Fig. 9. Composite videograph of vortex generation by a pupa during the performance of a turn. The vortex is produced by a downstroke that begins at the 0ms position and is complete by the 40ms position of the pupa. Its production is associated with a change in swimming direction of approximately 60° (the angle between the two smaller arrows). The vortex travels to the left in the direction of the large white arrow. The dye pattern corresponds to the position of the vortex 1s after its production, by which time it has travelled approximately 3cm from the 0ms position of the pupa. During the same period, the pupa disappeared from the lower right of the screen. Superimposed on the videograph are the images of the vortex 2s and 4s after its formation. Note that the vortex corresponds to the turning vortex shown in Fig.8; the stopping vortex also illustrated in that figure is not visible here because of the absence of dye in the relevant area. The dye pattern in the middle of the videograph is a streamer falling slowly from the surface of the water.