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 their amazing acrobatics and brutal predatory proficiency. Indeed, the alternative common name for the family is “Assassin Flies.”
Robber fly larvae are also predaceous. They live in the soil, or in decaying wood and other organic matter, where they feed on insect larvae.
Many species of these unusual-looking flies are on the wing at this time of the year. The flies come in a wide range of sizes as illustrated by the images below. The first image was taken this past weekend and 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.
One of the largest-sized robber flies found in Ohio is the Maroon-legged Lion Fly (Promachus hinei) with adults measuring over 1 1/4” long. This robber fly does not have a common name approved by the Entomological Society of America (ESA), so I’m using the common name applied by iNaturalist.
I spotted the lion fly pictured in the lead photo for this Alert during a recent site visit to photograph box tree moth (BTM) (Cydalima perspectalis, family Crambidae) caterpillars in Loveland, OH. The fly was perched on the netting of a BTM adult trap. Of course, there were no connections between the moths, the trap, or the fly other than the netting was a nice place for the fly to perch as it surveyed its surroundings for food.
Like all robber flies, lion flies are natural-born killers with their entire body designed to support their predatory lifestyle. Their long, narrow bodies aid in stream-lined 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.
Like many robber flies, lion flies take their prey out of the air. This requires aeronautical skills matched by few other flying animals. In an amazing display of aerial acrobatics, the fly slams at high speed into an airborne quarry stupefying the hapless insect victim.
The lion fly makes a distinct buzzing sound as it flies which is loud enough to draw your attention. 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. The flies are indiscriminate in their insecticidal pursuits and will attack a variety of insects including "armed" prey like bees and wasps.
Once the fly disables its target, it then grasps its dazed prey with its claws and uses its piercing-sucking mouthparts to inject saliva containing neurotoxic and proteolytic enzymes. As if being slammed by an airborne battering ram isn't enough, the neurotoxins paralyze the victim ending its days (daze?). The digestive enzymes dissolve the internal tissues robbing the ill-fated victim of its life. The fly then lands and uses its soda straw-like mouthparts to have an insect Slurpee.
Hanging thief robber flies (Diogmites sp.) don't land on a horizontal surface to feed. Instead, they use their grappling hook-like tarsal claws to hang from a leaf or stem, thus their common name.
Robber fly adults and larvae are generally considered beneficial. However, in 2020, Tonda Pence from Garrett, IN, sent me a picture of a European honey bee (Apis mellifera) in the clutches of a lion fly illustrating the downside of these undiscerning predators.
Indeed, the lifestyle of some robber flies clearly fall outside the limits of "beneficial." As their common name indicates bee-killer robber flies (Mallophora spp.) commonly attack bees including honey bees. 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.