  The supporting forces of all the legs have been simultaneously measured
through a natural phenomenon inspired shadow method, by capturing water strider
leg shadows to calculate the expelled water volume and the equivalent floating
forces according to Archimedes' principle, with a sensitivity up to
nN~pN/pixel. The method was verified by comparing to electronic balance to
weight water striders. The supporting force acted on legs was found to linearly
proportional to its shadow area, which geometry clearly indicated the
hydrophobicity of legs. The pressed depth of legs, vertical weight focus
position change and body pitch angle during leg refreshing, sculling and
jumping forward of water striders have been achieved with resolutions of
5um/pixel, 2 um/pixel, and 0.02 degree, respectively, which is difficult for
general imaging technologies. The shadow method was also developed into a force
measuring device and characterized the adhesion force of a hair/polymer surface
contact in a few uN with a 0.3 nN/pixel resolution.
