F. M. CAMPBELL—THE HESSIAN FLY. 
189 
not yot entered the pupal stage. Unlike their hosts, they obtain 
freedom by gnawing a small hole in the sheath of the plant. The 
parasites I have reared may he classified as follows, viz. two species 
of the genus Pteromalus , two of the genus Eutedon , and one of 
each of the genera Eupelmus and Dacnusa. The Rev. T. A. 
Marshall has identified one of these species as Eutedon Epigonus 
Walk., which belongs to the British Fauna. The periods of its 
appearance were in April from the puparia of last year, and in 
July from the puparia of this year. The same entomologist informs 
me that the Eacnusa is closely allied to I). senilis , Hah, also a 
common species of which it may he a small variety.] 
It is interesting to see that the disappearance of the Hessian fly 
is sudden, inasmuch as we might infer this from the following 
theoretical calculation as to the action of the parasites. Let us 
begin with four pairs of Hessian flies and one pair of parasites, and 
let us suppose that every female lays, say, 70 eggs, which are 
productive of 35 pairs. As every egg of the parasites in its 
development cancels what would become a fly, we may deduct 
from each generation of flies the whole of the eggs of the synchronic 
generation of parasites. We thus find that the fourth brood of 
flies numbers 3,001,250, all of which would he required to receive 
the eggs of the parasites (even if there are only two broods in a 
year), and would therefore he destroyed.* 
A monograph by Riley f describes and figures five species of 
parasites of the fly in America, viz. Merisus destructor, Say; 
Merisus ( EEomoporus ) sub apt er us, n.sp.; Eupelmus Allynii, French ; 
Platygaster EEerrickii, Packard ; Tetrastichus productus, n.sp. By 
far the most abundant is Merisus destructor , Say ( =Semiotellus 
destructor , Say), and Packard attributes to this fly alone the general 
immunity in some regions (U.S.) from the pest.J Lindeman § has 
recently mentioned seven, and described six species as having 
emerged from the puparia of the Hessian fly in Russia, viz. 
Merisus intermedius , n.sp.; Tetrastichus Rileyi, n.sp.; Semiotellus 
nigripes, n.sp.; Eupelmus Karschii, n.sp.; Platygaster minutus, 
n.sp.; Euryscapus saltator, n.sp.; Platygaster (?) sp.; and he 
points out the interesting fact that these are all different 
from those of America enumerated by Riley. Such being the 
case, the question arises whether, if we observe the parasites 
which proceed from the puparia of C. destructor in our own country, 
we may not obtain some indication as to the source from which this 
* The calculation is theoretical. The figures are as follows : — 
Hessian Flies. Parasites. 
1 st Generation 280—70=210 70 
2nd „ 7,350 — 2,450=4,950 2,450 
3rd „ 171,500—85,750=85,750 85,750 
4th „ 3,001,250-3,001,250=0 3,001,250 
+ “ On the Parasites of the Hessian Fly,” ‘ Proc. U.S. National Museum ’ 
1885, p. 413. 
f ‘ Third Rep. U.S. Comm.,’ p. 218. 
§ “Die Pteromalinen der Hessenfliege,” ‘Bull. Soc. Imp. Naturaliste de 
Moscou,’ 1887, p. 178. 
