  The effect of air viscosity on the flow around an insect wing increases as
insect size decreases. For the smallest insects (wing length R below 1 mm), the
viscous effect is so large that lift-generation mechanisms used by their larger
counterparts become ineffective. How the weight-supporting vertical force is
generated is unknown. To elucidate the aerodynamic mechanisms responsible, we
measure the wing kinematics of the tiny wasp Encarsia formosa (0.6 mm R) in
hovering or very slow ascending flight and compute and analyze the aerodynamic
forces. We find that the insects perform two unusual wing-motions. One is
rowing: the wings move fast downward and backward, like stroking oars; the
other is the previously discovered Weis-Fogh fling. The rowing produces 70
percent of the required vertical force and the Weis-Fogh fling the other 30
percent. The oaring wing mainly produces an approximately up-pointing drag,
resulting in the vertical force. Because each oaring produces a starting flow,
the drag is unsteady in nature and much greater than that in steady motion at
the same velocities and angles of attack. Furthermore, our computation shows
that if the tiny wasps employed the usual wing kinematics of the larger insects
(flapping back and forth in a horizontal plane), vertical force produced would
be only one third of that by the real wing kinematics; i.e. they must use the
special wing movements to overcome the problem of large viscous effects
encountered by the commonly used flapping kinematics. We for the first time
observe very small insects using drag to support their weight and explain how a
net vertical force is generated when the drag principle is applied.
