172 
E. A. ORMEROD—THE HESSIAN ELY 
In the investigation, however, which threw light on whence the 
Hessian fly came to us, we had an instance of how in some cases 
what is needed is the most complete scientific knowledge of 
structure, and every point that can be observed of the appearance 
of the insect itself under consideration, and also knowledge of its 
geographical distribution. 
We knew that the Hessian fly was an insect which was found 
at the present day in many parts of America and Europe, but those 
who had to study the subject in extenso , knew also that this was 
not the case with its parasites. One kind of these parasites 
certainly is found both in America and Europe, but the others 
belonged respectively to North America, or to Eastern Europe, not 
to both countries; so to find which set of parasites accompanied 
our new arrival of Hessian flies, appeared to me would be equivalent 
to finding from what country the visitation had arrived. 
It is just possible that some may not know what a parasitic fly 
is, so I will just mention that in this case the parasites are small 
flies, four-winged or two-winged as the case may be, which lay 
their eggs on the Hessian-fly maggot, or at least in it in some early 
condition, and as the parasitic maggot preys on the young of the 
Hessian fly, it thus lessens the numbers of its hosts and our pest. 
It turns to its own chrysalis state within that of the Hessian fly, 
and in due time those who carefully keep the Cecidomyia destructor 
chrysalids, see produced from out of these flaxseeds a parasite or a 
Hessian fly. To identify these parasites was a labour of great 
difficulty, and was brought about by the skill and research, and I 
should add the kind and patient labour bestowed on the subject for 
our benefit, by Hr. Lindeman the well-known Russian Entomo¬ 
logist, and Prof. Riley the Entomologist of the United States. 
The presence of these parasites was noticed almost from the 
beginning of the attack, but there was great difficulty in certain 
identification, as there was nothing to aid us excepting published 
descriptions, and for some time we worked without the help of 
either American or Russian specimens. 
After long and careful examination of British specimens, at a 
Conference at which I had the advantage of being present, Prof. 
Riley stated that none of those submitted to him appeared to agree 
with specimens of the kinds known to him in North America, and 
that the points laid before him inclined him to consider that the 
attack was of European rather than American origin. As a further 
step I forwarded in the autumn of 1887 a collection of parasites 
bred from puparia of Cecidomyia destructor , that is to say from 
Hessian-fly flaxseeds, by Mr. Taylor our best North British observer, 
to Hr. Lindeman at Moscow, from whom I received the valuable 
communication that four of the five kinds sent were of Russian 
species, and the fifth he was inclined to consider was the Merisus 
destructor of America, a kind, I may remark in passing, which is 
distributed in Europe as well as on the other side of the Atlantic, 
and thus we became aware that all the geographical “ test-flies ” sent 
were solely European, except one which belonged to both continents. 
