Voi>- XXV Washington, D. C., August 25, 1 923 No. 8 
the TWINNING AND MONEMBRYONIC development 
H ESS I AN Y FLY ^ ER HIEMAUS ’ A PARASIT E OF THE 
By R. W. Leiby North Carolina Experiment Station, and C. C. Hill. 2 Cereal and 
Agriculture '' 1 1 nvesh 9 atlons • Bureau of Entomology, United States Department of 
• INTRODUCTION 
Those who are familiar with the polyembryonic method of development 
of some insects have always thought it desirable that a study be made of 
the development of a species which produces but a small number of 
individuals from a single egg. In the four species of insects in which the 
polyembryonic method of development has been previously described 
m greater or less detail, the number of individuals produced from a single 
egg has been- 150 to about 1,700. These species represent a highly 
specialized type of polyembryony, whereas a species in which one egg 
produced only two or four individuals would be expected to illustrate a 
more simple type of polyembryony. 
Platygaster hiemalis Forbes, a hymenopterous parasite of the Hessian 
fly, is a species which develops both monembryonically and polyem- 
bryomcally. This parasite deposits a group of from 5 to 8 eggs in the 
egg of its host, and an average of 6.31 individuals are developed from 
ttese eggs. Some of the eggs of the group fail to develop at all, others 
^* gr . OU ? de ^ el °P twms, while still other eggs of the same group give 
nse to single individuals. The objects of this paper will be to describe 
the twinning and monembryonic development of this insect, to show 
why some of the eggs fail to develop, and to point out the relation be- 
mcmernbryony^ SimpIe type ° f P ol yembryony here called twinning and 
RELATION OF PARASITE TO HOST 
Platygaster hiemalis is single-brooded, although its host, the Hessian 
liy, a serious wheat pest, is double-brooded throughout most of the 
winter-wheat region of the United States. The adults of P. hiemalis 
emerge from their cocoons in the host puparia (PI. 1, D) during the fall 
of the year, at which time the Hessian flies of one generation also emerge. 
The parasite deposits its eggs in the eggs of the fly which have meanwhile 
been deposited on the wheat plants. 
1 Accepted for publication Aug. n, 1923. 
Walton fa &udi£ pa^^ 011 °' the shown by Dr ' ^ O. Howard and W. R. 
3 Phytophaga destructor Say. 
SSnSS “aSr 158 ^^ “* itS eC ™ « - —W 
Journal of Agricultural Research, 
Washington, D. C. 
age 
Vol. XXV, No. 8 
Aug. 2 s, 1923 
Key No. K-m 
54486—23-1 
( 337 ) 
