Fig. 4. Progressive visualisation of a discrete ring vortex shed from the tail fin of a swimming mayfly larva. In a, the larva is swimming in the direction of the arrow and is viewed from the side (dorsal to the left). The tail fin is within 20 ms of the completion of its dorsal half-stroke. To the left of the larva is a curved streamer. Subsequent frames (b,c) show the progressive outlining of the ring vortex shed from the tail as it penetrates the streamer. See text for more details. The arrow in b shows the direction of propagation of the ring vortex.