FIGURE 1. Locomotion in fluids.

From the following article:

Animal locomotion: How to walk on water

Michael Dickinson

Nature 424, 621-622(7 August 2003)

doi:10.1038/424621a

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Vortices carry the momentum to move animals forwards in both air and water, but their pattern varies with the style and speed of locomotion. A slow-flying bird (pigeon) makes a single vortex ring with one downstroke. Its wake consists of a series of rings that move backwards and downwards. Each stroke of a fish's tail (sunfish) creates a vortex ring, which fuses with those of prior strokes to form a linked 'zig-zag' chain. The long legs of a water strider create tiny U-shaped vortices with their free ends attached to the water surface. In all cases, the circular flow around the vortex filament shoots fluid backwards through the centre of the ring, propelling the animal in the opposite direction.

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