how do bugs eyes work….specifically flies and dragonflies. Like what do they actually see and how does it work?
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Your eyes contain a lot of light sensitive cells on the back of the eye and a lens with a single small hole (pupil) in the front of the eye. The size, shape, curvature, and material the eye lens is made of create a situation where light from some particular direction that passes through the pupil lands on only one of the light sensitive cells.
But that design doesn’t scale down to small sizes very well. For instance, there has to be some distance between the lens and the photoreceptor. That distance can be pretty small if you make the lens out of high-refractive-index glass – like on your cell phone. But if the refractive material is organic it’s not going to be as good.
So compound eyes use a different method to make sure each photoreceptor cell only gets light from some particular direction. Each photoreceptor is at the bottom of a deep, narrow well. So it’s like looking through a long tube — it only gets light from the direction the tube is pointed. So each photoreceptor gets it’s own narrow tube and all the tubes point in different directions. The tubes are kind of cone-shaped — narrower at the bottom and wider at the top. And they’re stacked very close together, so the top looks like a honeycomb or similar pattern. Of course, there’s a transparent covering over the top to keep germs out and I think the tube is filled with a clear liquid similar to our eyes.
That design isn’t as good as your eye — the fact that there’s a structure built up around every photoreceptor means they have to be spaced out. And the number of “pixels” the insect eye can see is equal to the number of tubes. Also, all the light from some distant point source has to go through that one narrow tube that’s pointed the right way. Light that hits the other tubes just gets absorbed on the tube’s black inside walls. Compare that to your eye — at night, your pupil can be maybe a centimeter in diameter and all the light that hits that centimeter get’s bent so that it all goes to one cell.
So the horror movie trope of “lots of identical images in an array” is wrong. A better trope would be a medium-to-low-resolution image that’s really only sensitive to things that are fairly close.
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