Though a few spiders have six eyes, the default number is eight, usually arranged in two approximate rows, says Richard Jones.
Scorpions also have multiple eyes (6-12), but they are less obvious because they are smaller. It seems likely that the original proto-arthropod probably had two eyes (as did the extinct trilobites and giant sea scorpions), and this pair has been maintained in crustaceans (such as woodlice), millipedes and insects.
But as arachnid ancestors evolved, the eyes split into a front portion and a side portion, and then split again.
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Spiders then evolved so that rather than the multiple lens facets shown in insect compound eyes, a spider eye is a single lens with multiple receptors beneath for far better resolution of the final visual image.
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