Wing and body kinematics measurement and force analyses of landing in fruit flies.

In this paper, we analyzed the body and wing kinematics and mechanics in fruit flies' landings on a vertical pole. By high-speed video techniques and CFD method, we exhibited the detailed process of landing as follows. A fruit fly first decelerates and enters a near-hover state approximately 25 mm away from the vertical pole, then starts approaching the pole in acceleration till one of its front-legs touches the pole (touchdown). Just before touchdown, the fly's acceleration is around 6 ms<sup>-2</sup>; at touchdown, it experiences an almost instant deceleration of several g (gravitational acceleration), approximately all of which results from the leg force. After touchdown, the fly's acceleration fluctuates about zero, and the acceleration is also mainly due to the leg force; meanwhile the aerodynamic force decreases to zero and the fly's weight is gradually shifted to legs. In this period, the fly's legs act as a compression spring, and the fly can be regarded as a legs-body oscillator. In conclusion, during landing, the fly keeps accelerating towards and "impinges" on the target on its legs, and the leg force, not aerodynamic force, is used to stop the insect's motion.