THE VISUAL CONTROL OF BEHAVIOR IN FIDDLER-CRABS .2. TRACKING CONTROL-SYSTEMS IN COURTSHIP AND DEFENSE

Male fiddler crabs (Uca pugilator Bose) have visual control systems th at enable them to track other crabs in front or behind, and to keep po tential predators to the side, where escape is easiest. The system for tracking conspecifics appears to be double, with a low-gain velocity- sensitive mechanism operating over about a 90 degrees range, backed up by a position-sensitive mechanism at the ends of this range which is responsible for recentring the target. This system has separate front and rear ranges, with a gap in the direction of the claw. The crabs se parately fixate the burrow entrance, keeping it in the direction oppos ite the claw. Predator evasion employs two systems simultaneously. An open-loop mechanism directs the crab's translatory movements directly away from the stimulus, and a rotational mechanism using continuous fe edback turns the crab so that the stimulus is kept at near 90 degrees to the body axis. Both systems are sensitive to the angular position o f the stimulus, not its velocity. Eye movements have little or no role in object tracking. An attempt is made to list Uca's known visual con trol systems.