my question has always been why (I think most vertebrates) stop at two? It seems that an extra eye here and there could be really helpful. Maybe it's because all verterbrates evolved from an ancestor that had two eyes, and once the template is in place, it was simply too deep a local maximum to evolve out of? Similar to the 5-digit hand design that all vertebrates share.
TFA is about the fact that originally the vertebrates had at least 3 eyes, 2 lateral eyes and 1 median eye (pointing upwards towards the sky, in the middle of the head).
Most vertebrates, with the exception of a few species, like tuatara, have lost the middle eye.
The subject of the parent article is that it was expected that if the third eye was lost, the retinas of the 2 lateral eyes that have been preserved are derived from the retinas of the 2 ancient lateral eyes, but despite this expectation, the retinas of the modern lateral eyes of the vertebrates are derived from the retina of the ancient middle eye.
Spiders have 8 eyes. As with vertebrates, this number doesn't change, but there is variation in what it means.
A "normal" spider doesn't really use its eyes. It just has them.
Some spiders are different and rely on their vision. Those spiders have two primary eyes, which they rely on, and six secondary ones, which they don't.
Moving to insects, they often have compound eyes. Two compound eyes. A mantis has two primary compound eyes and three secondary non-compound eyes.
All this convergence suggests to me that even if you have the option to grow more eyes, the correct number is two.
