Fig. 1. The three classes of leading-edge vortex (LEV) that have been described to date. (A) Class I: the Maxworthy (1979) description from his model experiments based on the `fling' of the chalcid wasp Encarsia formosa. The LEV inflects into tip and root vortices on each wing. The tip vortices connect to form a vortex ring behind the model, and the root vortices also connect so that the wake consists of one continuous vortex loop of complex shape. (B) Class II: the structure described by Luttges (1989) with a single LEV extending across the thorax of a hawkmoth and inflecting to form both tip vortices. This structure implies a free-slip critical point (a 3D focus) over the centreline of the thorax, as described by Srygley and Thomas (2002) for butterflies Vanessa atalanta. The topology is similar to that in A except that the root vortex is replaced by a continuous LEV over the thorax and there is no significant spanwise flow. (C) Class III: the structure described by Ellington and colleagues (Ellington et al., 1996; Van den Berg and Ellington, 1997a) where the LEV on Manduca is similar to that found on a delta wing. In this model there must be a surface-bound focus at the base of each wing and attached flow over the thorax. (D) Also Class III: the flow, topologically similar to C, scaled for Drosophila by Birch and Dickinson (2001). The flows in C and D differ qualitatively from A and B in the absence of the linkage between the LEVs via either wing root vortices or a continuation of the LEV across the centreline. Spanwise (axial) flow, if present, is marked by orange arrows in each case. Vertical planes show the simplified flow topology at the centreline and midwing positions.