88 THE HOUSE FLY—DISEASE CARRIER 
flies, sometimes in considerable numbers. It is not an 
uncommon sight to see any one of several different 
species of wasps flying about houses, capturing flies 
both on the wall and on the wing. The robber flies of 
the family Asilidae also catch house flies, on porches 
sometimes. On the whole, however, the predatory in¬ 
sect enemies of the house fly are negligible, so far as 
the beneficial result of their work is concerned. 
Parasitic Enemies 
To a certain extent the same may be said of the para¬ 
sitic enemies of this species, but these are perhaps more 
numerous than the predatory insect enemies, and sev¬ 
eral of them are accustomed to frequent excreta in the 
search of larvae in which to deposit their eggs. This 
is especially true of cow dung, and many minute hy- 
menopterous parasites may be found frequenting drop¬ 
pings in the pasture in order to lay their eggs in some 
one of the many species of maggots which are to be 
found there in a very short time. 
These very minute, active, four-winged parasites be¬ 
long either to the subfamily Figitinae of the gall-fly 
family Cynipidae or to the superfamily of true parasites 
known as Chalcidoidea. 
In the gall-fly family, Cynipidae, most of the species 
of which produce galls upon living plants and very 
numerously upon the oak, there is one subfamily of 
minute forms, the Figitinae, parasitic upon other insects 
and for the most part upon dipterous maggots. Those 
frequenting cow dung will lay their eggs in apparently 
