66 



DIPTERA OF MINNESOTA. 



The imago and larvae of thi^ insect are shown in Figs. 53 and 54. 

 The gall like growths caused by many flies in this, and by insects 

 in other gall-making families, said growths being seen on willow% 

 oak, goldenrod, sumac, maple and countless other trees and plants, 

 are caused by the tissue of the plant in the immediate vicinity of the 

 egg or larvae making an abnormal growth, due evidently to the 

 presence of an irritating agent placed there by the parent insect, or 

 by a secretion from the newly hatched larva, or in some other way 

 not at this time thoroughly understood. 



Fig. 55. Unsightly galls on box elder leaves. Caused by a Cecidomyid. Ruggles. 



Species of this family collected in Minnesota are : Contarinia 

 vioUcola, Coq. ; Mayctiola (Cecidomyia) destructor, Say.; C. aceris, 

 Shim.; C. robiniae, Hald ; C. (?) negundinis, Gill.; Asphondylia, sp., 

 "bred from cabbages in October ;" a Cecidomyid bred from Polyporns. 

 There are doubtless many others not yet encountered. 



