U.S. military researchers have turned to the insect world for inspiration in designing tiny drones that could be dropped off in swarms from an aircraft to glide to a target and spy on an enemy's activity.

The palm-sized aircraft are called Cicadas, inspired by the insects that spend years underground before emerging in huge swarms to reproduce, after which they die off in mass numbers.

"The idea was why can't we make UAVs (unmanned aerial vehicles) that have the same sort of profile," said Aaron Kahn of the Naval Research Laboratory regarding the Cicadas, or Covert Autonomous Disposable Aircraft.

The disposable aircraft, looking like a paper airplane fitted with a circuit board, would glide to pre-programmed GPS coordinates after it is dropped from an aircraft or a balloon or even from a larger drone, the developers reported.

Having no engine or propulsion system, a Cicada drone would be almost completely silent as it glides at around 45 mph.

"It looks like a bird flying down," said Daniel Edwards, an NRL aerospace engineer, who believes the drone would be "very difficult to see."

If built in mass numbers, the Cicada drones—intended to be smaller, simpler and cheaper than current robotic drone aircraft—could cost just $250 each, the researchers said.

"We will put so many out there, it will be impossible for the enemy to pick them all up," said Kahn, a flight controls engineer.

In initial tests of a prototype dropped from 57,000 feet, the Cicada drone landed within 15 feet of its designated target.

The tiny drones have proven surprisingly robust, surviving hard landings on asphalt runways and tumbles in gravel, the researchers said.

In addition to military uses, the drones could have civilian applications, including tornado tracking, weather forecasting or monitoring traffic, they said.

During the initial tests, the Cicadas were equipped with sensors that collected weather readings, including temperature, humidity and air pressure.

Equipping them with other lightweight sensors, such as microphones, would make the drones suitable for a variety of missions, the researchers said.

They did acknowledge that giving the drones video feed capability is at present a technical challenge because of the high bandwidth required.

Every branch of the government has expressed an interest in the Cicada drones, as have a number of academic institutions, Edwards said. "Everyone is interested," he said. "Everyone."

That's not surprising, given the wide range of possible applications they could be used for, he said.

"They are robotic carrier pigeons," Edwards noted. "You tell them where to go, and they will go there."