Subject: robber fly identification
Geographic location of the bug: New Jersey
Time: 01:18 PM EDT
Your letter to the bugman: Greetings:
I’m having trouble identifying Robber Flies. I’ve uploaded 2 fairly similar pictures (more pictures can be found at https://www.inaturalist.org/
P.S. I think Robber Flies are interesting, but don’t really know much about them except that they’re considered fairly tough predators in the arthropod world.
How you want your letter signed: Baffled By Robberflies
Sometimes it is not possible to identify a species conclusively from a photograph alone, and the Giant Robber Flies in the genus Promachus can be a challenge. We are somewhat inclined to doubt the accuracy of the individual identified as Promachus hinei on iNaturalist because BugGuide lists the range of the species as being considerably west of New Jersey with Ohio being the easternmost sighting. Because of the proximity of your two sightings in both location and time, we would be most inclined to suppose them to be the same species, and the Red Footed Cannibalfly is a likely possibility, especially since both of your individuals have red “feet” and the physical description on Encyclopedia of Life states: “Adults are 28 – 35 mm long. Typical of robberflies, the eyes are large and separated by a deep trough on top of the head. The body is covered in yellow bristles, particularly on the head and abdomen. The legs are black, except for orange tibia and pulvilli. As with other members of the genus Promachus, this species has sharp claws and an abdomen that extends beyond their folded wings. ” We apologize for not being able to provide a conclusive identification and we suspect that if actual specimens were to be identified by a dipterist specializing in Robber Flies, many of the individuals identified on our site as Red Footed Cannibalflies might actually be members of a similar looking species in the same genus. Like many other large Robber Flies, Red Footed Cannibalflies and other members of the genus are able to take large prey, often stinging wasps and bees, on the wing. Thank you for becoming a Patreon member and helping to support the free service we are able to supply on the World Wide Web.
Thanks for telling me what you know about these guys. I suspect that I’m going to have this problem with a lot of insects; too many similar-looking relatives. (I was kind of hoping you’d be able to tell me that no other species that looks like the Red-footed Cannibalfly lives in NJ.)
We located this comment on BugGuide “As I understand it, there are three ‘tiger-striped’ species of Promachus in the eastern U.S., with P. hinei being the most common in the central U.S. It is distinguished from the more southern P. rufipes by the reddish rather than black femora and from the more northern P. vertebratus by the larger dark areas dorsally on the abdominal segments and distinctly contrasting two-toned legs.” That supports our supposition that both of your individuals are Red Footed Cannibalflies, Promachus rufipes.
That was helpful. Just for due diligence, I then checked out BugGuide’s info on Promachus vertebratus to see if that could be a good candidate for my Robber Flies. And it really isn’t; P. vertebratus’s range is typically outside NJ, and it’s green or red eyes aren’t a match for any of my Robber Flies.
I’m cautiously optimistic that the Red-footed Cannibalfly is what I’m seeing.
And we agree, so we are changing the subject line of the posting to reflect that.