Fig. 7. Analysis of flow field velocities around the hindwing using DPIV techniques reveals the elaborate flow structures. The wing sections (white lines) are viewed at 50% distance between the wing base and tip, and have a geometric angle of attack of 45°. The direction of wing translation is from left to right in the hindwing (lower wing in panels) and, where present in the images, from right to left for the forewing (upper wing). The rotational axis for the fore- and hindwing is 0.33 and 0.22 chord widths, respectively. The DPIV images show wake velocity fields at two different times within the stroke cycle (A-C, 15% of stroke cycle; D-F, 35% of stroke cycle) when the hindwing flaps free from forewing downwash (A,D), when both wings flap but the forewing (B,E) or the hindwing (C,F) leads wing motion by a quarter stroke cycle. White open boxes in A-F indicate the region, located between the lower surface of the wing and the free stream, from which downwash flow velocities and angles were measured (see text for details). T, stroke cycle.