Whirligig Beetles Inspire Energy-Efficient Robots

The whirligig beetles that play the role of scavengers by keeping the surface of the waterways clean have inspired researchers from the University of Tennessee, Knoxville, to develop an energy-efficient propulsion mechanism for swimming vehicles and robots. 

The team led by Mingjun Zhang, associate professor of mechanical, aerospace and biomedical engineering, at the University of Tennessee, identified the key to a beetle's inner mechanism. The factors contributing to it were separate leg functions, alternative patterns of leg propulsion, a unique take-off technique and maximizing surface.

"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," Zhang said. "But nobody knew exactly why, so we conducted a quantitative study with experiment support that uncovered this mystery."

The whirligig beetles like to swim in large groups on the surface of the water. Apart from the way they swim in water, they have unusual characteristics."

In order to unlock the beetle's secret mechanism, the team conducted a combination of microscopic high-speed imaging, dynamics modeling and simulations.

On doing so they discovered that each of the beetles' three pairs of legs performs a different function. Their curved swimming trajectories gained energy efficiency over linear trajectories by alternating the ways the legs propelled.

With the help of high speed cameras, the researchers observed a peculiar behavior in these beetles. They noticed how they beat their legs in different directions in order to transition from swimming to diving. This activity provides the strength that is required to change 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 a larger thrust.

"Nature folds the laminae, or a thin tissue, after the beetle is done moving its legs," Zhang said. "It extends it when it is propelling to generate thrust. The legs may also be oriented at different angles, so that the maximum area is not perpendicular to the direction in which the beetle is moving. I am always amazed how nature does this with the small organism."

Zhang's team looked to nature for inspiration in engineering. By studying the movements of the whirligig beetle, the team applied nature's principles to bio-inspired swimming and diving robots. 

 The findings have been published in this month's journal PLOS.