The principal eyes of a jumping spider have a telephoto component
David S. Williams & Peter MeIntyre
Department of Neurobiology, Research School of Biological Sciences, Australian National University, Canberra, Australia
Jumping spiders are a cosmopolitan family (Salticidae) of predators that can make visual discrimination between prey and mates1,2. This task is mediated through the anterior median eyes, described by Land3 as consisting of a corneal lens and a motile retina that comprises four layers of receptors embedded in a matrix. The retinal matrix contains a pit distal to the receptors and symmetrically centred on-axis. We have now found that in Portia fimbriata (Doleschall) and some other species, this pit has a refracting interface that increases the focal length of the eye beyond its axial length, thereby magnifying the retinal image and increasing visual resolving power above that possible with only a corneal lens. The most effective part of the conical pit is its rounded apex, which augments the corneal lens to provide a telephoto system that increases the overall focal length by about 1½ times. This mechanism is of particular value to small spiders like P. fimbriata, for the possible axial length of their eyes is constrained by the small size of their prosomae (Fig.1).
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