These are both black, wasp-mimic species and can be easily confused. Bromley (1946) indicates that P. dorsatum flits its wings at rest like some spider wasps (Pompilidae). The all-black species in the Laphria canis complex are included on this page for comparison.
The proboscis in Laphria is vertically flattened; in Pogonosoma it is also vertically flattened, but it curves upward and is pointed at the tip; and in Orthogonis, it is cylindrical. Pogonosoma and Orthogonis have black wings, but members of the Laphria canis complex have brownish translucent wings.
All are rare in Florida, and Orthogonis stygia is known to be rare throughout its range.
Pogonosoma dorsatum
Proboscis short, upturned, flattened vertically and pointed at the tip. Black wings. Abdomen entirely dark. Legs with both black and white hairs, with white hairs most abundant on front and middle legs. Male has tooth-like projection on bottom margin of abdominal segment 6. It is very similar to Andrenosoma, though it has no red on the tip of the abdomen.
Found perched on horizontal pine logs or face-down on pine trunks (Bedell, 2010).
Size: 13-23 mm
Range: Eastern U.S.
FL Range: Panhandle and northern Florida
FL Season: April-July
Orthogonis stygia
Large compared to other black species. Proboscis cylindrical. Black with small whitish spots on the posterior lateral margins of abdominal segments 2-4. Black wings. Hairs black, but a small tuft of white hairs under the eyes. Similar to Laphria, and it was originally placed in that genus, but the proboscis differs.
Size: 25-27 mm
Range: Southeastern U.S. (TX and OK east to VA)
FL Range: Unknown
Season: May-August throughout range.
Laphria canis complex
This is a small, dark species complex that could be confused with robbers in other genera such as Pogonosoma dorsatum or Orthogonis stygia. Within the complex, they are only distinguishable by examination of the genitalia, so many photos cannot be identified.
”Laphria canis has a very wide genitalic bulb, wider than the distal abdominal tergites. L. sicula and winnemana have a much narrower one compared to the distal abdominal segments.’ (Bugguide)
See genitalic differences below.
Laphria canis
Very wide male genitalic bulb, wider than the distal abdominal tergites
Size: 7-12 mm
Range: Eastern U.S. and Canada
FL range: Panhandle (Liberty County, reported in Bullington (1986)) and possibly North Florida (Alachua County)
FL season: May
Laphria sicula
Male genitalic bulb much narrower than L. canis.
Size: 7-14 mm
Range: Eastern U.S. and Canada
FL range: Panhandle and North Florida (Jackson, Liberty, Levy, Alachua Counties)
FL season: May-June
Photo by Steve Daniel from San Felasco in Alachua County.
References
Baker, N. T. (1970). A taxonomic and ecologic study of the asilidae of Michigan (Doctoral dissertation, Michigan State University. Department of Entomology).
Bedell, P. (2010). A preliminary list of the robber flies (Diptera: Asilidae) of Virginia. Banisteria, 36, 3-19.
Bromley, S. W. (1931). New Asilidae, with a revised key to the genus Stenopogon Loew:(Diptera). Annals of the Entomological Society of America, 24(2), 427-435.
Bromley, S. W. (1946). Guide to the Insects of Connecticut, Part VI: The Diptera Or True Flies of Connecticut, Third Fascicle: Asilidae. State Geological and Natural History Survey.
Bullington, S.W. (1986). Two new genera related to Laphria Meigen (Diptera: Asilidae), with revisions of the included species in North America north of Mexico. Unpublished Doctoral Dissertation, University of Wyoming, Laramie, Wyoming, 275 pp.
McAtee, W. L. (1918). Key to the Nearctic species of the genus Laphria (Diptera, Asilidae).
Robber Flies of Florida 