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Small-headed Flies and the Tangle-veined Flies

(Families Acroceridae and Nemestrinidae.)

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 represented 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 were born, tilt themselves upwards and are blown away by the wind, falling to the ground and entering the bodies of the white grubs upon which they feed, or they may attach themselves to the bodies of the beetles and so be carried into the ground when the female enters to deposit her eggs.