Robber fly

2007 July 17

An Example of Convergent Evolution


This robber fly (subfamily Asilinae, I think[1]) was hanging out on our mulberry bush. It didn’t move when S. broke off the branch and brought it inside, and I actually thought it was dead while I was photographing it. But then, after getting several good pictures, it suddenly flew off. It was the most cooperative subject I’ve had since the millipede (and the dead ones, of course). These are carnivorous flies that prey on other insects. I suppose they would be capable of biting people, too, but I’ve never had one try to bite me.


Looking at the head and thorax, it is very clearly related to “standard” flies, with mouthparts that look a lot like what sandflies and horseflies use to suck our blood. The thing that I find interesting is that aside from the head shape and thorax coloration, they actually have a body plan that closely resembles damselflies, with the long wings, long legs that can be held in a sort of “basket”, enlarged thorax to provide room for larger wing muscles, and the very elongated abdomen, all of which are un-fly-like traits. As it turns out, both robber flies and damselflies have one thing in common: they are airborne hunters, grabbing their prey on the wing in mid-air. Most of the traits listed are obvious advantages in pursuing and snatching things out of the air[2]. The fact that these two distantly-related lineages both converged on the same solutions to the problem is very suggestive. For objects on this scale, this body plan is apparently ideal for high-speed pursuit and capture of small, rapidly-jinking targets.

[1] This seems to come up a lot with flies (Diptera), they all have clusters of species that look really similar. Judging from the Bug Guide, the possible genuses for this one are Neoitamus, Asilus, Efferia, Machimus, Proctacanthus, or possibly Promachus. I’m probably not picking up on some obvious distinguishing feature that would let me narrow it down a lot, it would be nice if Bug Guide had formal keys, and not just scads and scads of pictures.

[2] I’m not too sure what the advantage of the elongated abdomen is, whether it provides better streamlining for speed, or whether it gives a better moment arm for swinging the body around for better maneuverability. Maybe both. It would be interesting to film them with a high-speed camera to find out, has anyone heard of a researcher doing this?

One Response
  1. ALASTAIR KENNEDY permalink
    March 22, 2009

    I have been spending the morning trying to research flies in Singapore. I live in Sembawang and was biiten by a fly that looks rather like the Robber fly. The sting of the bite was similar to a bee sting. Initially there was no swelling (bitten on a Sat morning) but by Monday morning (now) my thumb has swollen quite alot and I have had to get anti-biotics from the doctor. IOt is also very itchy – hope there will bo no other after effects. Sometimes you just don’t know what bacteria they may have injected into you! Regards. Alastair.

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