Unconventional lift-generating mechanisms in free-flying butterflies

Flying insects generate forces that are too large to be accounted for by conventional steady-state aerodynamics(1,2). To investigate these mechanisms of force generation, we trained red admiral butterflies, Vanessa atalanta, to fly freely to and from artificial flowers in a wind tunnel, and used high-resolution, smoke-wire flow visualizations to obtain qualitative, high-speed digital images of the air flow around their wings. The images show that free-flying butterflies use a variety of unconventional aerodynamic mechanisms to generate force: wake capture(3), two different types of leading-edge vortex(3-7), active and inactive upstrokes(8), in addition to the use of rotational mechanisms 3 and the Weis-Fogh 'clap-and-fling' mechanism(9-12). Free-flying butterflies often used different aerodynamic mechanisms in successive strokes. There seems to be no one 'key' to insect flight, instead insects rely on a wide array of aerodynamic mechanisms to take off, manoeuvre, maintain steady flight, and for landing.