Fig. 7. The peak absolute value of the spanwise vorticity||_max (circles), rescaled by the wing chord c and mean speed U as a function of time t in wingbeat periods T. The normalised circulation (squares) is also plotted on the second ordinate. Filled symbols, positive vortices; open symbols, negative ones. The first four time steps correspond to the data in Fig. 6. The time series represents successive sections of the wake passing through the observation window. The field is strongly asymmetric in both the peak vorticity and its integrated total strength.