928 



AN INTRODUCTION TO ENTOMOLOGY 



The vein that persists in the wings of chalcid-flies consists probably 

 of the coalesced subcosta, radius, media, the stigma, the radial 

 cross-vein, and the base of vein Rs; but writers on the Chalcidoidea 

 make use of a special set of terms in describing the different parts of 

 this compound vein. These are the submarginal vein or subcostal vein 

 (Fig. 1 1 66, a), the marginal vein (Fig. 1166, b), the postmarginal vein 

 (Fig. 1 166, c), and the stigmal vein (Fig. 1166, d) 



The chalcid-flies constitute an exceedingly important group of in- 

 sects from an economic standpoint ; and nearly all of them are bene- 

 ficial, being parasites that do much to keep in check noxious insects. 

 A few species, however, are ph\i;ophagous ; among these are those of 

 the genus Isosoma that infest the stalks of growing grain, and species 

 of several genera that infest the seeds of various plants. While these 

 are noxious, the fig-insects, forming the subfamily Agaonins, although 

 phytophagous are very beneficial to man. 



Insects in all stages of their development suft'er from the attacks 

 of chalcid-flies, eggs, larvae, pupee, and even adults in a few cases 

 being attacked by them. The larvae of chalcid-flies usually feed 

 within the body of their host, but some species are external parasites 

 of other larvae. 



While the development of most species of chalcid-flies is a normal one, in 

 certain species remarkable modifications of the usual course exists. The phe- 

 nomenon of polyembryony which has been observed in several species is dis- 

 cussed on an earlier page. Another modification of the usual course of develop- 

 ment is the existence of a hypermetamorphosis that occurs in some genera. 



Fig. 1 167. — Planidium of Perilampns. From left to 

 right, dorsal, lateral, and ventral views. (After Ford.) 



Fig. ii68.—Peril- 

 ampus hyalinus. 

 Mature larva, 

 greatly enlarged. 

 (From Smith.) 



In these cases the larvas when they leave the egg differ greatly in form from 

 ordinary chalcid-larvae (Fig. 1167). and are active, moving about in search of 

 their prey. This active instar was termed by Professor Wheeler, who first de- 



