Horse Fly with Colorfully Striped Eyes

2014 May 7

Here is a horse fly that I caught on July 2, 2013 when it was trying to bite me on the back of the head[1]. I only squashed it a little, so it was still in OK shape for photographing.

This is one of the horse flies with brightly colored eyes. Incidentally, it is also clearly a female, because male horse flies have these massive eyes that cover their whole head, kind of like a helmet (and they also don’t go looking for blood).

In order to get pictures of the eye colors, it is best to photograph insects while they are either still alive, or very freshly dead. Once they die, the colors fade (although the colors can apparently be preserved if you store the fly in ethanol, so the fading is probably just due to the eyes drying out).

The color does not appear to be due to pigments. It is more of a diffraction/interference effect, kind of like the effects that give rise to the colors in Opals.

The exact pattern of coloration is somewhat useful for identification. The fact that this one has four blue-green stripes bordered with orange suggests that it is in the genus Hybomitra.

Unfortunately, the ID key doesn’t seem to quite match this specimen, so I don’t know which species it might be, although I expect it is some variant of either Hybomitra zonalis or Hybomitra aequetincta. Both of these are northerly species, more common in Canada than in most of the US.

Incidentally, this is probably the same species as the horse fly I posted back in 2009, which I had misidentified as being in the genus Tabanus. I don’t know what I was thinking, those don’t look right at all.

[1] And, in our never-ending quest to find ways to avoid being bitten by horseflies, here’s an interesting item: It seems that the latest hypothesis on why zebras have stripes[2], is that biting flies like horse flies and tsetse flies tend not to bite black-and-white-striped surfaces. Why this should be so, I don’t know. But it seems to me it would be simple enough to wear black-and-white-striped clothing to check it out!

[2] Zebra striping has puzzled people for a long time, because you’d expect them to want to be camouflaged, and those stripes are pretty much exactly the opposite of camouflage. So, there must be some pretty compelling reason to have stripes that has nothing to do with staying hidden. And since biting flies (and particularly tsetse flies) carry all sorts of potentially fatal diseases, avoiding fly bites would be a good reason.

3 Responses
  1. Carole permalink
    May 7, 2014

    You’ve given me new appreciation for horsefly eyes and zebra stripes.

  2. May 8, 2014

    So if it’s a diffraction effect, then the colors arise from the underlying structure of the eyes, right? Do the lines appear to move as you rotate the fly?

  3. May 8, 2014

    The stripes do shift slightly as the head moves. The intensity of the color, and even the shade of color, changes noticeably, too. If they didn’t bite, people would probably quite like these.

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