Fig. 6. (A) Pooled data from 62 individual pupal dives. For each data point, the x,y coordinates were plotted relative to the starting (0ms) position of the centre of gravity. Points were measured at 20ms intervals and show the motion of the centre of gravity (filled circles) and the dye front associated with the vortices (V1–V3; open triangles, open squares and filled triangles, respectively). The three profiles represent (from top to bottom) the configuration of the body in the resting position (0ms) and in the middle of the second and third half-strokes (40ms and 80ms positions of the centre of gravity). The S.E.M. for each data point was similar to or smaller than the symbol width, so error bars are not shown. Open-headed arrows show the angular motion of the abdomen about the centre of gravity; arrows on the vortices indicate the motion of the dye front along the axis of the vortex. The dashed circle indicates the size and position of the spherical vortex at the time of the final data point. The cartoon, on the right, is a scheme of the movement of the dye front into the vortex. Initially, the dye is drawn into the trailing edge of the vortex (cartoon 1); it then moves along the axis to the centre of the ring (cartoon 2). At a later stage (cartoon 3), the dye has recirculated into the ring: the leading edge of the dye front is now coincident with the leading edge of the vortex and travels with the same velocity as the vortex. R and R_e are the ring radius and external radius of the vortex, respectively. (B) The velocity of the dye front along the centre line of five individual ring vortices. It was difficult to obtain data over the trailing half of the vortex axis. The dashed line is drawn by hand. The cartoon on the right of B represents a median section through a vortex travelling in the direction of the straight arrow.