Saccadic tracking of targets mediated by the anterior-lateral eyes of jumping spiders.

The modular visual system of jumping spiders (Salticidae) divides characteristics such as high spatial acuity and wide-field motion detection between different pairs of eyes. A large pair of telescope-like anterior-median (AM) eyes is supported by 2-3 pairs of 'secondary' eyes, which provide almost 360 degrees of visual coverage at lower resolution. The AM retinae are moveable and can be pointed at stimuli within their range of motion, but salticids have to turn to bring targets into this frontal zone in the first place. We describe how the front-facing pair of secondary eyes (anterior lateral, AL) mediates this through a series of whole-body 'tracking saccades' in response to computer-generated stimuli. We investigated the 'response area' of the AL eyes and show a clear correspondence between the physical margins of the retina and stimulus position at the onset of the first saccade. Saccade frequency is maximal at the margin of AL and AM fields of view. Furthermore, spiders markedly increase the velocity with which higher magnitude tracking saccades are carried out. This has the effect that the time during which vision is impaired due to motion blur is kept at an almost constant low level, even during saccades of large magnitude.