Robber Flies

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Robber flies are aerial predators of a variety of insects. They often perch in conspicuous locations and make short flights chasing prey.

Donna Brunet

Other Common Name:

Assassin Flies

Family:

Asilidae (robber flies) in the order Diptera (flies)

Description:

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.

Size:

Body length (not including appendages): most are ½ to ¾ inch; varies with species; some reach 1¼ inches.

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Robber Fly

Most robber flies have a long, tapered abdomen, a rather humpbacked appearance, and spiny legs, and typically rest with wings folded down the back.

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Photo of a green-eyed robber fly depositing eggs into Missouri ironweed flowers.

Green-Eyed Robber Fly on Missouri Ironweed

Green-eyed robber fly (Promachus vertebratus) on Missouri ironweed (Vernonia missurica)

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Photo of a robber fly, genus Ommatius, perched on a wall.

Ommatius Robber Fly

In North America, there are about 13 species of robber flies in genus Ommatius. A hallmark of this genus are the somewhat feathery, branching antennae.

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Photo of a robber fly holding a bumblebee, eating it

Robber Fly Eating Bumblebee

Robber flies’ audacity is legendary; on the Internet, one can find many stories of robber flies attacking bees, wasps, large grasshoppers, even hummingbirds.

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.

Foods:

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.

Distribution in Missouri:

Statewide.

Status:

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.

Life cycle:

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.

Human connections:

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.

Ecosystem connections:

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.