These large predators are conspicuous and are typically a little larger than Promachus. Some, like P. fulviventris, are fairly distinctive, and others can be difficult to ID.
Proctacanthus fulviventris
Based on records in iNaturalist, this may be one of the most common robber flies in Florida. It may also be that they are large and beautiful and often perch on white sandy trails where people are hiking.
Note the bicolored legs with black femurs, orange hairs on face and thorax, and red on abdomen. There is less red on the abdomen of females, but it’s usually visible under the wings. The thorax is also typically black. Males also have the last few abdominal segments thickened – a trait shared by P. hinei. The hypopygium (male genitalia) are short. Note that the proboscis curved upward.
Size: 25-30 mm
FL Range: Throughout. These love white sand, so they occur primarily from central Florida and south, but have also been recorded from north Florida and coastal areas of the Panhandle.
FL Season: April – November
Male 
Female ovipositing 
Female
Proctacanthus fulviventris variation
The primary character distinguishing P. fulviventris in the literature is that they have black femurs. However, some individuals like this one have red femurs and the scutum (top of thorax) is also red. Note this individual is found in the same white-sand habitat where P. fuliventris is expected, and the hairs on the face (occipital hairs, mystax, beard) are all orangey-yellow as is expected on P. fulviventris. Perhaps this is a fresh individual that may turn darker with age.
Female 
Female
Giff Beaton also has found P. fulviventris with entirely red legs.
Proctacanthus rufus
I typically find these in riparian areas along sandy rivers where they can be abundant. They are very similar to P. hinei which may not be confirmed east of the Mississippi. The last few abdominal segments bulge from beneath in male P. hinei and there is no bulge in P. rufus. Entirely reddish legs and scutums (top disc of thorax). Red abdomens. The proboscis curves upward. White or yellowish mystax, beard, and occipital bristles.
Size: 30-36 mm
FL Range: Panhandle and north Florida south to Gainesville
FL Season: June – July
Male 
Male 
Female
Proctacanthus brevipennis
Bicolored legs. Strongly marked scutum (top of thorax). Ovipositor of females can be black or red.
Size: 20-28 mm
FL Range: Throughout
FL Season: March – June. Records from later in the summer may be misidentified. This is one of the first robber flies on the wing in spring and is typically the only Proctacanthus in March and April.
Male 
Female 
Male 
Male 
Male
Proctacanthus gracilis
These often have well-defined violin-shaped markings on the scutum (top disc of thorax). These markings may be less distinct on other individuals perhaps because of age. Note yellowish pruinosity on top of reddish/purple ground color of thorax and reddish wing veins.
Size: 26-30 mm
FL Range: Panhandle, north and central Florida south to Hillsborough County.
FL Season: May – September
Male 
Female 
Female
Male 
Male
Proctacanthus longus
Mystax yellow, palpi black with pale hairs. Occipito-orbital hairs part black and part pale. Thorax brown, gray pollinose. Abdomen brown. Wings uniformly brown and long. Male genitalia shorter than last two abdominal segments combined.
Size: 32-36 mm
FL Range: Panhandle, north and central Florida south to Highlands County
FL Season: May – September
Proctacanthus milberti
This is very similar to P. longus. Mystax yellow, palpi black with black hairs. Thorax gray. Legs also grayish purple. Male genitalia shorter than last two abdominal segments. Wings uniformly light brown.
Size: 28-40 mm
FL Range: Unknown. Likely panhandle and north Florida. Bromley (1950) said “northwestern part of the state.” This species is widely distributed throughout the U.S. and Canada.
FL Season: Late July – September. Season may be one of the better ways to distinguish this species.
Male 
Female
Proctacanthus heros
These large robbers are often found on dead trees in sandhills. Note the entirely bright orange legs, yellow facial hairs, dark thorax contrasting with yellow pollinose abdomen ending with a couple entirely pale segments.
Size: Huge. 32-45 mm
FL Range: Throughout, though apparently uncommon
FL Season: May – September
Male 
Male 
Female
Female by Rick Owen.
Male and female by Tony DiTerlizzi.
Proctacanthus nigriventris
Dark abdomen with the last couple segments mostly white.
Size: 30-35 mm
FL Range: Panhandle and north Florida
FL Season: May – August
Proctacanthus hinei
Dark purple thorax and pale red abdomens like P. rufus, but the last few abdominal segments on the males are swollen underneath.
Size: 33 mm
FL Range: Likely erroneous. I cannot find any modern confirmation that P. hinei occurs east of the Mississippi River.
Proctacanthus philadelphicus
Yellow mystax. Dark brown body and wings.
Size: 28-36 mm
FL Range: Erroneous in Florida?
Proctacanthus vittatus
“The color of the thorax of vittatus is quite variable. In some specimens the mesonotal vittae are conspicuous; in others they are quite absent, the whole mesonotum being rusty reddish. The tarsi are dark reddish, more castaneous apically” (Curran, 1951).
First abdominal segment black.
FL Range: Erroneous? Known from Dominican Republic and Haiti.
References:
Curran, C. H. (1951). The West Indian species of Mydas and Proctacanthus (Diptera, Mydaidae and Asilidae). American Museum novitates; no. 1507.
Bromley, S. W. (1928). Notes on the Genus Proctacanthus with the Descriptions of Two New Species (Diptera; Asilidae). Psyche, 35(1), 12-15.
Hine J.S. (1911). Robberflies of the genera Promachus and Proctacanthus. Ann. Ent. Soc. Amer. 4(2): 153-172.
Robber Flies of Florida 