To escape water, bees go surfing

Nov. 19 (UPI) — Every once in a while, bees end up stranded in a puddle of water or a picnic lemonade. To escape, new research suggests the insects surf their way to safety. The discovery — published this week in the journal PNAS — could help engineers develop robots capable of flying and swimming.

While watching a bee struggle to escape from Millikan Pond on the campus of the California Institute of Technology, research engineer Chris Roh noticed the shadows of waves generated by bees wings dancing across the pond’s bottom. The waves produced a unique interference pattern as they crashed into one another.

“I was very excited to see this behavior and so I brought the honeybee back to the lab to take a look at it more closely,” Roh, who earned a doctorate from Caltech in 2017, said in a news release.

In the lab, Roh and Gharib, a professor of aeronautics and bioinspired engineering at Caltech, recreated the water landing and escape. The researchers placed bees on the surface of perfectly still water resting in a shallow pan. They shined filtered light directly down on the bee and watched as the insect’s beating wings produced waves, casting patterned shadows across the inside of the pan.

After watching 33 bees mount a watery escape for several minutes, Roh and Gharib picked up on several patterns. The bees water-soaked wings prevent it from simply flying away, but the stickiness of their wet wings produced drag, which helped the insects generate waves that propelled them forward, towards the pan’s edge.

The scientists noticed the pattern created by their wing-generated waves was symmetrical from left to right. At the rear end of the struggling bee, the researchers observed a large-amplitude wave with an interference pattern. In front of the bee, scientists realized, the wave and interference pattern were missing. The asymmetry, researchers determined, was key to the escape.

“The motion of the bee’s wings creates a wave that its body is able to ride forward,” Gharib said. “It hydrofoils, or surfs, toward safety.”

High-definition video footage revealed the source of the life-saving asymmetry. Instead of simply flapping their wings up and down, the bees wings curve downward, or pronate, on the down strike and curve upward, or supinate, on the upstroke.

The bees flap their wings rather slowly, the video showed, and the bottom of their wings remains in the water, while the top of the wings stay dry. Keeping the bottom half of their wings wet amplifies the force of the strokes.

“Water is three orders of magnitude heavier than air, which is why it traps bees. But that weight is what also makes it useful for propulsion,” Roh said.

The force generated by their unique wing pattern isn’t enough to free them directly from the water’s surface, but the motion helps them hydrofoil toward safety. Flapping wet wings is taxing, however, and Roh and Gharib estimate bees have about ten minutes after a water landing to find dry land before they’re too exhausted to escape.

Water landings aren’t always accidents.

“On hot days, bee hives require water to cool off,” Roh said. “So when the temperature rises, workers are sent out to gather water instead of pollen.”

Roh and Gharib are currently developing a small robot that uses the bees unique wing-flapping technique to move across the surface of water.

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