Fig. 5. Vorticity and velocity when viewed from behind. Color represents vorticity, arrows represent velocity (arrow scale upper right). Panels i–iv show successive slices starting just behind the leading edge (i) and moving toward the trailing edge in 1 cm increments (see inset at top). The solid horizontal line indicates the laser sheet intersection with the wing; vectors above this line are above and behind the wing, vectors below represent fluid movement as seen through the wing (i.e. below and in front of the wing). Columns (A–D and E–H) represent two experimental protocols with identical wing size, flapping frequency and kinematic pattern; only oil viscosity and Re are different.