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The effect of air viscosity on the flow around an insect wing increases asinsect size decreases. For the smallest insects (wing length R below 1 mm), theviscous effect is so large that lift-generation mechanisms used by their largercounterparts become ineffective. How the weight-supporting vertical force isgenerated is unknown. To elucidate the aerodynamic mechanisms responsible, wemeasure the wing kinematics of the tiny wasp Encarsia formosa (0.6 mm R) inhovering or very slow ascending flight and compute and analyze the aerodynamicforces. We find that the insects perform two unusual wing-motions. One isrowing: the wings move fast downward and backward, like stroking oars; theother is the previously discovered Weis-Fogh fling. The rowing produces 70percent of the required vertical force and the Weis-Fogh fling the other 30percent. 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 atthe same velocities and angles of attack. Furthermore, our computation showsthat if the tiny wasps employed the usual wing kinematics of the larger insects(flapping back and forth in a horizontal plane), vertical force produced wouldbe only one third of that by the real wing kinematics; i.e. they must use thespecial wing movements to overcome the problem of large viscous effectsencountered by the commonly used flapping kinematics. We for the first timeobserve very small insects using drag to support their weight and explain how anet vertical force is generated when the drag principle is applied.

Very small insects use novel wing flapping and drag principle to generate the weight-supporting vertical force