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Subfamily Conopinae

Thick-headed Fly (Physocephala tibialis)? - Physocephala tibialis Thick-headed Fly - Physocephala tibialis Curated specimen from the Essig Museum - Physoconops townsendi - female blue winged beauty - Physocephala floridana Physocephala sp - Physocephala tibialis thick-headed fly - Physocephala furcillata - female Physoconops sylvosus from east of the Sierras - Physoconops sylvosus Thick-headed Fly 688A 9913 & 9919 - Physocephala marginata - female
Kingdom Animalia (Animals)
Phylum Arthropoda (Arthropods)
Subphylum Hexapoda (Hexapods)
Class Insecta (Insects)
Order Diptera (Flies)
No Taxon ("Acalyptratae")
Superfamily Sciomyzoidea
Family Conopidae (Thick-headed Flies)
Subfamily Conopinae
2 genera in our area: Physocephala and Physoconops (1)(2). Many other genera occur from Mexico south into South America, and elsewhere worldwide.
In our area (North America, north of Mexico):
  Physocephala has 7 species; and
  Physoconops has 13 species (12 are listed here and in Stone(2); the 13th, Physoconops weemsi, was described in 2007(3)).
Physocephala and Physoconops closely resemble each other and are often confused. They can usually be distinguished by examining the wing venation and the shape of the hind femora.

Wing Venation In Physocephala, the anterior crossvein (denoted "r-m" in the diagram below), is located well beyond the middle of discal cell (denoted "dm"); and the discal cell is strongly "indented" just basad of the crossvien r-m. Note that the tiny crossvein bounding the discal cell at its base is usually hard to see, but it is always just beyond where the vein on the lower edge of the discal cell meets cell "cup" (cf. 2nd diagram below):

In Physoconops, the crossvein r-m is near or before the middle of cell dm; and the discal cell is typically less strongly indented basad of crossvein r-m, and often more-or-less straight along its upper edge:

Hind femora In Physocephala the the hind femur is irregularly thickened and widest in its basal portion:

In Physoconops, the hind femur has a more uniform thickness and is widest near the middle:

Note that the hind femur character is best employed with a profile view. From some angles the basal swelling in Physocephala may not be clearly apparent.
To be confident of an identification, it's best to have a number of characters available for verification, since some may be difficult to see and/or interpret (and some may not even "work", due to variation). While the above characters are the primary ones used to separate Physocephala and Physoconops in most keys, and are presumably the most consistent and dependable ones, the following additional characters are also useful in distinguishing the two genera:
1) Physocephala do not have ocelli, whereas Physoconops have very small ocelli located on a swelling at the vertex(1) [see comment here by Jeff Skevington]. 2) Nearly all Physocephala have a sharply defined "T"-shaped marking on the frons, whereas only one uncommon species on Physoconops does in our area (P. nigrimanus). 3) In Physoconops the last few tarsi on all legs are typically dark black and the pulvilli are large and light yellow; whereas in Physocephala the tarsi are typically narrower, lighter reddish or yellowish, and the pulvilli and darker (with little or no yellow) and typically smaller. 4) In many (but not all!) Physoconops the 3rd antennal segment is noticeably much longer than it is in Physocephala. 5) In many (but not all!) Physoconops the female's theca (see thumbnail below) is noticeably much longer than it is in Physocephala:
See Also
A small group of genera in Syrphidae (tribe Cerioidini) are often mistaken for Conopinae.
Works Cited
1.Manual of Nearctic Diptera Volume 2
Varies for each chapter; edited by J.F. McAlpine, B.V. Petersen, G.E. Shewell, H.J. Teskey, J.R. Vockeroth, D.M. Wood. 1987. Research Branch Agriculture Canada.
2.A Catalog of the Diptera of America North of Mexico
Alan Stone, et al. 1965. United States Department of Agriculture.
3.A new conopid fly from Florida and Georgia (Diptera: Conopidae)
Sidney Camras. 2007. Insecta Mundi 0007: 1-4.