|MadSci Network: Zoology|
Actually, the reason for the number of eyes and eye rows in spiders is difficult to determine. It seems primarily to be a result of the need to preceive at least light and dark, and probably motion, in as many directions as possible in order to find prey, escape enemies and in at least some families to find mates (although chemicals and vibrations may be more important in many spiders in mate finding). However, other than the eye rows being at the front end, there is a wide variety of arrangements that are apparently acceptable for these functions. Most spiders have eight eyes, but some have six (spitting spiders and violin spiders for example), a few have four eyes, a few have two eyes (some Caponiidae), and some cave spiders have no eyes at all. The eyes are usually in two rows, but in some families the rows have become curved either forward or backward so that there are three or even four rows. In many six-eyed spiders the eyes are in three clumps of two each. In a few families at least one pair of eyes are highly enlarged and these are able to distinguish not only light and dark and motion, but colors and shapes as well. This is most highly developed in the jumping spiders (family Salticidae), but also occurs (in different eyes!) in several other families.
To find some pictures of some of these spiders see
Try Yahoo's Spider section for links to websites about them.
Try the links in the MadSci Library for more information on Zoology.