and larval developments may both be passed on the exterior. 



2. The egg may be laid and the embryonic development 

 passed through, outside the host, but the parasite on hatching 

 may enter the host, so that the post-embryonic development is 

 passed in the lymph of the host. 



3. The egg may be laid inside the host, both embryonic and 

 post-embryonic developments being gone through in the fluids of 

 the host. 



4. The egg may be laid inside another egg, the embryonic 

 and post-embryonic developments being passed therein. 



We shall find that all these conditions exist in the Insects 

 we are about to consider. 



We shall treat the series as composed of ten families ; but 

 we must remind the student that this great subject is still in a 

 very unadvanced state ; the combined efforts of generations of 

 naturalists will be required to perfect it. Of the ten families 

 five are comparatively insignificant in number of species. Many 

 of the Cynipidae are not parasitic in habits, but live in galls. 



