Fig. 7. Free-flight smoke visualization of the flow around the wings of Sympetrum sanguineum accelerating vertically with the wings stroking in-phase. A leading edge vortex (yellow arrows) forms and grows to extend over both sets of wings. (A–H) Consecutive images from a 250 Hz high-speed video recording. The dragonfly is moving from left to right through the smoke plane and the smoke is approximately 1/4 wing-length in (A) and coincident with the wing hinge in (H). (A) The end of the upstroke. (B) During the forewing rotation prior to the downstroke, there is some evidence of the start of LEV formation. In (C) the LEV is already clearly formed (yellow arrow). In (D–F) the LEV rapidly grows, the smoke streams within the LEV are thinned by the increased velocities in that region making it darker, and the stagnation point where the separatrix touches down moves aft from the forewing onto the hindwing. In (F–H), as the downstroke ends and the wing rotates, the LEV is shed into the wake. There is a saddle-point (red arrows) in the wake where smoke-streams bifurcate in the shear layer between the current LEV and the wake-vortex representing the LEV shed from the previous wake. There is no evidence of any spanwise flow.