Fig. 6. Vorticity plots at half-wing length at various times during one cycle. Solid and broken lines indicate positive and negative vorticity, respectively. The magnitude of the non-dimensional vorticity at the outer contour is 2 and the contour interval is 3. A–D, downstroke; E–H, upstroke. Reynolds number (Re)=200, stroke amplitude ()=150°, midstroke angle of attack (_m)=40° and non-dimensional duration of wing rotation (_r)=1.87 [mean angular velocity of rotation ()=0.93]; symmetrical rotation.