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. Description The Acroceridae vary in size from small to fairly large, about the size of large bees, with a wingspan over 25 mm in some species. As a rule, both sexes have tiny heads and a characteristic hump-backed appearance because of the large, rounded thorax. In appearance, they are compact flies without major bristles, but many species have a bee-like hairiness on their bodies, and some are bee or wasp mimics. In most species, the eyes are holoptic in both sexes, the heads seemingly composed mainly of the large faceted eyes. This is in contrast to many insects in which the males have larger (even holoptic) eyes, whereas the females have normal eyes. The squamae are disproportionately large, completely covering the halteres, and the abdomen has an inflated appearance, often practically globular. The tarsi are equipped with large claws with three pulvilli below them. Taxonomy The Acroceridae are a small family in the Brachycera. They are members of the infraorder Muscomorpha, and DNA studies suggest that they are most closely related to the families Nemestrinidae and Bombyliidae. A 2013 analysis of morphological data suggested the Acroceridae were a sister group to the Asiloidea and Eremoneura. The roughly 520 species are placed in 50 genera. In 2019, a revised classification of the family based on phylogenetic studies was published, listing five extant subfamilies and one extinct subfamily containing Archocyrtus from the Late Jurassic Karabastau Formation of Kazakhstan. Obsolete synonyms for Acroceridae include Cyrtidae, Oncodidae, and Ogcodidae. Distribution and habitat Acroceridae are cosmopolitan in distribution, but nowhere abundant. They appear episodically and in most places are rarely observed; of more than 500 species described, most are known from fewer than 10 specimens. They occur most commonly in semiarid tropical locations. Behaviour As far as is known, all Acroceridae are parasitoids of spiders. They are most commonly collected when a spider from the field is brought into captivity. As in the related families, Bombyliidae and Nemestrinidae, members of the family undergo hypermetamorphosis: the adults do not seek