Fig. 7. Temporal evolution of the upstream wake. (A) Time t=0; (B) t=(7/5)T; (C) t =(14/5)T, where T is the duration of a single swimming cycle. See Movie 4 in supplementary material. Fluid particles in each of three adjacent repelling LCS (red curves) are labeled magenta or green in order to track their evolution. After interaction with the swimmer (shown here in a reference frame fixed on the swimmer; the swimmer moves from right to left in a laboratory reference frame; see Fig. 5), the fluid particles in the repelling LCS are shown to comprise the subsequent downstream wake, illustrated by the attracting LCS (blue curves). Fluid particles initially separated by the repelling LCS do not mix and are only moderately deformed. S, swimmer; w(x), the width of the upstream wake; RLCS, repelling LCS; ALCS, attracting LCS.