Spider vision

The specific arrangement and structure of the eyes is one of the features used in the identification and classification of different species, genera, and families.

[2] The retina is concave[3] and composed of visual and pigment cells, which lie beneath a cellular vitreous body.

The exact arrangement of the eyes varies significantly by family, and to a lesser degree genus, so is often used as a diagnostic feature to identify or categorise spiders, especially in the field.

[5] They lack a tapetum entirely, and the retina is everted, meaning the rhabdomeres (light-sensitive parts of the visual cells) point towards the incoming light.

No actual focusing takes place; it is unnecessary, since the small lenses and short focal length of most spiders' principal eyes lead to a wide depth of field.

[2][3] Despite this variation, it can be divided into three main groups: Most spiders' eyes can detect little more than brightness and motion, so vision plays only a minor role in behaviour.

This is significant, because this orthogonal arrangement is also present in the 'dorsal rim area' used by some insects to detect the polarisation of light.

Despite this fact only one species, Drassodes cupreus, has been observed to detect polarised light with their secondary eyes.