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The formation of helical structures outside the wing-tip vortices of the hindwing from a non-phase-locked sequence. (a) The transverse vortex structures, highlighted in red, as they roll up behind the wings as a result from Kelvin–Helmholtz instabilities in the shear layer. (b) Shortly after formation the vortices get entrained around the strong hindwing downstroke vortices and form helical structures around these.
