316 EMBRYOLOGY OF INSECTS AND MYRIAPODS 



trophamnion becomes relatively thinner, and its paranuclear masses are 

 absorbed until the former is represented merely by a very thin membrane 

 surrounding the young larva. In late spring the young larvae rupture 

 the trophamniotic membrane and begin to feed on the body content of the 

 host. By the time the content of the host is devoured, the larval para- 

 sites are full grown. The parasites remain as pupae or adults in the 

 carcass of the host, emerging as adults in early fall. 



Some eggs develop monembryonically. In this case the embryonic 

 region does not divide into two parts, the germ forming a single blastula, 

 as in some other platygastrids. Egg structure, maturation, and fecunda- 

 tion of Platygaster hiemalis are similar to that of P. dryomyiae as described 

 by Silvestri, though Silvestri (1937) considers twinning in the former as 

 questionable. 



Platygaster vernalis (P. zosine Walker), another parasite studied by 

 Leiby and Hill (1924), differs from P. hiemalis in that it develops only by 

 the polyembiyonic process, one egg giving rise to approximately eight 

 individuals. There is but one generation annually. The adult parasites 

 emerge from their pupal chambers within the host carcass in the spring 

 and oviposit in the eggs of the spring brood of Hessian flies. The larval 

 and pupal stages are completed by late August. 



A single egg is deposited in the egg of the host in such a way that it 

 always becomes lodged in the mid-gut of the embryo host where develop- 

 ment to the primary larval stage of the parasite is completed. Matura- 

 tion is similar to that of P. hiemalis, the two paranuclear masses being the 

 product of the polar bodies. The embiyonic nucleus divides, each 

 daughter nucleus becoming differentiated into a germ. Growth of the 

 trophamnion permits each one-celled germ to divide until each reaches 

 the blastula stage. Organogeny of the parasite then follows, whereupon 

 the trophamnion becomes vacuolated, thinned, and finally breaks down to 

 release the parasites in their primary larval stage. The liberated primary 

 larvae within the stomach of the full-grown larval host now feed directly 

 on the host's tissues. When the parasite larva is mature, it pupates 

 inside the host puparium to emerge the following spring. 



The Cabbage-looper Parasite (Litomastix floridana) 



Litomastix floridana (Paracopidosomopsis floridanus) , a chalcid parasite 

 of the common cabbage looper {Autographa brassicae) studied by Patter- 

 son (1921), deposits its egg in the host egg at any time but does not 

 parasitize the young caterpillar after hatching. It lays one or two eggs at 

 each oviposition in any part of the host egg. Unless the egg is included 

 in the tissues of the embiyo host, it will not complete its development. 

 Eggs develop whether fertilized or not, those laid by virgin females always 



