86 



DIPTERA OF MINNESOTA. 



stage. Williston states that the larvae, so far as known, are parasitic 

 upon spiders and their cocoons. Aldrich, who, by the way, retains 

 the name Cyrtidae for this family, lists forty-two species for North 

 America. We figure Opsebius pterodontimis, O. S., to illustrate the 

 family. 



NEMISTRINIDAE, 



Tangle-Vein Flies. 



Not met with in Minnesota. Sharp and Williston state that about 

 lOO species occur in this family, widely distributed. Aldrich, in his 

 catalogue for 1905, mentions only six species as occurring in North 

 America. The veins of the wings of these flies unite in such a com- 

 plex manner as to give to the group the name "Tangle- Vein Flies." 



ASILIDAE, 



Rohher-Flies. 



These rapacious and really useful flies prey upon insects weaker 

 than themselves, carrying off bodily flies, wasps, and even beetles and 



Fig. 78. Cyrtopogon bimacula. Original. 



large grasshoppers, to be killed and devoured. Williston reports hav- 

 ing observed a female seize and carry off a pair of her own species. 



