Robber Flies

Top Photo: Giant robber fly

Some call them assassin flies. I first learned of them as robber flies. Whatever you call them, they’re predatory flies which perch out in the open, often near areas with heavy insect traffic in order to wait in ambush to snatch other insects out of the air. Visit a flowering group of Tithonia, milkweed, Joe Pye weed, or other rich nectar source with insects busily flying from flower to flower and you might see one of these swift flying, insect-hawking flies lying in wait.

If you’re really lucky you’ll see one quickly dash out, catch another insect, perch nearby and devour the unfortunate victim in front of you.

The robber fly injects its victim with neurotoxic and proteolytic enzymes which immobilizes the insect and liquifies its insides allowing the predator to suck the fluids out of the victim with the same proboscis it used to inject the prey.

Waiting to ambush whatever flies by.

The one pictured above, I believe, is a red-footed robber fly, or red-footed cannibalfly (Promachus rupifes). It’s certainly in the Genus Promachus. The giant robber fly below is also in the Genus Promachus but notice how its femora are nearly all white. The other Promachus robber flies pictured here have dark femora. Or, does the difference in leg color indicate age?

Head-on view of giant robber fly. Note white femora.

The robber fly below is in the Genus Diogmites. I’m not confident enough to give it a species name but it’s in a group of robber flies called hanging thieves.

Another genus (Diogmites), a hanging thief robber fly.

Hanging thieves get their name from their habit of hanging by two legs from vegetation or other objects while handling and consuming captured prey. There are about 26 species of hanging thieves in North America.

The 1,000 or so species of robber fly in North America range in size from about 5 mm to 30 mm in length. Most bear a physical resemblance to bees or wasps. Some are near perfect mimics of bees. They all have some sort of beard and or mustache and in general are quite hairy and spiked.

Hairy, spiky and resemble bees.

It’s always a thrill to see a robber fly, with or without prey.

All the robber flies belong to the family Asilidae the root of which (asilus) translates to gadfly, a fly that bites livestock. A curious name to give an insect family with such behavior as the robber flies. This group of insects is no mere collection of bothersome horse flies. They’re fierce predators. They catch and eat horse flies.

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