Spider vision
The eyes of spiders vary significantly in their structure, arrangement, and function. They usually have eight, each being a simple eye with a single lens rather than multiple units as in the compound eyes of insects. 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. Most haplogynes have six eyes, although some have eight, four or even two. In some cave species, there are no eyes at all. Sometimes one pair of eyes is better developed than the rest. 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 colour.
Structure and anatomy
Spiders' eyes are simple eyes, or ocelli, meaning their eyes have a single cuticular lens above a simple retina. The retina is concave and composed of visual and pigment cells, which lie beneath a cellular vitreous body.Categorisation
Most spiders have eight eyes, which tend to be arranged into two rows of four eyes on the head region. The eyes can be categorised by their location and are divided into the anterior median eyes, anterior lateral eyes, posterior median eyes, and posterior lateral eyes. 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. They can also be categorised by structure into the principal eyes, which are always the AME, and the secondary eyes, which are always the PME, ALE, and PLE. In six-eyed species, it is always the principal eyes which are absent.The secondary eyes normally have a light-reflecting layer, the tapetum, that makes the eyes appear pale. The tapetum differs considerably between spider families but can be separated into three main types: PT, CT, and GT. Some spiders, such as jumping spiders, have no tapetum in their secondary eyes.
Principal eyes
In most species, the principal eyes are more visually acute than the secondary eyes, at the cost of their sensitivity. They lack a tapetum entirely, and the retina is everted, meaning the rhabdomeres point towards the incoming light. Some species can move the retina using between one and six muscles, greatly increasing the field of view. 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. When a spider has colour vision, it is typically the principal eyes which are responsible.Secondary eyes
The secondary eyes have an inverted retina, meaning the rhabdomeres face away from the incoming light. They also typically have a tapetum made up of crystals, which are likely guanine. Because incoming light passes through the rhabdomeres both before and after being reflected by the tapetum, the sensitivity of the eyes is essentially doubled. The exact structure of the tapetum varies significantly between families and genera, so is sometimes used as a diagnostic feature. Despite this variation, it can be divided into three main groups:- The primitive-type tapetum fills the entire eye cup with holes only for the nerves; it is found mainly in the haplogyne, Mesothelae, and Mygalomorph spiders.
- The canoe-type tapetum is formed by two walls divided by a median gap, where the nerves exit; it is found in many families, including Theridiidae, Clubionidae, and Amaurobiidae.
- The grate-type tapetum is the most efficient; the long vitreous body and spherical lens combine to produce a sharp, in focus image. It forms rows of crystals which lie beneath the visual cells, and the nerves exit through the gaps in the 'grate'. This type is found primarily in hunting spiders such as wolf spiders and fishing spiders.
Function