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Abstract

Apparent motion was simulated in the visual system of the tethered fruitfly Drosophila melanogaster by projecting moving stripe patterns onto stationary screens positioned in front of the lateral eye regions. The reactions of the animal were recorded under conditions of stationary flight in still air. It was found that visual stimulation modifies, independently, torque and thrust of the flight system. The responses appear suitable to counteract involuntary changes of direction and altitude in free flight. Concerning the sensory system for visual flight control, the following was established: Both eyes are functionally equal, and sensitive to pattern motion in any direction. The motion detecting subunits possess a certain orientation on the eye surface, and discriminate between pattern motions that are progressive or regressive relative to this orientation. Progressive and regressive stimuli elicit opposite responses in the flight system. The subunit orientations are expected to group in at least two different directions that share a common line of symmetry with the internal eye structure. A minimum of two contralateral and two ipsilateral nerve connections between the visual system and the motor system is required for the various torque and thrust responses. Concerning the effect of pattern motion on the flight system, the following was found: The motion detectors control only the magnitude of the force of flight. With the tethered animal in still air, the inclination of the force vector remains constant. Consequently, the stroke plane and the wing pitch should be invariant to visual stimulation. Possible influences of pattern motion on the wing-beat frequency were ruled out by frequency measurements. The only major variables in wing articulation that respond to pattern motion are the wing-beat amplitudes on either side of the insect. In-flight photographs show that the difference and the sum of these amplitudes are, in fact, representative for the torque and the thrust of the flight system. The responses of the body posture may become important to flight performance at increased airspeed. Comparative experiments with the housefly Musca domestica indicate that the principle of independent torque and thrust control by vision is adopted in at least two different species.