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Robber Flies Active

8-3-18 robber fly with butterfly2_U1A3399

You may well have seen a robber fly (also known as an assassin fly) and not recognized it as such.  Big eyes, pointed mouthparts, a hairy face and a fondness for insects are robber fly characteristics to look for.

Robber flies are among our largest flies and are predatorial ambushers.  They tend to perch for minutes at a time on the tips of leaves, or other sunny viewpoints, where they keep a lookout for unsuspecting prey.  Once they spot an insect they lose no time in darting after it.  With their beak-like mouthparts they spear the insect and inject a mixture of nerve poisons and enzymes that liquefy the tissues of their victim.  They then drink the innards of their prey.

Note that there is a prominent “beard” or tuft of bristles (mystax) in the front of the pictured robber fly’s face.  All robber flies have this tuft which serves to protect the face and eyes of the predator from the struggling prey as it dines.

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16 responses

  1. Guy Stoye

    I discovered robber flies many years ago when I was working with a buddy in the tree surgery business on Long Island. It was a hot, humid morning and we were sweating and being harassed and bitten by deer flies. A deer fly landed on my bare arm and as I raised my other arm to swat the fly, an insect swooped down, grabbed the deer fly and flew off with it.
    That surprising incident got me looking through my nature books where I made the positive ID. These are amazing insects which I now regard as friends and am always glad to see one around though this happens all too rarely.
    Unfortunately, their appearance gives one to believe that they are stinging, dangerous critters so I know that the knee jerk reaction of most people is to kill them. Very sad. They will never bite humans or other mammals but instead prey on the insects which do bite us and helpless cattle and other livestock in the fields.

    August 3, 2018 at 8:03 am

    • Interesting thought on this Guy.

      August 3, 2018 at 8:07 am

    • We share a fondness for these little predators, Guy. By far my favorite fly!

      August 3, 2018 at 9:14 am

  2. Yikes…very sneaky. BTW how big is it (approx?). Thanks for telling us about it.

    August 3, 2018 at 8:04 am

    • Roughly 3/4″ to 1″, depending upon species!

      August 3, 2018 at 9:12 am

  3. Alice Pratt

    I’m going to write my thoughts: “that’s gruesome…and do they sting/bite humans?” Guy, above answered my question….

    August 3, 2018 at 9:09 am

  4. Bill on the hill

    Hi Mary… Outstanding detail on both the predators & preys eye. This is the kind of stuff that really excites me. Mary, you have got to get yourself a set of ” tube extenders ” to put behind the macro lens. You will get close to microscopic detail then! And of coarse, the subject has to stay put, it can be challenging at times, but I know you can do this!
    It will bring your macro photography to a whole new level…
    Bill Farr…

    August 3, 2018 at 9:13 am

    • Hi Bill,
      Thanks for all your great photographic advice! I should sign up for a lesson from you! I do have extenders, but I’ve never tried using them in the field with a handheld camera. Will do! Many thanks.

      August 3, 2018 at 4:15 pm

  5. Jo

    Wow, this is new to me. Fascinating!
    And, what a photo!

    August 3, 2018 at 9:18 am

  6. Ruth Gross

    Mary, This the most unusual fly I have seen!! God’s Creation certainly not man!

    Thank you for taking your time to share with us, Ruth Gross

    Sent from my iPad


    August 3, 2018 at 10:23 am

  7. Donna

    What a photo! Mary, did you take that yourself?

    August 3, 2018 at 11:54 am

    • Yes, I did, Donna. With a 100mm macro that I purchased with the incredibly generous donations from my blog readers after my photography equipment was stolen!

      August 3, 2018 at 1:19 pm

  8. Kathryn

    Quite an interesting creature though I honestly can’t say if I’d rather be the bug being liquefied or the fly drinking it all up!

    August 3, 2018 at 8:42 pm

  9. Barry Avery

    I’ve seen plenty of them. We have 6 cicada Killer Wasps in the stone dust between Granite pavers at our place. I have been fascinated watching them.

    August 4, 2018 at 8:56 am

  10. Mac Everett

    I couldn’t help but think of Darth Vader!

    August 20, 2018 at 11:33 am

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