Harvard professor Robert Wood has demoed a robotic fly, which is the first to be both actually fly-sized and have a higher power/weight density than an real fly. From this good article about the project:
"...how to power those wings to beat 120 times per second? To keep this 60-milligram robot (the weight of a few grains of rice) with a 3-centimeter wingspan to a minimal size and weight, Wood says, you can’t simply use a shrunken version of the heavy DC (direct current) motors used in most robots. So he and his team settled on a simple actuator: in this case, a layered composite that bends when electricity is applied, thereby powering a micro-scale gearbox hooked up to the wings. Wood says the actuator works even better than its biological inspiration. The power density—a measure of power output as a function of mass—of a fly’s wing muscles is around 80 watts per kilogram; Wood’s wing design produces more than 400 watts per kilogram.
The first takeoff occurred late one evening last March, as Wood worked alone in his office, his colleagues gone for the evening. As the fly rose, Wood jumped
up in celebration, quickly verified that his camera had captured the flight, and let out a sigh of relief."