An analytical study is presented to determine if the persistency of the leading-edge vortex in an insect wing can be explained as the balance between vorticity generation at the leading edge and advection plus effects of vorticity stretching and tilting by the flow along the wing span. It is found that a spanwise flow of the required magnitude is produced by the simple rotation of the wing about its root at a constant angle of attack (no supination or pronation), and that the regions where this equilibrium exists in stable form are well localized, independent of the rotation velocity, almost independent of the position along the wing, and weakly dependent on the angle of attack, for angles below approximately equal to 70 degrees. In contrast, extended regions of vorticity are expected for angles of attack above approximately equal to 75 degrees.