ABSTRACT
Insect wings are mounted on hinges, restricting the extent to which their bases can be supinated for the upstroke. The forewings of many insects therefore include devices that allow the distal part of the wing to twist relative to the base under aerodynamic loading in the upstroke, but restrict such twisting in the downstroke where the effect would be detrimental. In the broad forewings of butterflies, this asymmetric resistance to aerodynamic twisting seems to be a consequence of the curved section of the leading edge. The wing can be modelled as a cantilevered, thin cambered plate. Torsional tests on the forewings of four butterfly species and on a paper wing of curved section confirm the effect. Differences between the results for the four species appear to fit their morphological and kinematic differences. The nature of the mechanism is outlined.
