Swimming of the Japanese beetle

Japanese beetles (Popillia japonica) are one-centimeter-long terrestrial insects that are able to swim for survival when trapped on the water surface. We recently observed that, during free swimming, they move their hind-legs backward and downward. The backward motion resembles traditional rowing; however, the downward motion penetrates the water surface, which is potentially a novel propulsion mechanism. In this work, we experimentally investigate body and leg kinematics and their correlation with force generation in swimming. In addition, we present the shadowgraph visualization of the wave field generated in the propulsive stroke. Based on the observation we formulated two possible ways the surface penetration could contribute to the thrust generation. First, it creates asymmetry in both the power and recovery stroke. The large dimple during power-stroke (pre-penetration) could aid in generating large drag, while the fully submerged leg during the recovery stroke experiences less drag. Second, the leg breaking the surface generates sudden leg and body motion. This imparts momentum on the surrounding fluid in the form of flow and capillary wave.