152 Forty-first Report on the St4.te MvsEim. 



of Isosoma hordei (referred by tliem to the parasitic genus Eurytoma), 

 were of the belief that it must necessarily be parasitic, and that its 

 occurrence within the straw was that of a destroyer of the real cause 

 of the injury, which they supposed to be a dipterous insect — a 

 .Gecidomyia, allied to the Hessian fly. 



The parasitism of Isosoma is not yet entirely removed from the 

 sphere of discussion and doubt, for although it may be claimed as 

 effectually disproved by recent observations upon its actual oviposi- 

 tion, it is still maintained by eminent European writers. 



Other Species of Isosoma. 

 The associates in the genus of the joint-worm fly are not numerous. 

 The following have been described: 



Isosoma tritici, described by Professor Riley, in 1882,* from speci- 

 mens obtained in Illinois and other Western States, and found feed- 

 ing singly within the upper joints of wheat. In some publications it 

 has been named the " wingless joint-worm fly," as nearly all the 

 examples are without wings, or having them greatly aborted. This 

 species should not be confounded with Eurytoma tritici of the Fitch 

 Reports (vii, pp. 144-155 of vi-ix), which has been referred to 

 /. hordei, as before stated. 



Isosoma grande Riley, f named the " larger wheat fly," as it is a 



larger and stouter form than its 

 congenors. Its habits and general 

 features are so much like those of 

 /. tritici that it is thought it may 

 prove, upon further study, to be 

 but a dimorphic form of that 

 species. In its original descrip- 

 tion, in the Report of the Depart- 

 ment of Agriculture for 1884, page 

 "358, it is described by comparison 

 with /. tritici, from which its prin- 

 cipal differences may be seen. No 

 males have yet been found of 

 either. Figure 12 is a represen- 

 tation of /. grande, much enlarged. 

 Fig. 12.- Isosoma GEANDB- female. Isosoma elymi French,§ feeding 



in the middle internodes of Elymus Canadensis — a perennial grass of 



* American Naturalist, for March, 1882, xvi, p. 248. 



t Bulletin of the Brooklyn Entomological Society, for December, 188i, vii, p. 111. 



1 Canadian Entomologist, for January, 1882, xiv, p. 10. 



