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Abstract

The properties of the navigation system are derived from the open loop optomotor reactions of the fruitfly Drosophila which is either flying or walking under stationary conditions. The horizontal component of a movement stimulus controls the difference of the propulsive forces of legs and wings on either side and enables the freely moving fly to counteract involuntary deviations from a straight course. The vertical component controls the sum of the propulsive forces of the wings and enables the fly to maintain a given level of flight. Different properties of the navigation system are found in normal and mutant fruitflies. Comparative studies of open loop and closed loop behaviour disprove the separation theorem of KALMUS according to which the fly is always minimizing the rotatory component, and maximizing the translatory component, of a movement stimulus. There is evidence that inversion of the rotatory response can be elicited by the translatory component of the stimulus. Interaction of the stimulus components must be considered in order to determine the closed loop responses of the freely moving insect.