Visual control of the partition of flight force between lift and thrust in free-flying Drosophila

Flying fruit flies have a system for stabilizing height and speed which has two main components. They can change the total force generated by their wings and they can adjust the division of this force between lift and thrust by changing the angle of their body axis to the horizontal 1–3 . The optomotor reactions determining the force from the wings are governed by the vertical components of the image flow velocity 2,4–6 . I show here that the partition between lift and thrust is determined by the horizontal components of the image flow velocity. The resolution of the image flow velocity into components at right angles, each controlling a different output, produces a simple height and speed stabilizing system.