The Window Flies and the Stiletto Flies 



prey turned out to be more aggressive and ate up the Scenopinus 

 larva. They are apparently always especially abundant, as I am 

 informed by Mr. Chittenden, in the sweepings in feed stores and 

 the flies are always to be found around the windows in such 

 establishments. The probability is very strong that they feed 

 upon such small, soft-bodied insects as flour-mites and book-lice. 

 Mr. Pergande tells me that he has seen them eat the pupae of one 

 of the little stored-grain beetles (Silvanus surinamensis) and also 

 disabled house-flies which he had offered them, as well as their 

 comrades of their ov/n species. 



The family Therevidae comprises a group of rather small and 

 rather slender flies, frequently of variegated color, and looking 

 something like robber-flies. They have been called stiletto-flies. 

 The flies themselves prey upon other insects, principally other 

 flies. They are not as active as the true robber-flies, nor are they as 

 strong, and as a result they choose weaker prey and have the habit 

 of lying in wait upon leaves and bushes and even upon the 

 ground for their prey to come near them instead of flying about 

 actively in search of it as do the robber-flies. Their larvae are 

 very curious and are found in rotting wood or in earth which is 

 full of vegetable mold, and seem to feed upon both decaying 

 animal and vegetable matter. They have been found feeding 

 upon dead caterpillars and pupae. They are very long and 

 slender, and have apparently twenty segments to the body in- 

 cluding the head. This appearance, however, is deceptive, and 

 is due to a seeming division of the anterior segments. 



140 



