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Fig. 2. Free-flight flow visualization of the wake of the dragonfly Sympetrum
sanguineum in counterstroking flight. (A–F) Composite figure of
sequential images extracted from a 250 Hz high speed video recording (video S1
in supplementary material). The dragonfly is moving from left to right through
the smoke plane, which is approximately at the near wing hinge in (A). Wake
structure is incoherent. There is no sign of a starting vortex, but some sort
of vortex structure (stopping vortex? Wingtip vortex in oblique view?) is
apparent in (C–E) (green arrows) and a wake element of sorts can be seen
between the green arrows in (D). This wake element rapidly loses its identity
after it is shed, being hard to detect after two frames (1/125th of a second).
The visualised wake is not consistent with a series of discrete vortex
elements such as, for example, vortex rings.
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