This is a requested post on the basics of spider eyes; for more on spiders, check out my spider lecture.
Spiders only have ocelli, simple eyes consisting of a lens covering a vitreous fluid-filled pit with a retina (pigment cells + visual cells) at the bottom. The ocelli come in two types: the main eyes and the secondary eyes.
Eyes in spiders are named after their respective position on the head. Therefore, we distinguish between the following:
- Anterior median eyes: front, center;
- Anterior lateral eyes: front, side;
- Posterior median eyes: back, center;
- Posterior lateral eyes: back, side.
The anterior median eyes (AMEs) are always the main eyes, present in all spiders except the Dysderidae, Sicariidae, and Oonopidae. You can easily recognise them in pictures because they’re black, a consequence of not having a tapetum that reflects light back out. Main eyes are fairly uniform in all spiders and differ in their structure from secondary eyes in that they’re everted eyes – the light-sensitive parts of the retina (the rhabdomeres) are pointed towards the light.
All the rest of the eyes are secondary eyes. Secondary eyes are inverted, with their rhabdomeres pointing away from the light, same as in vertebrates (including the human eye). The number and arrangements of secondary eyes can differ significantly, as can their structure: for example, a typical garden spider has lateral eyes with a tapetum, while median ones lack it. All of these differences mean that secondary eyes are incredibly useful for taxonomic purposes, and it’s often so that merely looking at the eyes of a spider will give you a reliable identification of its family.
These differences result in highly-variable image qualities, and have most probably evolved due to ecology. Large secondary eyes can contain several thousand rhabdomeres, resulting in very high sensitivity to light that is very useful to hunters and/or nocturnal spiders. In contrast, small secondary eyes contain a couple hundred rhabdomeres at best, rendering them fairly useless for much beyond movement detection, which is why web-building spiders tend to have them: they don’t need fancy eyes for hunting.
Main eyes are immobile, small, and have a short focal length, granting the spider a large depth of field, making it unnecessary for them to have a focusing mechanism for short ranges. The best main eyes are found in salticids and thomisids, where they are enlarged, resulting in a clear, crisp picture.
This pictures is then combined with the 3D perspective given by the mobile and widely-spread secondary eyes in order to allow the spider to judge distances, most useful for hunting or ambushing spiders.