Drag is the primary force that
spiders and water striders use to move across the
water surface. Although we have several lines of
evidence that show this, one of the most compelling
is the demonstration of momentum transfer from the
leg to the water during a rowing stroke. One form
of this demonstration is shown below: as the leg
moves from right to left, the turbulent flow behind
it indicates that the water there has changed both
in direction and speed.
