DARPA just released this video that documents the steps taken to develop a functional nano air vehicle (NAV) , a project that started in 2005 and was also featured here a while ago. The flying robot is similar to a hummingbird and uses the flapping action of its wings to hover and move in the 3d space. It is a very challenging project because of the extremely small size (~15cm length and 20gr weight) and the dynamic demands of such a small machine. The viscosity of the air in these dimensions and flapping speed plays a very important role that should be taken into account. Propulsion, controls and electronics should also fit and function in a 20g package. The NAV is already capable of stable flight and video feed, you can read more info in DARPA’s website.