Why most insects have more than 2 eyes but mostly mammals have only 2 eyes?
In: 3
I suspect it’s because you can position eyes to cover what you need as a species (prey have side-facing eyes for full field-of-view, predators front-facing for better binocular). And eyes are ridiculously expensive, evolutionarily speaking. It’s not the eye itself, but a huge fraction of the brain ([this article](https://www.rochester.edu/pr/Review/V74N4/0402_brainscience.html) says over half the cortex) goes towards processing visual data, and brains are *very* expensive. The brain uses 20-25% of all the energy our body uses, so adding more eyes means you need to eat quite a bit more. It’s not a coincidence that eyes are the first thing to go in cave-dwelling species. That means ditching eyes when you don’t need them helps *a lot*.
I’m not sure how many eyes we’re counting for insects, but in compound eyes, each individual micro-eye is more like a single rod/cone in a vertebrate retina. Now, as to why spiders usually have 8 eyes…
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