The wing-beat kinematics and power output associated with circular motion in an insect flight-mill

Flight-mills provide a simple way to evaluate the flight potential of insects. However, the mechanical constraints imposed by the flight-mill limit the ability to translate the measured flight distance to natural flight.  On the one hand, the insects need to provide extra energy to rotate the flight-mill, while on the other hand, due to tethering, there is no assurance that the insects have invested sufficient energy to provide enough lift to support their body weight in air. To understand how flight-mill flight is affected by the design of the device we used high-speed video to extract wing-beat kinematics of the red palm weevil (Rynchophorous ferruginneus) as it flew in four variants of a flight-mill. The variants differed in the attachment angle of the insect and in the requirement to support body weight in air. We found that varying the type of flight-mill did not affect flight speed or wing-beat frequency, but did affect the wing-beat kinematics. In all flight-mills and body angle combinations, the wing internal to the circular trajectory moved faster relative to air, suggesting that the beetles were attempting to steer in the opposite direction to the curved trajectory forced by the flight mill. The average lift generated by the beating wings was sufficient to support 64-78% of the body weight of the beetle, with the remaining up-thrust derived from centrifugal force. Flight-mills will continue to offer an important tool in insect flight research, but their limitations for evaluating natural flight must be recognized and taken into account.