That’s the shape of their compound eye lenses, and thus the image it projects onto their many retinas.
What their eyes see looks like that.
Now what does their fly brain actually do to this image in post processing?
That’s more mysterious. Your brain does a ton of image editing to flip the image and stitch your two eyes together into one field of view. Their brain is a lot smaller and presumably doesn’t have the capacity to do a ton of intense image processing, but they’re probably not just perceiving the raw image either.
What you “see” (light hitting detector cells) and what you “perceive” (the image your brain actually generates) aren’t necessarily the same. You can tell what an animal sees by dissecting their eyes. Determining what they perceive is much more difficult.
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