Fig. 5 . Instantaneous particle traces at the beginning of the downstroke. Particles were released from a rake of rectangular grid of points in a plane 0.8 mm away from the bottom surface of the wing. Using the instantaneous velocity field, the positions of these particles were obtained by integrating the velocity at these rake points until the length of the traces exceeded a specified length or the particles ended on a solid boundary or exited the computational domain. These particle traces are colored according to the magnitude of the velocity (in cm s^-1) at that location. A leading edge vortex is seen rotating in the counterclockwise direction, and a stagnation line is shown near the z' axis of rotation (dark blue traces).