For years now, scientists have sought to build aerial robots inspired by bees and other flying insects. But they have always run into a fundamental problem: Flying takes a lot of energy.

Insects flap their wings, generating the thrust needed to move through the air by utilizing the energy stored in strong muscles. Their robot doppelgängers must rely on batteries, which are less efficient and tend to be heavy, or must be hooked up externally.

Now researchers at Harvard University have built a new type of robot that is capable of true, untethered flight. The unit, called the RoboBee X-Wing, is equipped with four tiny wings made of carbon fiber and polyester, and even tinier photovoltaic cells.