Sequential buckling mechanics of the ermine moth's aeroelastic tymbal: an origami-like creased shell analogue

Ermine moths produce bursts of ultrasonic clicks that protect them from their predators. The clicks are produced by a tymbal organ that features a series of corrugated striations in the membrane of the hindwings. In response to wing folding during the wingbeat cycle, individual stria snap-through sequentially to excite the natural frequencies of a scaleless patch adjacent to the striations. Based on morphological characterization of the aeroelastic tymbal of Yponomeuta moths, we propose an analogue origami-like creased shell model to reproduce the actuation and sequential click production of the biological structure. The origami-like model helps to explain the governing biomechanics of aeroelastic tymbals; namely, the burst of clicks occurs as a result of a series of snapping vertex folds, reminiscent of the origami waterbomb, that buckle and snap-through in sequence when actuated by a global stimulus (wing folding). Such sequential buckling behaviour often occurs in pattern formation events and in the structural failure of compressed cylindrical shells and sandwich panels. Interestingly, ermine moths instead use this instability phenomenon for functionality—namely, phased acoustic sound emission—creating opportunities for new engineering applications.