The Giant Robber Flies get their name for a reason. They are around an inch long, and the noise they make while flying is quite loud. The scientific name of the robber fly pictured is Promachus vertebratus. The Greek background of the name promachos is “defender, champion.” Another translation says promachus means “challenger”, and yet another says “who leads into battle.” No matter which you use, the robber fly sounds and looks very menacing.
Luckily these flies are more interested in their prey than us. It may look like they could sting, but what you see in the picture is a female laying eggs with her ovipositor (not a stinger). If you try to catch one with your bare hand, they will bite.
Adult robber flies eat just about every other insect on the prairie. This includes wasps and dragonflies, which as earned them a common name of bee killer flies. If they can catch and hold onto it, they will eat it. The eggs will hatch from within the plant where they were laid, and the larvae will burrow underground. Here they will eat grubs and overwinter. When spring arrives, the larvae pupate and adults will emerge again in the summer.
Over 1,000 species of robber flies can be found in North America alone. This species has vivid green eyes – sometimes with some red. The front and face have yellow hairs and bristles, and the proboscis is usually shining black like the antennae. The thorax is yellowish with brown and grey to reddish stripes/lines along. The abdomen is usually grey with black stripes.
They are likely to be found in or above prairies and fields. As stated earlier, they will be flying throughout the summer and early fall. If you see one of these beauties along the trail this fall, don’t catch it, but instead enjoy the show they put on. Maybe you will get to see them catch some prey.
Photo by Larry Reis