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The Q? Blog

Assassin or Robber, this Fly is on the Most Wanted List

by Devin Reese -- Mar 16, 2017

Devin is the lead digital science writer for the Q?rius website. She writes and gathers media for the Smithsonian Science How? webcast series,...

A robber fly, Microstylum morosu, with the facial bristles, or mytax (“moustache” in Greek), visible.  Photo by Eric Isley via iNaturalist, CC-BY-NC.
A robber fly, Microstylum morosu, with the facial bristles, or mytax (“moustache” in Greek), visible. Photo by Eric Isley via iNaturalist, CC-BY-NC.

What animal has been witnessed snatching a bee from mid-air, stabbing it with a sharp tool, and sucking out its insides? An assassin fly is the culprit. Also known as robber flies, they stand out in their penchant for preying on other insects. While you may not know it, you’ve probably crossed paths with one of these predatory flies, as there are more than 7,500 species of them distributed around the globe.

All flies are suction feeders. But, many of them, including the common house flies we see buzzing around our lunch, do not kill prey. If you stare at a house fly (Musca domestica) feeding before you swat it away, you can see it swabbing at your food with its long mouthparts. This “proboscis” is part paintbrush, part straw. It is specialized to sponge digestive enzymes onto food, then ingest the liquid food, whether a piece of your cantaloupe or some spilled sugar. 

Assassin flies, members of the family Asilidae, have evolved a predatory twist on this feeding behavior with a proboscis that is part injection needle. The sharp proboscis is used to pierce the hard bodies of other insects and inject a paralyzing venom. Digestive enzymes accompany the venom and turn the insides of their prey to liquid. Then, the typical sucking action is used to ingest the liquefied guts. While assassin flies can be tiny, their ambitious feeding mode allows them to consume insects larger than themselves.

Assassin flies prey on a variety of insects, including stinging bees and wasps. The bristles on their face and body may shield them from the dangers imposed by their prey. Like other flies, assassin flies benefit from oversized, compound eyes, which help them detect fine movements of prey. Their hindwings are converted into little gyroscopic devices that stabilize them during flight, conferring maneuverability.

Entomologist Torsten Dikow has described 68 new species of assassin flies and closely related flies, and continues to grow the collection at the Smithsonian’s National Museum of Natural History. Learn more about his work with these predators in a "Smithsonian Science How" webcast video. During Assassin Flies: Predators of the Insect World, Torsten will take you on a tour of his fly lab. You can also get teaching resources to use with the webcast.