THE SMALL-HEADED FLIES AND THE 

 TANGLE-REINED FLIES 



(Families Acrocerida and Nemestrinidce.) 



The Acrocerid flies, which have been called the "small- 

 headed " flies by Comstock and which in some books are known 

 under the family name of Cyrtidae, are of medium size and with 

 a strongly convex thorax so as to appear hump-backed. The 

 head is very small and is composed almost entirely of eyes. Both 

 the flies of this family and of the Nemestrinidae would be described 

 as little fat flies from their stout bodies. The Acrocerids are 

 sometimes of brilliant metallic colors and are rather well repre- 

 sented in North America since we have representatives of ten 

 genera comprising more than thirty species. The flies of the 

 genus Pterodontia, of which one species (P. analis) occurs in our 

 Southern States, are of very extraordinary form. They look like 

 minute inflated bladders, the head being extremely small. The 

 larvae of the small-headed flies are chiefly parasitic, living in 

 spiders or in their cocoons. In Europe one species lives in the 

 body of a spider, leaving it to pupate, while in this country 

 Emerton has found the larva of one of these flies living in the 

 webs of a common spider, presumably having eaten the spider 

 itself. 



The flies of the family Nemestrinidae resemble somewhat in 

 general appearance certain wild bees or the bee-flies of the family 

 Bombyliidae. Their mouth parts are frequently of great length 

 and they are used in gathering nectar from the flowers. They 

 are rare in the United States, only four species being known to 

 occur here. The life history of none of them has been worked 

 out. One of the European species of the genus Hirmoneura is in 

 its early stages parasitic in a beetle larva. The parent fly lays 

 her eggs in the burrows of some wood-boring insect. When 

 the larvae hatch they come to the surface of the log in which they 



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