Fig. 8. Velocity vectors from a synchronous kick based on a ventral view (horizontal light sheet; xz plane) plotted on a vorticity background. The reference vector equals 1 m s^–1. Red indicates counter-clockwise and blue indicates clockwise vorticity (s^–1). The three images are separated by 0.04 s, showing the build up of the vortex ring shed by the feet. No central jet is visible between the feet, but instead each foot produces its own vortex ring with a corresponding posteriorly directed jet. The forward directed vectors (to the left of the image) and the associated vorticity are caused by the feet still being in the image. Free-stream flow velocity has been subtracted from each vector.