Robber Flies
What are they?
The robber flies are a very interesting family of predatory flies that feed voraciously on a wide range of other insect species. Adults can be found in a wide range of habitats, but are most often seen hunting along open, sandy tracks, where they patrol in search of prey. Many species are bee or wasp mimics, being relatively large and hairy in appearance. Life cycles of many species are not completely known, but the larvae of at least some species are parasitic on the larvae of other insect species. A beneficial family to us, as they take many horse flies and other biting insects.
Identification
Robber flies are mostly rather hairy species, often resembling bees or wasps in color. They typically have furry faces and an obvious dip between the eyes on the top of the head. Identifying individuals to the genus level is usually pretty straightforward, but identifying to species can be problematic and the identification of some species groups based on external appearances are not fully resolved.
Asilus sericeus
A relatively hairless species with a long, tapered abdomen and hairy legs. This species has a rather long flight period and may often be the earliest robber fly to be seen in the spring.
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Diogmites sp.
The Diogmites robber flies are pale tan or straw-colored with three dark lines on the thorax. They have long, gangly legs and have a habit of hanging from the underside of twig by two of their legs while eating their prey.
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Efferia sp.
The Efferia robber flies may be pale tan or bluish-gray in color and are often seen throughout the summer months hunting from conspicuous perches. They often come to light at night where they prey on other insects that have also come to the light. Males have black, bulbous tips to the abdomen, while females have long, slender ovipositors, with which they lay their eggs
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Efferia aestuans
One of the commonest robber flies in the northeastern USA and often quite bold and approachable. A dull, brownish species with black legs that have reddish brown mid-sections to them. Hairy face has black hairs mixed with white ones.
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Laphria flavicollis
A remarkable bumblebee mimic, but the shape of the eyes and the forked antennae clearly show this to be a fly. The distinctive abdomen is slender, but is carried arched to make it appear fuller and more rounded, like that of a bumblebee.
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Megaphorus clausicellus
The Megaphorus robber flies are relatively small species that seem to be mimicking the leaf-cutter bees. They are densely hairy and are most often found hunting from grass stems in open areas.
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Ommatius sp.
This is a relatively small genus of robber flies, but the various species can be very difficult to tell apart and there seems to be much overlap in external appearance. Coloration of the legs is an important aspect of species identification. Ommatius species often perch conspicuously on exposed twigs, fence tops, parked cars and similar places.
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Proctacanthus nigriventris
A genus of large but rather plain robber flies, Proctacanthus species are out and about during the hottest days of summer and are most often found on the edges of wet woods. Being relative large, they hunt larger prey and will often take tiger beetles or grasshoppers..
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Promachus rufipes
Our biggest robber fly and a really impressive insect! Unfortunately this species' resemblance to a wasp or bee makes it feared by many people, but it is totally harmless to us. Uncommon but widespread in sunny habitats, this is one of our most impressive insects. They are readily approachable and can be studied at close range as they hunt from a perch, often dashing out to bring down prey in flight.
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