Ocellus

A simple eye (sometimes called a pigment pit) refers to a type of eye form or optical arrangement that contains a single lens. A "simple eye" is so called in distinction from a multi-lensed "compound eye", and is not necessarily at all simple in the usual sense of the word. The eyes of humans and large animals, and camera lenses are classed as "simple" because in both cases a single lens collects and focuses light onto the retina or film. Many insects have compound eyes consisting of multiple lenses (up to tens of thousands), each focusing light onto a small number of retinula cells.

The structure of an animal's eye is determined by the environment in which it lives, and the behavioural tasks it must fulfill to survive. Arthropods differ widely in the habitats in which they live, as well as their visual requirements for finding food or conspecifics, and avoiding predators. Consequently, an enormous variety of eye designs are found in arthropods: they possess a wide variety of novel solutions to overcome visual problems or limitations.

Some jellyfish, sea stars, and flatworms bear the simplest eyes, pigment spot ocelli, which have pigment distributed randomly and which have no additional structures such as a cornea and lens. The apparent eye color in these animals is therefore red or black. However, other cnidaria have more complex eyes, including those of Cubomedusae which have distinct retina, lens, and cornea.

Many snails and slugs (gastropod mollusks) also have ocelli, either at the tips or at the bases of the tentacles. However, some other gastropods, such as the Strombidae, have much more sophisticated eyes. Giant clams (Tridacna) have ocelli that allow light to penetrate their mantles.

Spiders do not have compound eyes, but instead have several pairs of simple eyes with each pair adapted for a specific task or tasks. The principal and secondary eyes in spiders are arranged in four or more pairs. Only the principal eyes have moveable retinas. The secondary eyes have a reflector at the back of the eyes. The light-sensitive part of the receptor cells is next to this, so they get direct and reflected light. In hunting or jumping spiders, for example, a forward-facing pair possesses the best resolution (and even telescopic components) to see the (often small) prey at a large distance. Night-hunting spiders' eyes are very sensitive in low light levels with a large aperture, f/0.58.

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