Vortex bursting corresponds to reduced gradients in wing flexibility

The leading-edge vortex (LEV) is a well-known flight mechanism for flapping insects, but the interplay between the bound LEV and the flexible wing it attaches to is not yet understood. On rigid wings, the LEV often bursts but remains attached at Re $\sim\mathcal{O}$($10^3$). However, this has not been seen on flexible insect wings. Force production increases with decreasing flexural stiffness and further increases when the wing exhibits chord- and spanwise gradients in flexural stiffness. We mounted real hawkmoth wings onto a motor that revolves the wings at a constant frequency and generates a coherent LEV. Using smoke-wire visualization, we observed the qualitative structure of the LEV. On freshly-mounted wings, LEV diameter is consistently 50\% or less than the local chord length with well-defined reattachment streaklines. After desiccating, the wings stiffen in both the chord- and spanwise directions, and the LEV formed on stiffened wings has a diameter of at least 80\% of the local chord length. The reattachment streaklines were also disrupted on stiffened wings, suggesting that the loss of flexibility gradients contributes to LEV bursting. Flexibility gradients on insect wings may compensate for changes in spanwise vorticity that induce vortex bursting.