Gently fluttering above a field of flowers on short, broad wings, butterflies are unlike any other animal aviator. With such large wings relative to their body size, these unusual fliers flap slowly, but inefficiently. So what can explain this unique body plan? Many different aerodynamic mechanisms have been previously suggested to play an important role in butterfly flight, but little data existed to assess their importance. Christoffer Johansson and Per Henningsson of Lund University, Sweden, set out to understand how these insects generate the forces of flight and shed light on the function of their iconic wings.
Johansson and Henningsson focused on the role of the wing upstroke in butterfly flight. Specifically, they looked for an important function at the end of the upstroke when the wings clap together, a phenomenon found in other flying insects that helps generate the lift that keeps them aloft. The team first filmed the butterflies at high speed, taking off in a wind tunnel, and used an aerosol mist to reveal the air movements generated by their wings. They found a clear difference between the downstroke of the wings, which created lift to support the body against gravity, and the upstroke, which created thrust to move the insects forward. These measurements also revealed that the wing clap, which generates lift in other insects, contributes primarily to thrust in butterflies; at the end of the upstroke, the surfaces of the wings meet above the insect, forcing a trapped volume of air out into a jet that propels the butterflies forward.
When analysing recordings of butterfly flapping, Johansson and Henningsson also observed that the wings formed a cup shape as they came together. The researchers hypothesized that the cup shape was important for improving the function and performance of the wing clap. Specifically, cupped wings could trap more air, while also forming a better seal at the edges, which together could result in a stronger, more efficient and more directed jet of air. To test this prediction, they constructed a mechanical clapper that mimicked butterfly flapping with either flexible or rigid wings. They found that the flexible wings, which were able to form a cupped shape, produced stronger and more efficient jets than the rigid wings, which slapped together.
Using relatively large wings to flap slowly is not an efficient way of generating lift and creates a great deal of aerodynamic drag. Therefore, the ability of large flexible wings to generate thrust through such an effective clap may be key to balancing the apparent inefficiencies of butterfly flight. The strong jets generated by the wing clap may also contribute to the erratic nature of butterfly flight, which is useful to escape predators. This could explain the persistence of the butterflies’ unusual body plan and flight style. Johansson and Henningsson have also identified the cupped wing clap as an alternative means of jet propulsion that may be exploited by other flying, or swimming, animals and robots. Clearly, butterflies and their exceptional wings are worthy of acclaim not just as graceful beauties but also as mechanical marvels.
