Fig. 16. LEV formation and growth in dragonflies. (A–D) Composite sequence of high-resolution centreline flow visualizations of tethered flight in Aeshna grandis. At the top of the forewing upstroke (A) the LEV shed after the previous downstroke is visible behind the wings in the wake (yellow arrow). There is a smoke bifurcation in the smoke streams behind the LEV (red arrow). In (B) at the start of the downstroke a LEV has formed between the forewings (left yellow arrow), and there is a second vortex in the wake (right yellow arrow), but this has the same sense of rotation as the LEV – as is clearly demonstrated by the pattern of smoke at the red arrow. Thus this second vortex is the shed LEV from the previous downstroke – representing a stopping vortex – and there is no evidence of the existence of any form of starting vortex. The wings clearly operate in a region influenced by the upwards flow to the left of the clockwise rotating shed vortex in the wake. By mid-downstroke (C), the LEV extends over the entire wing chord, and again there are only two coherent vortex structures visible (yellow arrows), and they have the same clockwise sense of rotation (as evidenced by the smoke at the red arrows). The LEV is transferred from forewing to hindwing at the end of the downstroke (D).