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This is a small family of odd looking flies most closely related to Nemestrinidae. There are about 520 species in 50 genera. They are characterized by a humpbacked appearance and a small head, sometimes with a long proboscis for nectar. As such, acrocerids are commonly known as small-headed flies or hunchback-flies. Many are bee or wasp mimics. Their eyes are often holoptic; their heads seem to be composed primarily of ommatidia. They are cosmopolitan in distribution but rarely observed in most places; the majority of the over 500 species are known from fewer than 10 specimens. They are found most commonly in semi-arid tropical locations.
All small headed flies are parasitoids of spiders. They are most commonly collected when a spider from the field is brought into captivity. The adults do not seek out their hosts; instead the first instar larvae is a planidium that seeks out spiders. When a spider contacts an acrocerid planidium, the larva grabs hold of the spider, crawls up the spider's legs to its body, and forces its way through the body wall, often lodging near the book lung, where it may remain for years before completing its development. Some obsolete names for Acroceridae include "Cyrtidae" and "Oncodidae".