Fig. 4. (A-H) Vorticity plots at half-wing length at various times during one cycle (advance ratio J=0). , non-dimensional time; _c, non-dimensional period of one cycle; , angle of attack of wing. Solid and broken lines indicate positive and negative vorticity, respectively. The magnitude of the non-dimensional vorticity at the outer contour is 1 and the contour interval is 1. (A-D), downstroke; (E-H), upstroke.