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

ABSTRACT The motion of the thorax of the blowfly Calliphora vicina was measured during cruising flight inside a cage measuring 40 cm×40 cm×40 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 ° and 1 mm. Flight velocities inside the cage were up to 1.2 m s−1, and accelerations were up to 1 g (≈10m 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 °, but 90 % were smaller than 50 °. Maximum angular velocities were 2000 ° s−1, and maximum accelerations were 105 ° s−2. The latter correspond to torques consistent with the maximal force (2×10−3 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.