Figure 3.

Graphical explanation of the basic concepts in the model of sliding water strider. (A,B) schematics of asymmetric (A) and symmetric (B) sliding, and variables used in the model: anterior and posterior normal forces ($N_a$, $N_p$), the anterior and posterior resistance forces ($R_a$, $R_p$), wetted leg lengths ($L$) and diameters ($D$), horizontal distance along line parallel to the moving direction from the centre of the mass to the centre of the anterior and posterior supporting wetted legs ($a$,$b$), body height above water surface ($h$), body velocity ($U$). (C,D) the two main forces contributing to the total resistance: hydrodynamic drag ($F_h$; brown) and wave drag ($F_w$; blue). The light blue area (C) and dotted line (D) represent the wetted area of a sliding leg. $C_D$ is the surface tension coefficient, $\rho$ is the density of water, $U$ is the relative velocity of the water strider to the water and $A$ is the wetted area of the leg. The wave drag, $F_w$, is induced by the wave generated by the cylindrical leg as shown in C. $N$ is the normal force on the leg from the water (A), $L$ is the leg length, $\Psi$ is the shape of wetted area depending on the shape of the leg and its moving direction. $\xi$ is designed to simplify the formula. See details in electronic supplementary material 1, parts 3 and 4 (electronic supplementary material, figures S13–S27).