Robber flies are a diverse group of predatory flies that typically perch in an open area, swiveling their heads to look around, then flitting out to chase insect prey. Most have a long, tapered abdomen, a rather humpbacked appearance, and spiny legs, and typically rest with wings folded down the back. The face often appears bearded, and the mouthparts are a pointy knifelike proboscis for injecting saliva into prey. Between the two large compound eyes, the head is depressed, with three tiny eyes (ocelli) in that little valley. Many make a distinctive buzz or rattle in flight.
Larvae are grublike and live in or on soil, among decaying materials, or in rotting wood.
Similar species: Some robber flies are so long and slender they resemble damselflies. Many other robber flies have black and yellow striped patterns and make loud buzzing sounds as they fly; these are bee and wasp mimics. Some robber flies in the genus Laphria look almost exactly like bumble bees, complete with yellow fuzzy patches on their rather pudgy bodies. But true flies always have the same characteristics: only one pair of wings (bees and wasps have two pairs); the second pair of “wings” on true flies are reduced to tiny knobby structures called halteres (which function like gyroscopes, assisting flight). Also, flies usually have huge eyes and short antennae.
Body length (not including appendages): most are ½ to ¾ inch; varies with species; some reach 1¼ inches.
Habitat and Conservation
The different species have their preferred habitats, but most are found in open places, including grasslands, old fields, openings in woods, marshes, and sunny areas near ponds and streams. Be careful if you plan to collect robber flies; they can inflict a painful bite if mishandled.
Both adults and larvae are predators on other insects. The grublike larvae, as they move around in decaying organic materials, eat the eggs and larvae of other insects. As adults, robber flies dart from an exposed perch, snag, and stab insects with their pointy proboscis (tubelike mouth) as they fly past. The bite injects a digestive saliva into the prey, immobilizing it, and the fly returns to its perch to finish its meal. Robber flies’ audacity is legendary; on the Internet, one can find many stories of robber flies attacking bees, wasps, large grasshoppers, even hummingbirds.
The predatory habits of robber flies, both as adults and larvae, tend to put them into the same “beneficial” category as lady beetles, lacewings, mantises, and assassin bugs, since they devour many pest insects, including insects that damage crops. One species is known to prey on Japanese beetles. Some, however, eat honey bees, as well as wasps and bumble bees.
As a general rule, robber flies that hunt on the ground lay eggs on or in the soil, and those that hunt from plants deposit eggs into the stems, flowers, or other parts of plants. Larvae are grublike and generally live on the ground among decaying organic materials, or in decaying wood. When they’ve grown large enough, they pupate and emerge as winged adults.
Robber flies deliver a painful bite, so be careful with them. They don’t “go after” humans, but if you mishandle them, they might bite. Their bite, adapted for their predatory lifestyle, injects digestive enzymes and a painful venom. If one lands on you, don’t slap it — instead, brush it lightly away.
The typical foraging strategy of robber flies is similar to that of birds in the flycatcher group: They perch in exposed locations, then flit out to snatch prey, often returning to the same perch. Both are visual predators. The spines lining the legs of robber flies undoubtedly help them snag prey, as do the small, forward-pointing bristles beside a flycatcher’s bill.