Drowning bees use their wings as hydrofoils to maneuver on the surface of water, creating waves with their wingbeats to “surf” towards safety, according to new research. Engineers at the California Institute of Technology report for the first time that this method can help bees move at speeds up to three body lengths, allowing them to overcome hydrodynamic drag.
It all started in summer when research engineer Chris Roh saw a bee that was stuck in a pond struggling to swim. The afternoon Sun overhead cast shadows of the creature, allowing Roh to see how the bee's flailing wings created waves in the water.
Using this same concept, Roh put more than 30 bees individually in a pan filled with water and aimed a filtered light directly down on the bees in order to see how their shadows move along the bottom of the pan. The researchers found that when a bee lands in the water, the liquid sticks to its wings and hinders the insects’ aerodynamic ability. This stickiness also allows the bees to drag water and create waves that propel them forward.
But these waves are not symmetrical. Rather, a large-amplitude wave is generated in the water behind the bee while the water in front of it remains relatively still. This asymmetry propels the bee forward a teensy, tiny amount – just 20 millionths of a Newtown. (By comparison, the gravity of catching an apple in your palm exerts about one Newton of force.)
"The motion of the bee's wings creates a wave that its body is able to ride forward," said researcher Mory Gharib, whose study is published in Proceedings of the National Academy of Sciences, in a statement. "It hydrofoils, or surfs, toward safety."
Further analysis of the slow-motion videos shows that rather than flapping its wings up and down, a bee’s wings pronate, or curve downward to push the water, and curves upwards when the wings pull back and out of the water. This pulling motion provides thrust while the pushing motion makes up a recovery stroke. Wings were also observed beating more slowly and at a shorter range when in the water. In the air, a wing will travel between 90 and 120 degrees compared to less than 10 degrees when underwater.
This hydrofoil method does not allow for the bee to propel itself out of the water but allows it to “surf” towards the water’s edge so that it can eventually pull itself out. The researchers estimate that the insect can keep it up this motion for about 10 minutes, or just enough time to escape a watery death.
"On hot days, beehives require water to cool off," said Roh. "So when the temperature rises, workers are sent out to gather water instead of pollen." The bees will find a water source, swallow some into a special chamber in their bodies, and then fly off. Sometimes, however, they fall in. And if they cannot free themselves, they die.
Besides helping to inform our understanding of locomotion, the researchers say that their findings help inform robotics research. The team is currently working on a small robotic aerial-aquatic hybrid vehicle that uses the same strategy to navigate the water.
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