Window Flies, and Compound Eyes

2009 February 14

We found the first of these little flies on May 23 of 2007[1], on our window:

A second one turned up a few days later, on June 2.

At first, I thought these were Syrphid flies, maybe in the genus Pipiza, but as Richard Vernier and Keith Bayless pointed out on Bug Guide, I was Wrong, Wrong, Wrong! They are actually “Window Flies”, family Scenopinidae. It turns out I should have been paying more attention to the shape of the antennae and the veins in the wings, which were all wrong for Syrpid flies[2].

Unfortunately, Keith Bayless says that my pictures don’t show the wing veins well enough to identify it down to species. In the BugGuide description, it says that the cosmopolitan species Scenopinus fenestralis is commonly found in buildings on windows worldwide, and surprise, surprise, that’s where I found these. So it would not surprise me at all to find out that that’s what these are. It turns out that the larvae live in leaf litter and debris, but not to eat it: they are predatory on the other small arthropods in the leaf litter. They aren’t terribly well studied, but there are some reports that the larvae sometimes live in houses and prey on carpet beetle larvae in rugs.[3]

I dont remember now why I took pictures of the second one, I think I might not have realized that I already had an example of the species. I’m glad I did, though, because it turns out that the first one was a male, and the second one was a female. How can I tell? The eyes, look at the eyes!



Male flies tend to have bigger eyes than female flies, I think because they track their mates by sight. So, the males have eyes that run together, while the females have a band running up their forehead that clearly separates the right eye from the left eye.

Which brings us to the topic of insect eyes. This is something I’ve been interested in, because, well, look at all the difficulties that I have doing macro photography and trying to get pictures of insects. And think of the difficulty that you have focusing your eyes at very close range, say less than a centimeter. And consider: for small arthropods like insects, everything they see is something that we would consider to be a “macro shot”. And on top of it all, they are highly size-limited, they can’t have the honking huge lenses that we use on our cameras and microscopes for looking at things close up. For the smaller insects, like these, their whole eye has to be on the order of the size of a pinhead. So, how can they do it?

Some other arthropods, like spiders, basically can’t do it. They do the best they can with simple eyes (a single lens, and an array of light receptors to make an image), but even the best spider eyes are evidently low-resolution and what we would consider fantastically near-sighted. Which is why a lot of spiders pretty much give up on the whole “vision thing” altogether, settling for enough eyesight to basically detect light and movement, and letting it go at that.

Insects, though (and some other arthropods, like crustaceans) have actually come up with a solution: the compound eye. As near as I can understand, what they have done is this: instead of just a few eyes, they have an array of hundreds, or even thousands of tiny eyes. Each sub-eye has a very restricted field of view, and hardly any resolution. But, by patching them together in the brain, the insect can stitch together the individual images to make a full image. Since each element is pointing in a slightly different direction, this also gives the insect a full panoramic view. And. since the individual elements are practically pinhole cameras, they have essentially infinite depth-of-field, and so the insect can see both close-up objects and far-away objects with reasonable clarity.

I’ve been thinking for some time that it would be nice to have a camera that uses the same principle as the compound eye, so that I could get around the depth-of-field problems that plague me when taking pictures of bugs. It turns out that there is some work ongoing in this area. In this PDF document the authors describe how they have done just that. They have actually made an electronic compound eye, and extracted decent images from it. The resolution is still not nearly as good as what one gets from conventional camera optics, but they are doing it with a camera that is about the thickness of heavy paper, and not some monster lens assembly several inches long. One of their objectives is to make a camera that can just stick on flat surfaces. There are other research groups working on other aspects of making electronic compound eyes, and I fully expect that eventually they will be able to match the best insect compound eyes. This is probably not as good as our eyes as far as image resolution, but will be much more compact, will allow for panoramic views, and will be able to simultaneously view near and far objects without refocusing.


[1] “But this is 2009!” I hear you cry. “What’s with the nearly-two-year-old picture?” Well, as you can see I’ve got a bit of a backlog. I didn’t get a chance to work through all of the 2007 pictures before winter of 2008 ended, so now this winter I need to work through these old bug pictures. Otherwise, I may never get to some of them at all.

[2] Of course, I do have an excuse for not correctly identifying these from pictures on BugGuide in the first place: the pictures I put up were evidently the first examples of their genus to be submitted to BugGuide. I’ve actually been the first submitter for a surprisingly large number of insect species, about half of what I submit doesn’t even have a page for the appropriate genus yet. This is mainly because I’m taking a lot of pictures of little bugs that aren’t very visually striking or distinctive. The pictures that people take of bugs tend to over-represent the big, showy ones pretty severely. So, if you want to submit unique pictures on BugGuide (or any other insect identification site), concentrate on the little guys.

[3] This should be good news for all the people who desperately want to get rid of carpet beetles in their house. Which is apparently a lot of people. During the winter, that page is not only the single most popular page on this site, it appears to get about as many visitors as all of my other archived pages combined. Although, once tick seasons starts up again around May, I expect the Wood Ticks will once again give them a run for their money.

3 Responses
  1. February 19, 2009

    I like the way many common names are so intuitive (let’s call these flies that hang out by the windows “window flies” until we figure out what they are) we tend to dismiss them. I’m excited about the reports of these predating carpet beetle larvae. Someone had a question for me about bio-control for carpet beetles, but I had no answer to offer, not even a grunt. Thanks for the info on that.

  2. February 20, 2009

    I kind of like the idea of using them for bio-control of carpet beetles, but the unfortunate part of it is that the type of people who freak out over carpet beetles will probably also freak out over having a bunch of these flies suddenly appear on their windows. Sometimes you just can’t win. Although, the people who are actually concerned about carpet beetles because of the damage they do, and not just because they are “icky”, might be willing to give it a whirl with these flies. I wonder how hard it would be to rear them in captivity?

  3. February 21, 2009

    Linked with a little bit of added research on spider brains.

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