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Answer: 99% have Eight
Most spiders have eight eyes, but some species have six, four, two, or even no eyes. Even within a single species, the number of eyes may vary, but it's always an even number.
⦁ Spiders have two types of eyes. The large pair of primary eyes forms images. The secondary eyes help the spider track movement and gauge distance.
⦁
⦁ The number and arrangement of spider eyes helps an arachnologist identify the spider's species.
A spider needs so many eyes because it cannot twist its head to see. Rather, the eyes are fixed in place. In order to hunt and evade predators, spiders need to be able to sense movement all around them.
In other arthropods, the ocelli eyes only detect light direction, but in spiders these eyes form true images.
The principal eyes contain muscles that move the retina to focus and track an image.
Most spiders have poor visual acuity, but ocelli in jumping spiders exceeds that of dragonflies (insects with the best vision) and approaches that of humans.
In some cases, the lateral eyes expand the range of the primary eyes, giving the arachnid a wide angle image.
The secondary eyes act as motion detectors and provide depth perception information, helping the spider locate the distance as well as direction of prey or threats.
In nocturnal species, the eyes reflect light and help the spider see in dim light. These spiders show eye-shine when illuminated at night.
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