Most spiders are known for excellent mechanoreception, which helps them detect tiny vibrations useful in finding web-entangled prey, but some have remarkable vision rivaling that of humans. Jumping spiders belonging to the genus Portia have the highest optical spatial acuity and most complex behavior. They do not spin webs like other spiders; instead they hunt much like cats by spotting, stalking, and pouncing on their prey. These spiders have 2 large principal eyes in the center of their faces (Figure). These eyes are capable of seeing size, color, and shape at distances of up to a foot. There are also 6 secondary eyes located around the sides and back of the body that provide wide-angle vision used for detecting movement. The principal eye of the spider is a telescopic eye tube that has a corneal lens in the front and a second lens in the back (as in a Galilean telescope) that focus light onto a 4-layered retina.1 The fovea is small and provides only a narrow field of vision that the spiders can build into a detailed image by scanning objects. The visual acuity generated is one-fifth of the visual acuity of human beings, which is enough to recognize (and attack) video images of prey. The spiders can track prey by using muscles to pivot their eye tubes inside their heads. The intricate eye movements may be the key to explaining the complex behavior of animals with such tiny brains. Scientists hope that studying Portia's vision and complex behavior will help in understanding the mechanisms of intelligent behavior and in designing better robots.
Rozenbaum I, Ritch R. Eye on the Web. Arch Ophthalmol. 2007;125(11):1557. doi:10.1001/archopht.125.11.1557