Fig. 14. Characteristic smoke patterns associated with the forewing downstroke in normal counterstroking flight. The video images show a tethered hawker Aeshna grandis; the topological interpretation is the same for all three species. The critical points in the 3D flow field are denoted by black spots (N=node; F=focus; S=saddle); dotted lines represent hypothetical surface streamlines. Visualizations are shown for 5 spanwise stations along the wing (A–E), marked by colour-coded slices in the figure. The LEV is continuous with the vortices trailing from the wingtips (A). The LEV diameter is similar across the wing, and the flow is topologically similar at all three stations inboard of the wingtip (B–D). The flow over the midline of the insect clearly shows that the LEV is continuous across the midline (E), indicating the existence of a free-slip focus above the thorax. The topology is the same throughout the downstroke: we have chosen those images that show the downstroke flow structures most clearly for each spanwise station.