Nick Carne
When a honeybee falls onto water it can essentially surf to safety, research engineers in the US have discovered.
While the water sticking to its wings robs it of the ability to fly, that very stickiness allows it to drag water, creating waves that propel it forward. The wings act much like hydrofoils.
The motion has never been documented in other insects, and may represent a unique adaptation by bees, says Chris Roh, who worked with California Institute of Technology colleague Mory Gharib.
Writing in the journal Proceedings of the National Academy of Sciences, they report that the bees don’t seem able to generate sufficient force to free themselves directly from the water, but their wing motion can propel them to the edge of pond, where they can pull themselves onto dry land.
The research stemmed from a chance observation at the right time of day. As Roh noticed a bee stuck on the still water of a pond, the overhead sun cast the shadows of the bee and the waves created by its flailing wings directly onto the bottom.
A surface streaming flow pattern generated by a horizontally tethered bee.
The shadows showed the amplitude of the waves generated by the bee’s wings, as well as the interference pattern created as the waves from each wing crashed into each other.
They then recreated the conditions in their lab, studying 33 bees individually for a few minutes at a time, before giving them time to rest and recover.
They noted that the generated wave pattern was symmetrical from left to right. A strong, large-amplitude wave with an interference pattern was generated in the water at the rear of the bee, while the surface in front of the bee lacked the large wave and interference.
This asymmetry propels the bees forward with the slightest of force – about 20 millionths of a Newton. (For reference, they say, an apple held in your hand exerts about one Newton of force on your palm due to gravity.)
Slow-motion video revealed the source of the 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 – the measure of how far their wings travel when they flap – of less than 10 degrees, as opposed to 90-120 degrees when they are flying through the air.
The dorsal (top) side of the wing remains dry throughout, 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,” Roh says.
Hydrofoiling is a lot more taxing for the bees than is flying, however. Roh estimates bees could only keep up the activity for about 10 minutes.
The authors say they have started applying the findings to their robotics research, developing a small robot that uses a similar motion to navigate the surface of water.
