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



The egg differs in its manner of development from those of 

 most insects by being polyembryonic in nature. The original 

 germ-cell, instead of giving rise to a single embryo, develops 

 many separate embryos. At the same time the nutritive 

 functioning plasm of the egg develops in a remarkable way to 

 accommodate the rapidly multiplying embryos. The egg 

 always develops in the mid-intestine of the host and during its 

 early growth, is constantly tossed back and forth in the chyle 

 of the mid-intestine by peristaltic action. 



The primary larva is bluntly rounded at extremities, with 

 length of body about three times its width. Before it has 

 become very much inflated with food, a broad, deep constriction 

 is evident on each side slightly posterior to the mouth. This 

 is caused by the greatly .enlarged .and projecting bases of the 

 mandibles. The large size of the mandibles is characteristic 

 of this stage, the length of each being nearly one-third the 

 width of the head. They are slightly curved, setosous toward 

 the extremity, and practically coiorless. During this stage the 

 body segments are not distinct. 



In the succeeding larval development the mandibles are 

 replaced by small, d : stinctly chitinized mandibles, the body 

 segments become apparent, and the body assumes a more 

 obtusely ell psoidal form. 



When the primary larvae have freed themselves from the 

 surrounding gelatinous-like mass, they commence to imbibe the 

 chyle from the host stomach and soon ingest particles of the 

 stomach wall itself. They shortly pass into the advanced 

 larval stage of development, and it is not long before nearly the 

 entire contents of the host is consumed. At this point each 

 larva forms for itself a separate cocoon in which it pupates and 

 transforms to the adult. The parasitized host larva very seldom 

 succeeds in pupating. 



The cocoons are broadly ellipsoidal, pale yellowish brown in 

 color, and made of a tough, flexible material. The number 

 which develop in a single flaxseed varies somewhat. An 

 examination of 100 Hessian fly puparia taken from the field and 

 containing Platygaster vernalis cocoons showed an average of 

 7.91 cocoons per puparium with a maximum number of 13 and 

 a minimum of 3. The average number of cocoons in which 

 adults developed was 5.49 All these adults, however, did not 

 emerge. This was probably due to the artificial conditions 

 under which thev were reared. 



SEASONAL HISTORY 



Normally this parasite emerges in the early spring when the 

 Hessian flies are beginning to lay their eggs. It begins at once 

 to oviposit into the host eggs. Development progresses 



