Lift-based propulsion for fastest-swimming whirligig beetles (Gyrinidae)

Whirligig beetles, Gyrinidae, are the fastest swimmers among insects. This one-centimeter-long aquatic beetle can achieve a peak acceleration of 100 ms^-2 and a top velocity of 1 ms^-1. The swimming mechanisms of the whirligig have been previously studied in a tethered state, which suggested that their oar-like rudder legs create drag for propulsion. In our study, we present leg and body kinematics of Dineutus discolor to show that the thrust generation in high-speed free swimming relies on lift. We found that the rudder legs had nearly no motion relative to the surrounding fluid along the axis of swimming. Instead, the legs were subjected to drastic transverse motions, in which large lift force was generated. Our force estimation suggests that the lift associated with the transverse leg motions is comparable to the thrust required for overcoming hydrodynamic drag and generating large acceleration.