One of my favorite insects is the Red-Footed Cannibalfly (Promachus rufipes) which is a type of robber fly (family Asilidae). Few carnivorous insects can match the amazing acrobatics and predatory proficiency of robber flies in their pursuit of prey. Adults of all species are predators and they will attack a variety of insects; even "armed" prey and insects much larger than their own body size. One of the appropriately named Hanging Thief Robber Flies (Diogmites spp.) pictured below is chowing down on a yellowjacket. There are over a thousand species of robber flies in North America representing a wide range of forms and sizes.
The Red-Footed Cannibalfly is the largest sized species found in Ohio; the flies measure over 1 1/4" in length. The Cannibalfly makes a loud buzzing sound as it flies and alert observers may hear the buzz periodically punctuated by a very loud "snap" which means the fly has committed an insecticidal act. I heard the characteristic buzzing last Thursday while taking pictures of compass plant (a future Perennial of the Week), but wasn't able to snap a shot of the huge predator; the pictures in this report were taken a couple of years ago.
The entire body design of the Cannibalfly foretells its predatory lifestyle. Their long, narrow body aids in stream-lined flight; think fighter jet. Their stout thorax is packed with muscles to operate the wings and legs and the ball-like shape serves as a battering ram; think linebacker. Each of their long, dangling legs is tipped with two formidable tarsal claws that function like grappling hooks. In a smashing display of aerial acrobatics, the fly slams into its airborne quarry stupefying the hapless insect victim. The high-speed collision produces a snapping sound that may be heard several feet away. The fly then grasps its dazed prey with its claws, and uses its piercing-sucking mouthparts to inject saliva containing neurotoxic and proteolytic enzymes. The enzymes paralyze the victim and digest the internal tissues. The fly then lands, and sips the life out of its victim.
Robber fly larvae are also predaceous and live in the soil, or in decaying wood and other organic matter, where they feed on insect larvae. Although robber fly adults and larvae rob insects of their lives and are generally considered beneficial, a few species test the limits of "beneficial." As their common name indicates Bee-Killer Robber Flies (Mallophora spp.) commonly attack bees including honeybees. 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; a twist on the adage, "you are what you eat."