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For Insects, Spiders & Their Kin
For the United States & Canada
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Family Acroceridae - Small-headed Flies

Small-headed flies mating - Eulonchus tristis - male - female strange humped fly - Ogcodes Acrocerid - Ogcodes Bee Fly? - Eulonchus marginatus Flies on Penstemon on Chandler Butte - Eulonchus tristis - male - female Unknown Insect - Pterodontia flavipes Ocnaea sp. seemingly out of range - Ocnaea - female Pterodontia?
Kingdom Animalia (Animals)
Phylum Arthropoda (Arthropods)
Subphylum Hexapoda (Hexapods)
Class Insecta (Insects)
Order Diptera (Flies)
No Taxon (Orthorrhapha)
Family Acroceridae (Small-headed Flies)
Other Common Names
Spider flies(1)
Synonyms and other taxonomic changes
Cyrtidae Newman 1834
Acroceratidae (an older spelling)
Explanation of Names
Acroceridae Leach 1815
from Greek, akros 'summit' + keras 'horn' referring to antennae of some species (e.g. Eulonchus)
~60 spp. in 7 genera in our area(2), ~400 spp. in 55 genera worldwide(3)(4)
Humpbacked flies with thorax and abdomen balloon-like; small heads with holoptic eyes (covering most of the head); and large, conspicuous calypters (membranous disk-like structures) tucked-in at the base of the wings. Also, the wings are pleated with many folds on the membrane. Many species mimic bees/wasps or even beetles.
Key to genera in(5); keys to nearctic species in Sabrosky (1948); for western species see(6)
Separating nearctic genera
▩ Proboscis very long; antennae inserted near middle of eyes; abdomen less swollen than in other genera:
◆ 3 ocelli on a tubercle ⇒ Eulonchus (above left)◇ 2 ocelli, no tubercle; abdomen more swollen and evenly convex ⇒ Lasia (above right)
▩ Mouthparts very short; antennae inserted otherwise; abdomen more swollen:

▶ Antennae distinctive, nearly as long as the face, somewhat "sausage-like": Ocnaea
▶ Antennae inserted near top of head ⇒ Turbopsebius, Acrocera        
◆ Eyes densely pilose ⇒ Turbopsebius      vs.       ◇ Eyes bare ⇒ Acrocera          
▶ Antennae inserted near bottom of the eyes, very short ⇒ Ogcodes, Pterodontia    
◇ Eyes bare; tibiae without spurs ⇒ Ogcodes
◆ Eyes pilose; tibiae with small apical males, costal edge of wing with a tooth-like projection ⇒ Pterodontia
Cosmopolitan; in our area, most diverse in sw. US(2)
Most common in semi-arid tropical locations
Spider parasitoids
Life Cycle
The first instar larva ('planidium') seeks out spiders. When a spider contacts a 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.
Adult longevity is usually rather short (3 days to ~1 month). Mating usually takes place in flight; female begin to lay up to 5000 eggs soon after mating and may continue during the following 2-10 days. The tiny, pear-shaped, black, microtype eggs are deposited either in flight upon the ground (Eulonchus), upon dead branches (Ogcodes), upon tree trunks (Pterodontia), or upon grass stems (Acrocera). Eggs hatch in 3-6 weeks giving rise to small planidial larvae. Most 1st instar planidia must seek out their spider hosts and can crawl or jump with "inchworm-like" movements. There is only one generation per year with the acrocerines (Acrocera, Ogcodes, Turbopsebius) on their araneomorph hosts; but many panopines (Eulonchus, Lasia, Ocnaea, Pterodontia) seem to have only one generation every 5-10 years due to the longer immature stages of their mygalomorph hosts.(7)
Most commonly collected when a spider from the field is brought into captivity.
Eulochus and Lasia are strong flyers, with a metallic sheen and very long mouthparts, and often seen nectaring on flowers. Most other genera are more cumbersome flyers with a duller sheen, often appearing to have no mouthparts at all, and more typically found on stems, leaves, or grass.
Print References
Internet References
Works Cited
1. Biology of Spiders 3rd Edition
Rainer F. Foelix. 2010. Oxford University Press.
2.American Insects: A Handbook of the Insects of America North of Mexico
Ross H. Arnett. 2000. CRC Press.
3.USDA Diptera Site (now closed)
4.Order Diptera Linnaeus, 1758. In: Zhang Z.-Q. (ed.) Animal biodiversity: An outline of higher-level classification...
Pape T., Blagoderov V., Mostovski M.B. 2011. Zootaxa 3148: 222–229.
5.Manual of Nearctic Diptera Volume 1
Varies for each chapter; edited by J.F. McAlpine, B.V. Petersen, G.E. Shewell, H.J. Teskey, J.R. Vockeroth, D.M. Wood. 1981. Research Branch Agriculture Canada.
6.The Flies of Western North America
Frank R. Cole and Evert I. Schlinger. 1969. University of California Press.
7.The biology of Acroceridae (Diptera): true endoparasitoids of spiders
Schlinger E.I. 1987. In: Nentwig W. (ed.) Ecophysiology of spiders. Springer, Berlin. pp. 319-327.
8.A revision of the American spider parasites of the genera Ogcodes and Acrocera (Diptera, Acroceridae)
Sabrosky C.W. 1944. Am. Midl. Nat. 31: 385-413.
9.The dipterous family Cyrtidae in North America
Cole F.R. 1919. Trans. Am. Entomol. Soc. 45: 1-79.
10.A further contribution to the classification of the North American spider parasites of the family Acroceratidae (Diptera)
Sabrosky C.W. 1948. Am. Midl. Nat. 39: 382-430.