Blowfly Flight and Optic Flow

The motion of the thorax of the blowfly Calliphora vicina is measured during cruising flight inside a cage of 40×40×40 cm. Sensor coils mounted on the thorax pick up externally generated magnetic fields, and yield measurements of position and orientation of the thorax with a resolution of 1 ms, 0.3°, and 1 mm. Flight velocities inside the cage are up to 1.2 m/s, accelerations are up to 1 g (≈10 m/s) vertically and 2 g horizontally. During flight, blowflies perform a series of short (~20-30 ms) saccade-like turns at a rate of about 10 per second. The saccades consist of a succession of rotations around all axes, occurring in a fixed order. First, a roll is started. Second, the rolled thorax pitches (pulling the nose up) and yaws, which results in a turn relative to the outside world. Finally, the thorax rolls back to a level position. Saccades have yaw amplitudes of up to 90°, but 90% is smaller than 50°. Maximum angular velocities are 2000 °/s, maximum accelerations 10 °/s. The latter correspond to torques consistent with the maximal force (2⋅10 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.