Optomotor control of the force of flight in Drosophila and Musca. II. Covariance of lift and thrust in still air

Drift of the retinal images of the surroundings elicits optomotor responses of flight control in the fruitfly, Drosophila melanogaster, and in the housefly, Musca domestica. The present investigation deals with the responses of tethered flies in still air. The responses were elicited by continuous movement of striped patterns in front of the eyes, and characterized by the magnitude and elevation of the resulting force of flight which is the average of the forces produced during a wingbeat cycle. The force of flight is resolved into the upward directed lift and the forward directed thrust. In either species, pattern movement acts upon the magnitude, but not upon the elevation of the force of flight. The elevation relative to the longitudinal body axis is almost invariably 24° in Drosophila, and 29° in Musca. The lift/thrust ratio in still air is fixed accordingly, and can be changed only by variation of the body angle. Keeping an angle of minimum body drag does not contribute significantly to the efficiency of insect flight at very low Reynolds numbers (Re). Control of the lift/thrust ratio by variation of the body angle is, therefore, less surprising in Drosophila where Re is in the order of 102, than in Musca, where Re is in the order of 103. Control of this ratio without variation of the body angle is actually established in insects flying at even higher Re. Covariance of lift and thrust in the investigated flies is achieved by control of wingbeat amplitude or wingbeat frequency, but not by control of wing pitch or stroke plane. A change in the latter parameters would have deflected the force of flight and is, therefore, inconsistent with the constant elevation found in the present experiments. The results obtained, so far, do not exclude active deflections of the force vector during occasional bouts of aerobatics, or passive deflections of this vector during flight at non-zero airspeed