Flies adaptively control flight to compensate for added inertia

Animal locomotion is highly adaptive, displaying a large degree of flexibility, yet how this flexibility arises from the integration of mechanics, sensing and neural control remains elusive. For instance, animals require flexible strategies to maintain performance as changes in mass or inertia impact stability. Compensatory strategies to mechanical loading are especially critical for animals that rely on flight for survival. To shed light on the capacity and flexibility of flight neuromechanics to mechanical loading, we pushed the performance of fruit flies (Drosophila) near its limit and implemented a control theoretic framework to quantify how flies compensated for added inertia. Flies with added inertia were placed inside a virtual reality arena which enabled free rotation about the vertical (yaw) axis. Adding inertia increased the flys response time yet had little influence on overall gaze performance. Flies maintained stability following the addition of inertia by adaptively modulating both visuomotor gain and damping. In contrast, mathematical modeling predicted a significant decrease in flight stability and performance. Adding inertia altered saccades, however flies compensated for the added inertia by increasing yaw torque production, indicating that flies sense that they are mechanically loaded. Taken together, in response to added inertia flies trade off reaction time to maintain flight performance through adaptive neural modulation. Our work highlights the flexibility and capacity of motor control in flight.