Fig. 7. Vortex production in (A) a pupa and (B) a final-stage larva. In A, the top videograph shows two consecutive stages, at 20ms intervals, in the execution of a downstroke. The two images have been superimposed on the same frame and separated horizontally for clarity. A vortex has been produced by the end of the stroke and is travelling towards the dye streamer on the left in the direction of the arrow. After 80ms, the leading edge of the vortex hits the streamer, and after a further 140ms (middle videograph) it emerges from the other side of the dye streamer with its outline faintly showing through the dye, a plug of dye being drawn into the centre of the vortex. After a further 100ms (bottom videograph), the dye that entered the vortex is now recirculating back into the ring. In B, the top videograph shows two superimposed and horizontally separated images of the larva viewed from above flexing to its right side. 20ms later, the leading edge of the vortex produced by the body contraction has reached the streamer, and after a further 100ms has almost passed through the streamer (middle videograph). The outline of the leading edge of the vortex is faintly visible, and a plug of dye is being drawn into the centre of the vortex. 200ms later (bottom videograph), the dye that entered the vortex is now recirculating back into the ring.