Most spiders have eight eyes, arranged in pairs at the front and on the sides of the head. The principal pair are known as pigment–cup ocelli and are similar to the ocelli of the hymenpotera (see Bee Eyes), except that they can form images. (In insects, this is done by the compound eyes.)
The other eyes (normally six in number) are known as secondary eyes, and while they are quite limited in function they can intensify light by reflecting it back off the retina, which makes them useful for night vision.
Some spiders have only six eyes, some have four, and some have only the two ocelli; while cave–dwelling spiders have only vestigial eyes, incapable of sight, or no eyes at all.
© Haydn Thompson 2022