14 FARMERS’ BULLETIN 640. . 
was removed. As a result of this experiment, from all of the flies 
developing from the transplanted wheat the offspring numbered but 
four individuals, showing quite conclusively that the Hessian fly can 
not develop in destructive abundance in that locality. This insect 
is destructively abundant west of the Cascade Mountains in Washing- 
ton and Oregon, but not in the semiarid section to the eastward. 
The distribution of the Hessian fly seems, therefore, to be controlled 
to a certain extent in the United States by conditions of bumidity. 
Furthermore, the development of the fall generation appears to be 
governed to a certain extent by the same factor. 
NATURAL ENEMIES. 
There can be no doubt that parasites play a most conspicuous part 
in the natural control of the Hessian fly, and if we only knew the whole 
truth of the matter we should find that these minute friends of the 
Fig. 11.—Polygnotus hiemalis, a parasite of the Hessian fly. Much enlarged. (Original.) 
farmer are worth many times their weight in gold. Not infrequently 
one species of these parasites will overcome the pest in a neighbor- 
hood so effectively as almost to exterminate it. Several times the 
writer has found, in attempting to breed the Hessian fly from young 
wheat plants that had been killed by the larve, that hundreds of 
these parasites would emerge from the flaxseeds, while only an 
occasional fly could be obtained. Nearly all of these deposit their 
eggs in the bodies of the maggots, but the fully developed parasites 
emerge from the flaxseeds. 
Prof. Herbert Osborn * has enumerated six species of these para 
sites, not including the English species Hntedon epigonus Walker. 
1 Osborn, Herbert. The Hessian Fly in the United States. U.S. Dept. Agr., Diy. Ent., Bul. 16, 58 
p., 2 pl., 8 figs., 1898. See p. 28. 
