Blowfly flight and optic flow : I. Thorax kinematics and flight dynamics

The motion of the thorax of the blowfly Calliphora vicina was measured during cruising flight inside a cage measuring 40 cmx40 cmx40 cm. Sensor coils mounted on the thorax picked up externally generated magnetic fields and yielded measurements of the position and orientation of the thorax with a resolution of 1 ms, 0.3 degrees and 1 mm. Flight velocities inside the cage were up to 1.2 m s-1, and accelerations were up to 1 g ( approximately 10 m s-2) vertically and 2 g horizontally. During flight, blowflies performed a series of short (approximately 20-30 ms) saccade-like turns at a rate of approximately 10 s-1. The saccades consisted of a succession of rotations around all axes, occurring in a fixed order. First, a roll was started. Second, the rolled thorax pitched (pulling the nose up) and yawed, resulting in a turn relative to the outside world. Finally, the thorax rolled back to a level position. Saccades had yaw amplitudes of up to 90 degrees, but 90 % were smaller than 50 degrees. Maximum angular velocities were 2000 degrees s-1, and maximum accelerations were 10(5 ) degrees s-2. The latter correspond to torques consistent with the maximal force (2x10(-3 )N) that can be generated by the flight motor as inferred from the maximal linear acceleration. Furthermore, the sequence of energy investment in consecutive rotations around different axes appears to be optimized during a saccade.