Fig. 2. Midline kinematics of zebrafish larvae (A–E; age 2, 3, 5 and 7 days post-fertilisation) during cyclic swimming. (i) Lateral view of the larva and midlines in a fish-based frame of reference. The superimposed midlines (time step 1 ms, scaled to the lateral view of the larva) of one tail beat cycle show the amplitude envelope of the body wave. The envelope has a minimum at 0.16–0.21 body length (L) located within the anterior third of the yolk sac (black oval: yolk sac; grey circle: otic placode at 0.16L). All larvae are scaled to be the same length. (ii) All midlines (black) of the recorded sequence (time step 1 ms) in an earth-based frame of reference. The path of the head and the tail are indicated in blue and red, respectively. The scale bar indicates 0.5L. The sequences were chosen for a similar swimming speed for the two categories `slow' and `fast' swimming.