114 PROC. ENT. SOC. WASH., VOL. 24, NO. 5, MAY, 1922 



Within these cocoons adults developed to the average number of 

 6.31 per host puparium, although, as in the case of P. vernalis ^ 

 all did not emerge. The host larva very seldom succeeds in 

 pupating, although a well developed puparium case is always 

 formed. 



SEASONAL HISTORY 



In the eastern states in winter wheat this parasite normally 

 has but one generation a year. The adults emerge in the fall 

 at the time the Hessian flies are laying their eggs for the over- 

 wintering generation. They begin at once to oviposit into the 

 eggs of the fly. Cold weather setting in now greatly retards 

 the development of the growing embryos, and usually the entire 

 winter and spring, and frequently the early summer are passed 

 in the embryonic stage. The larvae and pupae commence to 

 develop during July, and by the end of August most of them have 

 transformed into adults within their cocoons. Weather con- 

 ditions modify the rapidity of development, and sweeping 

 records prove that adults occasionally emerge throughout the 

 summer months. 



COMPARISON OF THE TWO SPECIES 



The immature stages of Platygaster vernalis may be distin- 

 guished from those of P. hiemalis by the following character- 

 istics and habits: 



P. vernalis 



Egg claviform; about 0.07 mm. 

 long by 0.016 mm. wide. 



Embryos held together in a spheri- 

 cal or ellipsoidal, elastic poly- 

 embryonic mass. 



Embryos always develop inside 

 the mid-intestine of the host. 



Primary larva with greatly en- 

 larged, setosous mandibles. 



Mandibles of mature larva dis- 

 tinctly curved. 



P. hiemalis 



Egg ellipsoidal, slightly flattened 

 along one side; about 0.02 mm. 

 long by 0.01 mm. wide. 



Embryos individual, spherical 

 masses separated one from the 

 other except in rare instances. 



Embryos always develop in the 

 haemolymph of the body cavity of 

 the host. 



Mandibles very small and smooth 

 throughout larval period. 



Mandibles of mature larva almost 

 straight. 



The two species may also be distinguished by certain 

 habits of the adults. Platygaster vernalis is rather sluggish 

 and seldom flies when in captivity. P. hiemalis, on the 

 other hand, is normally very active and flies readily. In 

 ovipositing, P. vernalis holds the body parallel to the 

 long axis of the Hessian fly egg, while P. hiemalis poises 



