The Acroceridae are a small family of odd-looking flies. They have a hump-backed appearance with a strikingly small head, generally with a long proboscis for accessing nectar. They are rare and not widely known. The most frequently applied common names are small-headed flies or hunch-back flies. Many are bee or wasp mimics. Because they are parasitoids of spiders, they also are sometimes known as spider flies.

Nemestrinidae, or tangle-veined flies is a family of flies in the superfamily Nemestrinoidea, closely related to Acroceridae. The family is small but distributed worldwide, with about 300 species in 34 genera. Larvae are endoparasitoids of either grasshoppers (Trichopsideinae) or scarab beetles (Hirmoneurinae). Some are considered important in the control of grasshopper populations. Adults are often observed on flowers.