July 7,1933 
Platygaster vernalis , Hessian Fly Parasite 
35 
developed from these eggs were killed on different dates and sectioned. 
Three such larvae killed at the end of 26 days contained normally develop¬ 
ing vernalis embryos; two larvae killed at the end of 43 days disclosed the 
presence of healthy, mature vernalis larvae; and two killed at the end of 
78 days contained vernalis cocoons. Two similar cages showed the 
development of healthy vernalis embryos. 
SEX RATIO 
From 1,169 adults which emerged in confinement, 48.59 per cent were 
females and 51.41 per cent males. The occurrence of parthenogenesis 
may explain the preponderance of males in this species. 
To determine whether the several individuals developing in single 
puparia usually comprised one or both sexes, records were kept of the 
sex of the adults (whether emerged or not), in the case of 48 host puparia. 
Of these 48 puparia, 40 yielded adult parasites, each brood of which was 
either pure male or pure female; while each of the remaining 8 puparia 
produced a mixed brood of both males and females. This indicates that 
in the polyembryonic development of Platygaster vernalis the adults 
produced from a single egg are usually of the same sex. The coming 
of both sexes from a single host could be explained on the grounds that 
more than one egg was deposited in a single host insect. 
OVIPOSITION 
A female of Platygaster vernalis , when seeking Hessian fly eggs in which 
to oviposit, travels at a moderate rate up and down the leaves of the 
wheat plant, repeatedly tapping the leaf before her with her antennae. 
Pig. i.—D iagram showing the tendency of Platygaster vernalis to avoid ovipositing twice into the same 
egg. The short lines represent the Hessian fly eggs; the crosses mark out the eggs oviposited into on the 
first visit of the parasite; the dotted line plots out the course of the second visit; the circles mark out 
the host eggs oviposited into during the second visit. 
As soon as her antennae come in contact with an egg, she halts and 
concentrates on the spot. At times she loses track of the egg, where¬ 
upon she turns in small circles until she finds it again. After finding 
the egg, the parasite strokes it rapidly with her antennae and meanwhile 
strides it with her body held parallel to the long axis of the egg, where¬ 
upon the abdomen is drawn up with the ovipositor exserted and in 
contact with the egg. After sliding the ovipositor back and forth over 
the surface of the egg several times, she begins to insert it, usually at 
about the middle of the egg. At this point she draws her antennae 
downward and remains motionless for the rest of the time, except for 
slight movements of her abdomen, while bearing down on her ovipositor. 
The entire act of oviposition requires about one minute, the average of 
10 ovipositions being 59.9 seconds. 
Although certain individuals in confinement have deliberately ovi¬ 
posited several times in one host egg, most females seem to avoid ovi¬ 
positing more than once in the same egg. This tendency is shown by 
the experiment illustrated in figure 1. 
