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 Figitinse of the gall-fly 

 family Cynipidse or to the superfamily of true parasites 

 known as Chalcidoidea. 



In the gall-fly family, Cynipida?, 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 Figitinse, parasitic upon other insects 

 and for the most part upon dipterous maggots. Those 

 frequenting cow dung will lay their eggs in apparently 



