Why aren't flies blinded by the sun?

[Beatsy]

I saw one sit completely still in a penny-sized spot of bright sunlight for 10 minutes before it flew off. It prompted this thought that I've never considered before. Their compound eyes have no eyelids or irises (right?) but do have lenses. So even if bright, visible light is no issue, surely there must be heat buildup!? Or do they have built-in IR filters?

[MarkSturtevant]

I have no idea. The facets you see on a compound eye are just the visible part of an 'ommatidium', which is a tiny light collecting organ with a cuticular lens at the surface and several kinds of cells that extend deep into the eye. Among these cells will be pigment cells that help channel the light down the ommatidium to the retina cells, and to block light from leaking in from adjacent ommatidia. Perhaps those retina cells play a key role in blocking light in a protective fashion.
But I don't know. One may have to consider matters of scale. These structures are tiny. How hot can something get if its very small?

[Lou Jost]

Good question

[mawyatt]

Wonder if they have a type of structure where the light transmission varies with intensity, like the sunglasses that get dark when exposed to bright light, then return to somewhat clear when the intensity drops?

Best,

[rjlittlefield]

I have read quite a bit about fly eyes, but I do not recall ever seeing this specific issue addressed.

Searching today, I found only that there is a "pupil" mechanism that limits light transfer to the receptors. "The fully light-adapted pupil diminishes the photoreceptor's acceptance angle by a factor of ∼0.6 due to selective absorption of higher order waveguide modes." (HERE)

I was amused, in passing, to run across one unusually pithy acknowledgement. It is by Adrian Horridge, in "The spatial resolutions of the apposition compound eye and its neuro-sensory feature detectors: observation versus theory" (HERE)

I am much obliged to two ardent referees who, with their contrasting conceptions of their duty, cast light and heat into this debate.

--Rik

[Beatsy]

Thank you for your comments everyone, and Rik for the definitive answer (definitive enough to satisfy my curiosity, anyway). I learnt a lot more about fly eyes in general over the course of this too. All good stuff. Cheers.

[Troels]

Very intersting question and answers.

I just want to add, that while we are used to enlarge everything with our wonderfull equipment we often forget the important factor of scaling in physics.

Collecting the light from the sun through our own contracted eye pupil with a diameter of, let us say 2 mm, and concentrating it on the retina generates enough energy to damage the eye permanent after a few seconds.

Collecting the sun light with a telscope with a 100 mm lens supplies us with enough energy to boil the inner eye instantly because the energy collecting area is so much bigger.

The single lens in front of a single ommatid in a compund eye of a fly can have a diameter of 25 micrometer. Even if the sunlight is concentrated on a smaller spot, the total amount of captured energy must be fairly small and perhaps easily absorbed and distributed?

Does that make sense or have I forgotten something?