Most insects create lift by the stiff-winged mechanism of insect flight with steady state aerodynamics; this creates a leading edge vortex continuously as the wing moves. The feathery wings of thrips, however, generate lift by clap and fling, a mechanism discovered by the Danish zoologist Torkel Weis-Fogh in 1973. In the clap part of the cycle, the wings approach each other over the insect's back, creating a circulation of air which sets up vortices and generates useful forces on the wings. The leading edges of the wings touch, and the wings rotate around their leading edges, bringing them together in the "clap". The wings close, expelling air from between them, giving more useful thrust. The wings rotate around their trailing edges to begin the "fling", creating useful forces. The leading edges move apart, making air rush in between them and setting up new vortices, generating more force on the wings. The trailing edge vortices however cancel each other out with opposing flows. Weis-Fogh suggested that this cancellation might help the circulation of air to grow more rapidly, by shutting down the Wagner effect which would otherwise counteract the growth of the circulation.