Fig. 3. Trail produced by the wake of a continuously swimming mayfly larva. The larva is swimming upright just above the bottom layer of milk and is viewed from above. (a) The larva is swimming in the direction indicated by the arrow: the larva is seen in dorsal view and the three tail fin bristles can be identified. At this point, there is no indication of the wake. (b) 0.4 s later, the larva has progressed beyond the field of view, and three roughly circular impressions have appeared in the bottom layer. As explained in the text, these were produced by three consecutive half-strokes executed to the ventral side of the swimming line.