Multiple eyes of a
jumping spider
Spiders have up to eight eyes, each with a single
lens rather than multiple units as in the
compound eyes of insects. The specific arrangement of the eyes is one of the features used in classifying different species. Most species of the
Haplogynae have six eyes, although some have eight (
Plectreuridae), four (e.g.,
Tetrablemma) or even two (most
Caponiidae). Sometimes one pair of eyes is better developed than the rest, or even, in some cave species, there are no eyes at all. Several families of hunting spiders, such as
jumping spiders and
wolf spiders, have fair to excellent vision. The main pair of eyes in jumping spiders even sees in color.
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Net-casting spiders of genus
Deinopis have their posterior median eyes enlarged into large forward-facing compound lenses. These eyes have a wide field of view and are able to gather available light more efficiently than the eyes of cats and owls. This is despite the fact that they lack a reflective layer (
tapetum lucidum); instead, each night, a large area of light-sensitive membrane is manufactured within the eyes, and since arachnid eyes do not have irises, it is rapidly destroyed again at dawn.
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However, most spiders that lurk on flowers, webs, and other fixed locations waiting for prey tend to have very poor eyesight; instead they possess an extreme sensitivity to vibrations, which aids in prey capture. Vibration sensitive spiders can sense vibrations from such various mediums as the water surface, the soil or their silk threads. Changes in the air pressure can also be detected in search of prey.
https://en.wikipedia.org/wiki/Spider_anatomy