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 that enable them to track other crabs in front or behind, and to keep potential 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 separately fixate the burrow entrance, keeping it in the direction opposite 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 feedback 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 of 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 control systems.