Jumping spider

The eight camera-type eyes of jumping spiders (Salticidae) endow these hunters with remarkably acute vision. Six of them - two rudimentary and four fixed to the carapace - are conventional eyes that detect motion. The large pair of tubular anterior median eyes, however, is rather more unusual. In both these eyes, the retina is a narrow boomerang-shaped strip oriented vertically. Its central region, where acuity is highest, is merely six or seven receptor rows wide. As the lens is anchored to the carapace, the retina needs to be moved for scanning, which is achieved by a set of muscles that orchestrate a complicated pattern of translations and rotations. Furthermore, the retina possesses a highly complex ultrastructure. Not only does a characteristic pit magnify the image (resulting in a telephoto eye), but the retina also consists of up to four layers of receptors. It has been suggested that this layered structure allows objects at different distances to be focussed at different levels, or different wavelengths to be focussed on receptor layers that are particularly sensitive to them. The scanning movements that direct the retina across a target seem to be related to pattern recognition, distinguishing potential mates from prey.