Whirligig beetles are named for their whirling movement on top of water, moving rapidly in and taking off into flight.
By studying the movements of the whirligig beetle, researchers are applying nature's principles to bio-inspired swimming and diving robots. They are designing the robots for the Office of Naval Research.
"The propulsive efficiency of the species has been claimed in literature to be one of the highest measured for a thrust-generating apparatus within the animal kingdom," said researcher Mingjun Zhang, from the University of Tennessee.
"But nobody knew exactly why, so we conducted a quantitative study with experiment support that uncovered this mystery," said Zhang.
Zhang saw the curious beetle as inspiration for developing energy-efficient propulsion mechanisms for swimming vehicles and robots.
His team discovered separate leg functions, alternative patterns of leg propulsion, a unique take-off technique and maximising surface area as key to the beetle's inner workings.
The findings were published in the journal PLOS Computational Biology.
Researchers performed a combination of microscopic high-speed imaging, dynamics modeling and simulations to unlock the beetle's secret.
They discovered each of the beetle's three pairs of legs conducts a different function.
Their curved swimming trajectories gained energy efficient over linear trajectories by alternating the ways leg propelled.
Using high-speed cameras, the researchers observed that the beetles beat their legs in different directions in order to transition from swimming to diving. This provides the force required to alter the angle of the body's tilt and break the surface tension of water.
Finally, the swimming legs rely on the extension of "swimming laminae" to increase the surface area and generate larger thrust.
"Nature folds the laminae, or a thin tissue, after the beetle is done moving its legs," Zhang said.