The Hessian Fly, and its Parasites. 157 



The piipas, which became such in and about June, evolve the 

 perfect insect, partly during the next autumn, and partly during 

 the year succeeding. 



Parasites. — There are in this region, four principal parasites 

 of the Hessian fly, one of which attacks the eggs, and the other 

 three the pupa3. They are all minute Hynienoptera. 



1. The egg-parasite, is a species of Platygastcr, Latr., and 

 may prove to be identical with some one of the hundred species 

 of this genus which are described. (Entom. Mag. Lond. iii, 217. 

 Cont. Macl. Lye. i, 81.) The insect is abundant in the autumn. 

 I first saw it Sept. 23, 1833, in the act of depositing its eggs in 

 the eggs of the Hessian fly. From subsequent observations it 

 appears that four or five eggs are laid in a single egg of the Hes- 

 sian fly. The latter egg hatches, and the animal advances to the 

 pupa state as usual, but from the puparium no Hessian fly ever 

 comes forth. This parasite forms within the puparium, a silky 

 cocoon of a brownish color. 



2. This is the chief parasite of the pupa. It is described by 

 Mr. Say, (Jour. Acad. Nat. Sci. Phil, i, 47,) as the Ceraphron 

 Destructor. It appears to me not to belong to the genus Cera- 

 phron, (Latr.) but to fall within the genus Eurytoma, of Illiger. 

 It pierces the sheath of the stalk, (making a hole too small to be 

 detected by a powerful microscope,) and deposits an egg in the 

 pupa within. This is done chiefly in June. The perfect insect 

 is evolved in the summer and autumn succeeding, eating its way 

 through the puparium and the sheath of the leaf. An insect (of 

 which I have seen females only) very similar to the Eurytoma 

 Destructor, but with mere rudiments of wings, is sometimes 

 evolved from the pupse of the Hessian fly. I am in doubt whether 

 it should be considered a distinct species or only a variety. The 

 winged individuals never throw ofl^ their wings. 



3. The next parasite of the pupa, is an insect of the tribe 

 Chalcidice, [Latr. in Cuv. Regne An.) whose genus I have not 

 determined. Its habits are like those of No. 2, but it is evolved 

 later. Apterous females of this species are also found. 



4. Another parasite of the pupa is an insect of the tribe Ox- 

 yuri, {Latr. in Cuv.) whose genus I have not determined. In 

 habits it agrees with Nos. 2 and 3, but it is evolved still later in 

 the year. All of these parasites are likewise evolved in the 

 spring, from Hessian fly pupae of the summer previous. 



