The arrangement of eyes in a spider help to categorize the type of spider one is. Mostly all species of spiders except a few have 8 eyes – some are arranged front-facing while the others are positioned on the upper region of the head. The other families of spiders like Sicariidae, Dysderidae, Oonopidae are six-eyed. Amazingly, in spite of having 6 to 8 eyes, spiders generally have poor eye sight and can see only dark or light shapes moving. Their excellent ability for taste and of sense vibration is what aids them to hunt their prey.

Spiders possess simple, singular eyes or eyes with just one lens and no tapetum called the ocelli. Spiders have two kinds of eyes, namely primary and secondar. The center pair of eyes is known as the main ones and is constructed differently from the others with an exception to the jumping spider and crab spider. The six-eyed spiders do not have any main or primary eyes. These eyes are generally small with high resolution and their light sensitive cells or rhabdomeres direct towards the incoming light. The primary eye focuses on the prey as the spider comes closer to it.

The secondary eyes help the spider to get a much wider ranging vision as compared to the primary eyes. They possess a tapetum and the rhabdomeres or light sensitive cells in the secondary eyes that are directed away from light. These eyes help the spider judge distances and track the movement of a potential prey.

Different kinds of spiders have their eyes arranged in different but set patterns. The identification of what species of spiders can be done by researchers on basis of this. The four placements defined for the spider’s eyes are anterior median eyes or AME, anterior lateral eyes or ALE, posterior median eyes, or PME, and posterior lateral eyes, or PLE. 

Spiders can identify with polarized light. This helps them know their where-about. The efficiency of an eye depends most importantly on the density and number of rhabdomeres present in its retina. The number varies from few hundreds to over sixteen thousand. Some spiders, such as thomisidae, lycosidae and salticidae, have very good vision while most of the others in this family do not.

Salticidae, also known as the jumping spider, is recognized for its exceptional eyesight. Their two primary eyes are significantly elongated and are pushed deep inside the spider’s head. They function similar to telephoto lenses and use the movable retina for increasing the visual area. Oddly, they have 4 sheets of rhabdomeres that are sensitive to certain wavelengths of light namely ultraviolet or 360, 480, 500 and 580 nanometers. Jumping spiders can differentiate objects at a distance of 30 to 40 centimeters with the help of their secondary eyes and once the jumping spider is about 20 centimeters from its prey, its primary, or AME eyes, spot it.

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Mark David: How Many Eyes Does A Spider Have?
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