478 Saw-flies, Gall-flies, Ichneumons, 



the Proctotrypoidea to the digger-wasps (Sphecoidea) and to the gall-flies 

 (Cynipoidea) than to the other parasitic groups (Chalcidoidea and Ichneu- 

 monoidea). This latter arrangement is based on structural unlikeness among 

 the parasitic groups to which Ashmead gives much classificatory importance. 

 Parasitism is a condition widely spread in the animal kingdom, parasitic 

 species being found in most of the invertebrate phyla. The importance of 

 these parasites in causing disease and death and their peculiar biological 

 interest have led to much special study of them and of the particular phe- 

 nomena of parasitic life. Parasites may be external or internal as they cling 

 to the outer surface of their host or burrow within the body; permanent or 

 temporary as they live their whole life or only part of it in or on the host; 

 but in almost all cases except in those of our parasitic Hymenoptera the 

 parasite shows a more or less marked degeneration or simplification by 

 loss of parts of its body structure. Lice and fleas are the degenerate wing- 

 less descendants of winged ancestors; the intestinal worms are for the most 

 part without sense-organs; the tumor-like Sacculina, parasite of crabs, has 

 a body made up of feeding and reproductive organs and little else. But 

 the parasitic hymenoptera show little or nothing of this insidious degenera- 

 tion due to the adoption of a parasitic life. The reasons for this, however, 

 are fairly obvious when the life-history and life-conditions of these insects 

 are inspected. 



The general course of the life and the character of the various stages of 

 a parasitic hymenopteron are as follows: the winged, free-flying female (the 



males are winged and free-flying also) 

 searches, often widely, for its special 

 host species in that stage, egg or larval, 

 on or in which its eggs are to be laid. 

 This host may be always an individ- 

 ual of a particular species or may be 

 one of any of several usually allied 

 species. The hosts represent most 

 of the larger insect orders, although 

 caterpillars of moths and butterflies 

 FIG. 672. A common parasite, Merisus furnish the great majority of hosts 

 destructor, iemale, of the Hessian fly (After f he parasitic Hymenoptera. On 

 Lugger; natural size indicated by line.) J r 



the surface of the body, or, more 



rarely, inserted beneath the skin, the parasite deposits one or several eggs. 

 The footless, maggot-like larvae soon hatch, and if not already inside the 

 host's body very soon burrow into it. Here they lie, feeding on its body, 

 tissues, growing and developing until ready to pupate. They may now 

 eat their way out of the enfeebled and probably dying host to pupate in little 

 silken cocoons or fluffy silken masses on or off its body-surface, or may pupate 



