OF WASHINGTON. 



15 



productus and carinatus, are listed by Professor Osborn in 

 Bulletin 16, new series, Division of Entomology, U. S. De- 

 partment of Agriculture, as parasitic on Polygnotus that 

 minute proctotrypid whose untiring hostility to the Hessian 

 fly renders it an important factor in keeping the world's table 

 supplied with bread. 



Evidence in support of either contention is difficult to se- 

 cure. Individuals reared from infested material prove as 

 much for one side as for the other, since their presence is 

 mainly inferential and their mission may be interpreted as the 

 student may see fit. Clearly the only satisfactory solution of 

 the problem, is to see the Tetrastichus at work, if possible, and 

 discover what it is doing. By a fortunate circumstance this 

 has actually happened in at least one very 

 interesting instance. 



During the month of April, 1907, a 

 large number of wheat plants containing 

 Hessian fly larvae known to be infested 

 with Polygnotus was transferred from 

 Marion, Pa., to a field of wheat on the 

 battlefield of Antietam near Sharpsburg, 

 Md., where the fly was prevalent and 

 where no Polygnotus had previously 

 been found. 



On July 8 of the same year this field 

 was visited by Prof. F. M. Webster and 

 Mr. E. G. Kelly, and a quantity of ripen- 

 ing straw containing numerous flaxseeds 

 was gathered and sent to Washington. 

 Examination showed that a large per- 

 centage of these flaxseeds contained 

 Polygnotus, which seems to indicate that 

 the imported parasites began operations 

 as soon as they emerged from the Marion 

 material in Maryland. 



This infested straw, collected in July, remained dry in the 

 office during the summer, and apparently no adults issued 

 therefrom. About Nov. 15 I removed a number of the Hes- 

 sian fly puparia, cleared them in carbol-xylol, and mounted 

 them in balsam. The pupa case of the fly is quite dense in 

 its structure, but the mounts gradually cleared in the balsam 

 until the contents of the puparia became distinctly visible. 

 The accompanying drawing (fig. 2) is a fair representa- 

 tion of the colony of Polygnotus that was in possession in 

 many of these flaxseeds, although the number of included 



FIG. i. " Flaxseed " or 

 puparium of Hessian fly 

 (Mayetiola destructor) 

 showing two adult Poly- 

 gnotus, and a hyperpara- 

 site, Tetrastichus sp. 



