Robber flies

2021 February 14

Robber flies generally are flighty little beasts. Normally I don’t even get one good photo before they fly off. In this case, on July 2, 2019, I did manage to get one good shot at least:


Then a bit over a year later, on July 20, 2020, we found one that was a bit more cooperative. It had fallen into some water, and when we rescued it with a stick it pretty much just stayed still for quite a while. Unfortunately, I had forgotten to reset my lens aperture from using the camera on non-insect subjects, and ended up with a bunch of pictures that were way too blurry to use because of bad depth-of-field. But there were a few that came out kind of OK. Here is the second robber fly from the side, looking a bit bedraggled:


And here’s a dorsal shot so we can see how they hold their wings:


And, finally, a closeup of the eyes:


I think they are both female, male robber flies tend to have structures at the tip of the abdomen. I’m not quite sure if they are the same species or not. The body shape and leg colors match, but the second one has more distinct striping on its abdomen. I think the best match might be Machimus sadyates, which is found in this area and looks to fly from July through September.

Anyway, robber flies are predatory. Their preferred technique is to hang out on twigs and leaves that provide a good field of view, and then quickly chase down any prey that passes by too closely. This particular species is clearly built for fast pursuit, with a long body for streamlining, and relatively short wings with powerful wing muscles to provide good acceleration and maneuverability. Once they catch their prey, they have a piercing/sucking mouth that allows them to quickly suck the contents out of it. The big eyes with large numbers of small optical elements makes it clear that they are visual hunters, with both a good field of view and good visual acuity.

The larvae live under the bark of dead trees and in decaying matter on the ground. I’m not seeing any indication of whether or not the larvae are carnivorous or not. They are apparently legless, eyeless maggots similar to the larvae of most other flies, so they probably are omnivores and eat whatever is available.

3 Responses
  1. February 17, 2021

    Eww. What a horrible sight to see when I first click over here. Horrible.

    Great photos despite your settings.

    Still, this is horrible. It’s one of the few insects I would kill on the spot, the wretched, biting brute.

  2. Steve Plumb permalink
    February 24, 2021

    The commenter who hates robber flies must be confusing them something else. A quick search confirmed that they don’t use humans for food and will only cause injury if one tries to capture them. I also was pleased to discover the great variety of robber flies species out there.
    Now the green head, horse and deer fly trying to feed on me, I’d squash the in a second if I could catch them!

    Thanks for keeping the posts coming.

  3. February 27, 2021

    Yes, robber flies look a lot like stretched deer flies and horse flies, and I suspect that KT might be mistaking the robber fly for one of them.

    Still, robber flies can bite defensively if they are roughly handled, so it is probably a good idea not to catch them in your hands.

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