Dragonfly larva's respiratory jet control using tri-leaflet anal valve

Previous studies on the dragonfly larva’s anal valve focused on the modulation of the aperture size. Both Tonner’s 1936 paper and Mill and Pickard’s 1972 paper report that the anal opening narrows during exhalation and widens during inhalation. However, the dragonfly larva’s anal valve is a tri-leaflet valve with individual leaflet retractor muscles that enable more sophisticated leaflet movements than a simple size control. To fully understand the intricate functional design of the valve, we investigated Anax sp. larvae’s detailed kinematics of the leaflets during the respiration. Simultaneously, the fluid flow was measured by illuminating particles in the water with a laser. The result shows that the majority of the exhalatory jets are deflected diagonally. The corresponding anal valve movement during the exhalation shows that the top leaflet is retracted while the bottom two leaflets remain closed. This asymmetric opening at the nozzle causes the jet deflection by the stagnation pressure at the closed two leaflets and by the Coandă effect–tendency of the jet to attach to a solid surface. The deflected jet influences the direction of the entrainment flow – a secondary flow induced by the jet flow – at the lower periphery of the jet to be directed towards the anal valve. The entrainment flow flowing towards the valve, coupled with the widening of the anal valve in the inhalation phase, appears to help the larva inhale clean water more efficiently.