The amazing robber fly

by Matthew Cobb

While I was asleep, my Tw**ps were chatting about robber flies. So I woke up to these great pictures. Robber flies – technically the family Asilidae – are strong and agile predators that as adults feed on other insects, sometimes much larger than themselves, catching them in the air. The result of such a chase is shown in this fantastic  photo by Sarah J. Semmler (@SarahSyrphid), showing a robber fly that had just grabbed a meadowhawk dragonfly:


What a fantastic photo! Sarah also tw**ted this photo of a robber fly by Seth Patterson from Bugguide, pointing out the well-developed ‘claws’ on the end of the tarsi, and the spines all over the legs which would make it easier to grab prey in the air.

Robber Fly - Efferia albibarbis - male

They will eat anything, including stuff much smaller than themselves, as seen in this photo by Muhammad Mahdi Karim of a Pegesimallus fly nomming what looks like a small fly:

File:Pegesimallus sp robberfly.jpg

And look at this, the massive ‘Florida bee-killer’,  Mallophora bomboides, photographed by Nancy West, University of Florida. That’s a honey bee it’s eating!

The "Florida bee killer," Mallophora bomboides (Wiedemann), with honey bee prey

And people think that mammals are beautiful predators!

According to Wikipedia (sorry) there are over 7,000 species of Asilidae. You can find tons of stunning photos on the internet.

Although the life-style of adult robber flies is well known, the ecology of the larvae – which interest me – is much less well understood. Larvae of some species appear to be classic detritus-feeding maggots, while others are reported to eat insects but mainly eggs and other larvae (i.e. they bumble across them, perhaps guided by smell and taste, rather than being cunning predators).


  1. Diane G.
    Posted September 24, 2013 at 12:20 am | Permalink

    Amazing indeed!

    And fabulous pictures. I love the way the robber fly in the third photo is just nonchalantly hanging by one leg.

    • Posted September 24, 2013 at 2:51 am | Permalink

      Bittacidae (hanging flies) are perhaps the most nonchalant of the Diptera. Some have elongate tarsi especially for gripping and dangling.

      • Posted September 24, 2013 at 7:50 am | Permalink

        Bittacids are actually a specialized family of scorpionflies (order Mecoptera), and thus not Diptera. Still very cool, and you’re correct that they’re specialist hangers.

        • Posted September 24, 2013 at 8:10 am | Permalink

          Classic mistake. And I’m a Mecoptera fan as well! Do you know of any other orders that enjoy ‘hanging around’ in this manner?

          • Posted September 24, 2013 at 11:02 am | Permalink

            I’m not sure! I know a lot of crane flies (Tipulomorpha) tend to hang from various substrates, but I’d guess this is more of a function of their delicate legs rather than a strategy.

        • Diane G.
          Posted September 24, 2013 at 10:28 am | Permalink

          Thanks for the info, Matthew and Morgan. This is the sort of stuff that can keep one web-surfing the rest of the day.

  2. Posted September 24, 2013 at 12:35 am | Permalink

    I think that Pegesimallus is having a go at a (mirid) bug, not a small fly. 😉

  3. rogercottonsr
    Posted September 24, 2013 at 1:23 am | Permalink

    Beautiful creatures indeed, however, now knowing their capabilities I shall pursue them with more vigor when they enter my home, knowing what a worthy adversary they are!

  4. Posted September 24, 2013 at 2:31 am | Permalink

    The bugguide robber fly is so beautiful in all its detail. Amazing.

  5. Adam M.
    Posted September 24, 2013 at 3:01 am | Permalink

    Found this amazing quote on the BugGuide site: “Cockroach wasps temporarily paralyze the adult cockroaches using venom. While paralyzed, the wasp makes a second sting, this time using venom inserted into a precise location in [the] cockroach’s brain. The antennae of the cockroaches are then clipped, and when the paralysis wears off, the wasp leads the cockroach to its lair. The cockroach makes no attempt to flee as an egg is laid upon it. When the eggs hatch, and the helpless cockroach is devoured by the young maggot-like wasps.”

    You can hardly make this stuff up…

    • Alex
      Posted September 24, 2013 at 3:30 am | Permalink

      > You can hardly make this stuff up…

      … or ever sleep again, for that matter.

      • JohnnieCanuck
        Posted September 24, 2013 at 4:16 am | Permalink

        What’s that white thing stuck on the back of your neck? Never mind, it’s just an empty egg shell…

        • Alex
          Posted September 24, 2013 at 7:25 am | Permalink

          Sound advice when in sects:

          When they lay one on you, eat your heart out… before they do.

    • jesse
      Posted September 24, 2013 at 6:56 am | Permalink

      It is amazing! If you would like to see a video of this, look at Carl Zimmer’s article from earlier this year, here:

  6. Posted September 24, 2013 at 4:11 am | Permalink

    Those are great photos. I did not know about the humongous ‘bee killer’ species, which is clearly bigger than our local robber flies that mimic bumble bees.

  7. ichneumonid
    Posted September 24, 2013 at 5:37 am | Permalink

    The Pegesimallus fly also looks (from the shape and colour of the abdomen) as if it might be mimicking an Ichneumonid wasp. A male wasp anyway, it lacks the jutting-out ovipositor of a female wasp. Intriguing!

  8. Diana MacPherson
    Posted September 24, 2013 at 5:51 am | Permalink

    The robber fly certainly has a fierce look with it’s spikely legs & ocular fringe. The size of it’s compact eyes are impressive as well. I wonder if they have hunting advantages over the many eyes of a spider.

    • Diana MacPherson
      Posted September 24, 2013 at 5:51 am | Permalink

      “spikey” and “its”. I haven’t had all my coffee this morning yet.

  9. darrelle
    Posted September 24, 2013 at 6:08 am | Permalink

    Robber flies are very interesting. Can I say cool? That’s what I really mean.

    I have one or two good pictures myself of an unknown (to me that is) robber fly my daughter and I came across. Not quite the quality of these pics though. One stand out feature was bright yellow feet on black legs. Similar to the third pic above, but otherwise different.

  10. Notagod
    Posted September 24, 2013 at 9:14 am | Permalink

    Thanks for posting on robber flies. I took some photos of one similar to Seth Patterson’s last June. I knew I had seen the type of fly before but couldn’t remember the name and my searching for a name wasn’t fruitful. Mine was apparently a male slurping on a house fly.

  11. marksolock
    Posted September 24, 2013 at 6:41 pm | Permalink

    Reblogged this on Mark Solock Blog.

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