spiders, depending on species, need all their eyes.
Some have excellent vision, i.e. the hunting species, usually "webless" species (wolf spiders, and jumping spiders). Their vision is considered quite good and can discern detail from quite a distance (sometimes 4' or maybe more).
Many spiders, however, have very simple eyes that recognise simple shapes (the largest two) and light and direction (the rest). What must be realised is that most spiders don't need to have good eyes. They lay a web, using wind and luck to start, after that, it's all about vibrations. Some even wait on a rock, or under the ground (trap-door spiders), for prey to come close.
Most spiders need few things to exist. Shelter, with which to make a web (easy to find, even with bad eyes.) Light, to attract insects looking for the moon (very easy to find, no matter what). Above and beyond that, it comes down to waiting for food to vibrate the web. Then, it's all action time! Imagine if you couldn't move your eyes. Wouldn't you want to have eyes around your head?
And, as in my previous post about scorpions, some species of spiders have no eyes at all. (though some have almost useless, rudimentary eyes). They live in caves where there is little or no light. Some also have 6 eyes as well. (no four-eyed spiders as far as I know) (sorry, bespeckled people). So, as to why they have 8, it's merely to allow them to be as observant as possible and survive. I hate to make it as simple as that, but there it is.
And, as far as the theory of legs=eyes, we, in all rights have 4 legs. Where are the 4 eyes on a horse? Or the 6 eyes on an ant? Just doesn't work out. Sorry CBL4.
