Honeybees (Apis mellifera) trapped on a water surface use their wetted wings as hydrofoils for their water surface propulsion, according to a paper published in the Proceedings of the National Academy of Sciences.
A honeybee fallen on a water surface creates a wave and hydrofoils atop it. Image credit: Roh & Gharib, Caltech.
When honeybees fall on the surface of water, their wetted wings lose aerodynamic ability. Nevertheless, the bees can move forward by beating their wings. According to the new study, such motion results from the wings acting as hydrofoils.
“I was very excited to see this behavior,” said Dr. Chris Roh, a research engineer in the Graduate Aerospace Laboratories at Caltech.
For the study, Dr. Roh and his colleague, Caltech Professor Mory Gharib, placed water in a pan, allowed it to become perfectly still, and then put bees, one at a time, in the water.
As each bee flapped about in the water, filtered light was aimed directly down onto it, to create shadows on the bottom of the pan.
The team studied 33 bees individually for a few minutes at a time, carefully scooping them out after a few minutes to let them recover from their swimming efforts.
When a bee lands on water, the water sticks to its wings, robbing it of the ability to fly. However, that stickiness allows the bee to drag water, creating waves that propel it forward.
The researchers noted that the generated wave pattern is symmetrical from left to right. A strong, large-amplitude wave with an interference pattern is generated in the water at the rear of the bee, while the surface in front of the bee lacks the large wave and interference.
This asymmetry propels the bees forward with the slightest of force — about 20 millionths of a Newton.
“The motion of the bee’s wings creates a wave that its body is able to ride forward. It hydrofoils, or surfs, toward safety,” Professor Gharib said.
Slow-motion video revealed the source of the potentially life-saving asymmetry: rather than just flapping up and down in the water, the bee’s wings pronate, or curve downward, when pushing down the water and supinate (curve upward) when pulling back up, out of the water. The pulling motion provides thrust, while the pushing motion is a recovery stroke.
In addition, the wingbeats in water are slower, with a stroke amplitude of less than 10 degrees, as opposed to 90-120 degrees when they are flying through the air.
Throughout the entire process, the top side of the wing remains dry while the underside clings to the water.
The water that remains attached to the underside of the wing gives the bees the extra force they use to propel themselves forward.
“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,” Dr. Roh said.
The bees do not seem to be able to generate enough force to free themselves directly from the water, but their wing motion can propel them to the edge of a pool or pond, where they can pull themselves onto dry land and fly off.
“Hydrofoiling is a lot more taxing for the bees than is flying,” Dr. Roh said.
“They could keep up the activity for about 10 minutes, giving them a fixed window to find the edge of the water and escape.”
“The motion has never been documented in other insects, and may represent a unique adaptation by bees.”
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Chris Roh & Morteza Gharib. Honeybees use their wings for water surface locomotion. PNAS, published online November 18, 2019; doi: 10.1073/pnas.1908857116
