Movements of the Retinae of Jumping Spiders (Salticidae: Dendryphantinae) in Response to Visual Stimuli

Abstract

1. Movements made by the principal eyes of jumping spiders (Phidippus and Metaphidippus spp.) have been investigated using an ophthalmoscopic technique which permits simultaneous observation and stimulation of the retinal surface. 2. The eye-movements are produced by six muscles. Four are attached to the carapace, and displace each retina latero-medially and dorso-ventrally. The remaining pair are thin bands of muscle which encircle the eye-tube. These twist the eye-tube, rotating the retina about the visual axis (torsion). 3. The nerve supplying these muscles contains only six axons. Each axon terminates in one of the six muscles. 4. Four types of eye-movements are observed. These are spontaneous activity, saccades, tracking and scanning. All movements are usually conjugate. 5. Spontaneous activity consists of a very variable, periodic side-to-side motion of the retinae. It is associated with states of high excitability, and occurs whether or not there is any structure in the field of view. 6. Saccades occur when a small stimulus (e.g. a dark dot) is presented to, or moved upon, the retinae of either the principal eyes or the antero-lateral eyes. In a saccade the retinae move towards the image of the target so that they come to rest with their central regions fixated on the target. 7. If the target moves the retinae track it, maintaining central fixation. 8. Scanning normally follows a saccade. It consists of an oscillatory, side-to-side movement of the retinae across the stimulus, with a period of 1-2 sec., and a simultaneous torsional movement in which the retinae partially rotate about the visual axes, through an angle of approximately 50° and with a period of 5-15 sec. 9. Jumping spiders distinguish other jumping spiders from potential prey by the geometry of their legs. It is suggested that scanning is a pattern-recognition procedure in which the torsional movements are concerned with the spatial alignment of line or edge detectors, and the horizontal component with providing relative motion between these detectors and the stationary stimulus.