John Timmer, writing for Ars Technica:
To get the wings to beat fast enough, the authors created two “muscles” made from a piezoelectric material, which changes shape when a voltage is applied. These flapped the wings at 120 beats a second. Not only is this rate similar to a fly’s, but it also created a resonance in the robot’s body that amplified the force of each beat. That resonant frequency was so important that the flight control system never changed it, even when it needed to change the force generated by the wing (to fly up or drop lower, for example). Instead, the force was controlled by changing how far the wing traveled with each beat.
The power source and control module for these mini drones are externally mounted, so it’s only capable of tethered flight at the present time. That and the wings wear out after just 10-15 minutes of flight. Fruit flies still have the upper hand.