Watch this high-speed video of a scared whirligig diving from the surface in just 0.2 seconds, and slowed to a speed were you can see the individual sets of legs at work.
Bugs from the whirligig beetle family are some of the most nimble on the planet. They are among the few organisms that can fly, crawl, and swim efficiently, and sometimes more than that—they are capable of precise sharp turns and evasive maneuvers, and their ranks include the fastest insect swimmer ever recorded.
The Office of Naval Research Young Investigator Award sponsored a study to model the dynamics of the whirligig beetles. University of Tennessee engineering professor Mingjun Zhang wanted to figure out how the beetle transitions from swimming on the surface of water to diving, hoping the secrets of the insect's maneuverability could lead to better biology-inspired robots. He published his results in the PLOS Journal of Computational Biology.
In the water, the best way of getting from point A to point B is not necessarily a straight line. Studying whilrigigs, Zhang and his team found that the insects would take a curved swimming trajectory between two points; even if the bugs covered more distance this way, they expended less energy because of the way they move through the water.
Using microscopes and high-speed imaging to zoom in on the whirligig beetle's legs, Zhang's team found that the beetle's angular velocity comes primarily from the hind legs, while the middle pair is used for steering (the forelegs are used for grabbing prey, not for swimming). The hind legs alternate beats breaststroke style at the sides of the body when swimming normally and dolphin-kick under the body during a dive.
Swimming on the surface the whirligig travels in an S shape—the most energy efficient way to get the job done—instead of a straight line. (A straight line is fastest, but also tires out the beetle the quickest. In an S shape, the beetle can move almost as fast and with an unpredictable trajectory, helpful for avoiding predators, without getting exhausted.) The hind legs have retractable, paddle-like layers of tissue that pop out for a propulsive stroke, to increase surface area, and retract for the recovery stroke, minimizing drag.
The team is currently building a beetle-inspired robot for energy-efficient swimming on a water surface, and increasing maneuverability, which will add to long line of existing bio-inspired robots, such as the water strider, the snake and the wall-climbing gecko robots.