Spider Eyes
Spiders do not have compound eyes. Instead, they have several pairs of simple eyes. Each pair is adapted for specific tasks. Spiders have principal and secondary pairs of eyes.
Only the principal eyes have movable retinas. Principal eyes may have compound lenses that give a wide field of view while efficaciously gathering available light.
Secondary eyes have a reflector (tapetum) at the back of the eye that affords detection of direct and reflected light. Tapeta improve light sensitivity.
Hunting and jumping spiders have 4 pairs of eyes. The large forward-facing principal pair have the best resolution. Some eyes may have telescopic ability to see small prey at considerable distance.
Most wolf spiders hunt at dusk and by moonlight. Besides a front principal pair, they have 2 pair of large posterior eyes with well-developed tapeta, letting them spot prey movement anywhere around them.
Deinopis, a genus of net-casting spiders, has 2 enormous night-hunting eyes which give a wide field of view as well as effectively gathering light. Unlike other night hunters, Deinopis‘ eyes lack tapeta. Yet the lenses of Deinopis‘ eyes detect light better (F 0.58) than cats (F 0.9) or owls (F 1.1). That is possible because a large area of light-sensitive membrane grows within the eyes each night. This membrane rapidly disintegrates at dawn.
