|Pterodontia sp. fly|
Acroceridae are a small family of odd-looking flies. They have a humpbacked appearance with a strikingly small head, generally with a long proboscis for accessing nectar. They tend to be rare and are not widely known outside entomological circles. The most frequently applied common names are small-headed flies or hunchback-flies. Many are bee or wasp mimics.
General appearance and common names
The Acroceridae vary in size from small to fairly large, about the size of large bees, with a wingspan of over 25 mm in some species. As a rule both sexes have tiny heads and a characteristic humpbacked appearance because of the large, rounded thorax. Accordingly Acroceridae are commonly known as hunchback-flies or small-headed flies. In reference to their biology, they also are known as spider flies.
In appearance they are compact flies without major bristles, but most species have a bee-like furriness on their bodies. In most species the eyes are holoptic in both sexes, the heads seemingly composed mainly of ommatidia. This is in contrast to many insects in which the males have larger (even holoptic) eyes, whereas the females have normal or even vestigial eyes. The squamae are disproportionately large and the abdomen has an inflated appearance, often practically globular.
Many are bee or wasp mimics.
Affinities and taxonomy
The Acroceridae Leach, 1815, are a small family of odd-looking flies 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. There are about 520 species in 50 genera.
Of the traditionally recognised subfamilies the Panopinae and Philopotinae appear to be monophyletic, but the Acrocerinae polyphyletic.
Obsolete synonyms for Acroceridae include Cyrtidae, Oncodidae, and Ogcodidae.
Distribution and biology
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 semi-arid tropical locations.
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 Nestrinidae, members of the family undergo hypermetamorphosis; the adults do not seek out their hosts; instead the first instar larvae is a planidium. In the Acroceridae the planidia seek out spiders. They do not resemble the triungulin of most beetles with a hypermetamorphosis, but do resemble the triungulin of Stylops. The larva can move with a looping movement like a leech or inchworm, and can leap several millimetres into the air. When a spider contacts an acrocerid planidium, the planidium grabs hold, crawls up the spider's legs to its body, and forces its way through the body wall, usually at an articulation membrane. Often it lodges near a book lung, where it may remain for years before completing its development.
The adults of most species, like various members of the Tabanidae, Nemestrinidae and Bombyliidae, are nectar feeders with exceptionally long proboscises, sometimes longer than the entire bodily length of the insect. Unlike the other families however, when not deploying the proboscis for feeding, the Acroceridae carry it lengthwise medially beneath the body, instead of projecting forward. As a result the proboscis might escape casual notice, though careful inspection may reveal it projecting slightly behind the abdomen. (See the attached photograph of a Psilodera species).
- Gillung, Jéssica P , Winterton, Shaun L. New genera of philopotine spider flies (Diptera, Acroceridae) with a key to living and fossil genera. Journal Article: ZooKeys 01/2011; doi:10.3897/zookeys.127.1824
- Richards, O. W.; Davies, R.G. (1977). Imms' General Textbook of Entomology: Volume 1: Structure, Physiology and Development Volume 2: Classification and Biology. Berlin: Springer. ISBN 0-412-61390-5.
- Wiegmann, Brian M; Yeates, David K; Thorne, Jeffrey L; Kishino, Hirohisa. Time Flies, a New Molecular Time-Scale for Brachyceran Fly Evolution Without a Clock. Syst. Biol. 52(6):745–756, 2003. ISSN: 1063-5157 print / 1076-836X. Online doi:10.1080/10635150390250965
- Winterton, Shaun L; Wiegmanna, Brian M; Schlinger, Evert I. Phylogeny and Bayesian divergence time estimations of small-headed flies (Diptera: Acroceridae) using multiple molecular markers. Molecular Phylogenetics and Evolution Volume 43, Issue 3, June 2007, Pages 808–832
|Wikimedia Commons has media related to Acroceridae.|
|Wikispecies has information related to: Acroceridae|
- http://delta-intkey.com/britin/dip/www/acroceri.htm - illustrations
- http://www.acroceridae.org - Yahoo group includes preliminary bibliography
- http://www.diptera.info/photogallery.php?album_id=59 Images at Diptera.info
- http://bugguide.net/node/view/7016/bgpage Images at BugGuide