I don't really like the "flying submarine" angle, as historically interest-piquing it might be. A lot of the success seems like it might come from the scale that the machine works at, i.e. there are non-linear cost functions involved when it comes to changing the scale at which the aerodynamics are treated at.
Why are all the robotic insects built with flapping wings, as opposed to propellers or rotors that drones, airplanes, boats, and helicopters are built with? Is there something inherently different at that scale that requires flapping wings?
Yes, the tendency to shed vortices is much greater at small scales. A rapid flapping motion is basically a vortex generator and more efficient than a wing (rotating or fixed) which relies more on steady state, laminar flow. See https://en.wikipedia.org/wiki/Reynolds_number for more explanation.
Additionally, the energy required (relevant to total mass) to generate lift is a lot smaller at this scale thanks to the Square-cube law[0], and it's a lot more efficient to flap a wing on an as-needed basis than to run a rotor at a continual and steady rotation.
I'm impressed by the simplicity of this design. How much control can you get out of the robot over that power cable? Is timing and computation done on the robot itself, or on the other side of the wire?
"While this RoboBee can move seamlessly from air to water, it cannot yet transition from water to air because it can’t generate enough lift without snapping one of its wings. Solving that design challenge is the next phase of the research, according to Chen."
But yeah, super cool.