Flies are not easily perturbed in their pursuit of prey, be it fruit or humans. The flies' insistence may be annoying, but their ability to maintain stable flight, despite all interference, is impressive. Leif Ristroph and colleagues have now studied in detail how fruit flies recover from flight disturbances, and propose a mechanism of autostabilization that keeps these — and possibly other — animals on course (Proc. Natl Acad. Sci. USA 107, 4820–4824; 2010).
In their experiments, Ristroph et al. mapped the free flight of common fruit flies (Drosophila melanogaster), using three orthogonally arranged high-speed cameras, each recording 8,000 frames per second. To perturb the flies' flight in a controlled manner, they glued 1.5-mm-long pieces of carbon steel wire onto the backs of the insects. Once inside the filming volume, magnet-field pulses applied through a pair of Helmholtz coils tilted the pins, and the flies with them.
For perturbations that caused deflections of up to 45°, the flies recovered remarkably quickly. Typically they were back on their original trajectory within fewer than 60 ms, with an accuracy of 2°. When thrown by more than 45°, the flies could still get back on track, but less precisely.
The time resolution of the recordings that Ristroph and co-workers made of these manoeuvres — roughly 35 frames per wing beat — is sufficiently high to see the mechanism at work. The flies counteract the imposed rotation by generating aerodynamical torque by adjusting the wing angles. The angles of attack are different for the two wings, causing the fly to turn.
But how does the animal sense that it has been derailed? The speed with which the correction happens rules out visual input. It takes the flies roughly 10 wing beats to react to visual stimuli — and yet in this time, they have already executed the entire correction manoeuvre. Instead, the flies sense body rotation through structures known as halteres. This pair of drumstick-shaped extensions (seen behind the wings in the picture) evolved from a pair of hind wings and acts as a vibrating-structure gyroscope.
Ristroph et al. propose a model in which the wing and body motions are coupled, leading to strong damping of rotations. Such damping would constitute efficient autostabilization, making it unnecessary to actively stop rotations. That this mechanism fails for large deflections might be caused by saturation of the brain structures that process information from the halteres.
Taking on board aspects such as sensor saturation, this study should serve as a basis for further exploring the neural circuitry that controls fruit-fly motion. And it may provide insight into the general physical principles that allow stable flapping-wing flight.
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Trabesinger, A. Correction on the fly. Nature Phys 6, 248 (2010). https://doi.org/10.1038/nphys1650