During this week’s BYGL Zoom Inservice, Erik Draper (OSU Extension, Geauga County) shared an image taken by his colleague Wendy Ward (OSU Extension, Geauga County) of a Red-footed Cannibal Fly (Promachus rufipes) caught in an insecticidal act robbing the life of an insect victim. I don’t have a photo of this impressive fly (it’s on my photo-bucket list), so I was thrilled that Wendy’s photo-prowess provided the perfect lead image for this Alert.
There are over a thousand species of Robber Flies (family Asilidae) in North America, and the adults of all species are fierce predators. Few carnivorous insects can match the amazing aerobatic skill and brutal predatory proficiency of robber flies (family Asilidae) in their pursuit of prey. Adults of all species are predators and will attack a variety of insects, including those armed with stingers like bees and wasps. Indeed, the alternative common name for the family is “Assassin Flies.”
Robber fly larvae are also predatory. They live in the soil, or in decaying wood and other organic matter, where they feed on insect larvae.
Giant Robber Flies
Many species of robber flies are currently on the wing in Ohio. However, members of the Promachus genus, which are collectively known as “Giant Robber Flies,” are arguably the most impressive in terms of size, sound, and fright factor.
Adults commonly measure over 1 1/4” long, and they make a loud, buzzing sound as they fly. They will sometimes land on or near people, inducing loud, screeching sounds, laced with expletives, and coupled with creative bug-eyed dance routines.
Two of the most common giant robber flies currently stalking flying insects (not people) in Ohio are the aforementioned red-footed cannibal fly and the Maroon-legged Lion Fly (P. hinei). Like all robber flies, these giant robber flies are natural-born killers with their entire body designed to support their predatory lifestyle.
Maroon-legged Lion Fly (Promachus hinei)
Matig Maroon-legged Lion Flies (Promachus hinei)
Giant robber flies have large, bulbous eyes (all the better to see you with). Their long, narrow bodies aid in streamlined flight. Their stout thorax is packed with muscles to operate the wings and legs. Their long, dangling legs are tipped with two formidable tarsal claws that function like grappling hooks. The ball-like shape of the thorax serves as a battering ram.
Maroon-legged Lion Fly Tarsal Claws
Like many robber flies, giant robber flies take their prey out of the air. This requires aeronautical skills matched by few other flying animals. In an amazing display of aerobatics, the fly slams at high speed into an airborne quarry, stupefying the hapless insect victim.
The giants draw your attention with their distinct buzzing sound. If you hear the buzz punctuated by a very loud "snap," like someone snapping their fingers, it means the fly has rammed its prey … at high speed.
I once observed and heard the dramatic prey-slamming behavior of a lion fly when it disabled a baldfaced hornet (Dolichovespula maculata) near where I was standing. It demonstrated the effectiveness of the prey-slamming behavior in countering a stinging defense.
Maroon-legged Lion Fly With Honey Bee
Once the robber fly disables its target, it then grasps its dazed prey with its tarsal claws and uses its piercing-sucking mouthparts to inject neurotoxic venom. As if being slammed by an airborne battering ram isn't enough, the neurotoxins paralyze the victim, ending its days (daze?). Next, it injects enzymes from its stomach to pre-digest the innards of its meat meal, robbing the ill-fated victim of its life. The fly then lands and uses its soda-straw-like mouthparts to have an insect Slurpee.
Other Robbers
Robber flies come in a wide range of sizes, as illustrated by the images below. The first image shows a tiny male and female of an unidentified species resting on the cut 6-gauge wires at the top of a chain link fence.
Unidentified Robber Flies on Cut 6-Gauge Wires
Hammertail Robber Flies (Efferia spp.) are so-named because of the black, rounded structure at the tip of the male’s abdomen. Females look like the males, but have long, slender ovipositors. The Eastern Hammertail (E. aestuans) is almost an inch long and may be observed perched on vegetation surveying its surroundings for prey. Once they spot a mobile meat morsel, they follow the same prey-dispatching playbook described with the giant robber flies. Hammertails may also show up at porch lights at night for a late-night snack.
Eastern Hammertail Male (Efferia aestuans)
Eastern Hammertail Female (Efferia aestuans)
Hanging Thief Robber Flies (Diogmites spp.) are so-named because of their peculiar feeding behavior. They don't land on a horizontal surface to feed, but instead, use their grappling hook-like tarsal claws to hang from a leaf or stem.
Plain-Tailed Hanging Thief (Diogmites neoternatus) with Yellowjacket
The lifestyle of some robber flies clearly falls outside the limits of "beneficial." As their common name implies, Bee-Killer Robber Flies (Mallophora spp.) commonly attack bees, including honey bees (Apis mellifera). Likewise, robber flies belonging to the genus Laphria will also attack bees. These robber flies mimic bees and are sometimes called bee-like robber flies.
Bee-Like Robber Flies Mating (Laphria sp.)
Selected References
Bromley, S. W. (1948). Honey-bee predators. Journal of the New York Entomological Society, 56(3), 195-199.
https://www.jstor.org/stable/25005271
Bromley, S.W. Ohio robber flies IV (Diptera: Asilidae). Ohio J. Sci. 1947, 47, 67–68
https://kb.osu.edu/server/api/core/bitstreams/f3f9ef49-92cb-535a-adf2-167a2988f1be/content
Cohen, C. M., Cole, T. J., & Brewer, M. S. (2020). Pick your poison: molecular evolution of venom proteins in asilidae (Insecta: Diptera). Toxins, 12(12), 738.
https://www.mdpi.com/2072-6651/12/12/738
Drukewitz, S. H., Fuhrmann, N., Undheim, E. A., Blanke, A., Giribaldi, J., Mary, R., ... & Von Reumont, B. M. (2018). A dipteran’s novel sucker punch: Evolution of arthropod atypical venom with a neurotoxic component in robber flies (Asilidae, Diptera). Toxins, 10(1), 29.
https://www.mdpi.com/2072-6651/10/1/29#
Fabian, S. T., Sumner, M. E., Wardill, T. J., Rossoni, S., & Gonzalez-Bellido, P. T. (2018). Interception by two predatory fly species is explained by a proportional navigation feedback controller. Journal of The Royal Society Interface, 15(147), 20180466.
https://doi.org/10.1098/rsif.2018.0466
Knight, K. (2022). Robber flies combine approaches to simplify airborne interceptions. J Exp Biol 15 February, 225 (4): jeb243931.
https://doi.org/10.1242/jeb.243931
Mccravy, K. W., & Baxa, K. A. (2011). Diversity, seasonal activity and habitat associations of robber flies (Diptera: Asilidae) in west-central Illinois. The American Midland Naturalist, 166(1), 85-97.
https://doi.org/10.1674/0003-0031-166.1.85
Wardill, T. J., Fabian, S. T., Pettigrew, A. C., Stavenga, D. G., Nordström, K., & Gonzalez-Bellido, P. T. (2017). A novel interception strategy in a miniature robber fly with extreme visual acuity. Current Biology, 27(6), 854-859.