Smart wing rotation and trailing-edge vortices enable high frequency mosquito flight

In addition to generating lift by leading-edge vortices (as used by most insects), mosquitoes also employ trailing-edge vortices and a lift mechanism from wing rotation, which enables them to stay airborne despite having a seemingly unlikely airframe. As anyone who has shared a bedroom with a mosquito will attest, mosquitos beat their wings fast enough for the irritating whine at about 800 Hz to be audible. Strangely, mosquito wings are long and thin, rather than the short and stubby wings expected to support rapid wing beats. The wing beat is also of rather low amplitude; the entire angular sweep of the wing is around 40 degrees, less than half that of the honey bee, whose 91 degree range is regarded as shallow. So how do mosquitoes fly at all? Bomphrey and colleagues show that in addition to generating lift by leading-edge vortices (as used by most insects) mosquitos employ trailing-edge vortices and a lift mechanism caused by the rotation of the wing. This adds to the expanding repertoire of mechanisms that insects use to stay airborne despite a seemingly unlikely airframe. Mosquitoes exhibit unusual wing kinematics; their long, slender wings flap at remarkably high frequencies for their size (>800 Hz)and with lower stroke amplitudes than any other insect group1. This shifts weight support away from the translation-dominated, aerodynamic mechanisms used by most insects2, as well as by helicopters and aeroplanes, towards poorly understood rotational mechanisms that occur when pitching at the end of each half-stroke. Here we report free-flight mosquito wing kinematics, solve the full Navier–Stokes equations using computational fluid dynamics with overset grids, and validate our results with in vivo flow measurements. We show that, although mosquitoes use familiar separated flow patterns, much of the aerodynamic force that supports their weight is generated in a manner unlike any previously described for a flying animal. There are three key features: leading-edge vortices (a well-known mechanism that appears to be almost ubiquitous in insect flight), trailing-edge vortices caused by a form of wake capture at stroke reversal, and rotational drag. The two new elements are largely independent of the wing velocity, instead relying on rapid changes in the pitch angle (wing rotation) at the end of each half-stroke, and they are therefore relatively immune to the shallow flapping amplitude. Moreover, these mechanisms are particularly well suited to high aspect ratio mosquito wings.