Fig. 2. Trail produced by the wake of a continuously swimming mayfly larva. The larva is swimming on its side just above the bottom layer of milk and is viewed from above. It is pursuing a slightly curved path from top left to bottom right. (a) The initial trace consists of a sinuous line coinciding with the motion of the tail fin. (b) The appearance of a fully formed trail 0.5 s after the events shown in a. Profiles of the larva are shown at successive 40 ms intervals. The mature trail consists of a ladder-like series of areas, each corresponding to a half-stroke, in which the general motion (as indicted by the arrows) of the dye is nearly at right angles to the swimming line.