A robber fly, Microstylum morosu, with the facial bristles, or mytax (“moustache” in Greek), visible.
Bottom-left: Up-close view of tiny assassin (or "robber") fly, genus Holcocephela, that eats tiny prey such as mites. (Eric Isley via iNaturalist, CC-BY-NC/Smithsonian image by Torsten Dikow)
Assassin or robber, this fly is on the most wanted list
March 27, 2017
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.
These are 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. There are more than 7,500 species of them. They are 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 it ingests the liquid food, whether it's a piece of your cantaloupe or some spilled sugar.
Assassin flies are members of the family Asilidae. They 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 paralyzing venom. Digestive enzymes accompany the venom. The venom turns the insides of their prey to liquid. Then, the typical sucking action is used to ingest the liquefied guts.
Assassin flies can be tiny. But 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. These help them detect fine movements of prey. Their hindwings are converted into little gyroscopic devices. These stabilize them during flight, conferring maneuverability.
Entomologist Torsten Dikow has described 68 new species of assassin flies and closely related flies. He continues to grow the collection at the Smithsonian National Museum of Natural History. Learn more about his work with these predators in the "Smithsonian Science How" webcast on Thursday, April 6, 2017. During Assassin Flies: Predators of the Insect World (airs at 11 a.m. and 2 p.m. EDT on the Q?rius website). Torsten will take you on a tour of his fly lab while answering your questions live. You can also get teaching resources to use with the webcast.
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CRITICAL THINKING QUESTION
Why do many insects have hard bodies?
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