Bees need water just like the rest of us. A honeybee might fly several miles to find a good water source, both for drinking and to help regulate the temperature of her hive. Sometimes, though, a thirsty honeybee gets more than she bargained for, and instead of water ending up in the bee, the bee ends up in the water.
That's worse for the bee than it might sound. Honeybees can't swim, and when their wings are wet, they can't fly, either. But as a new study reveals, honeybees do have another, less obvious option to save themselves from drowning: surfing.
This discovery started with a lucky accident. As research engineer Chris Roh was walking through the California Institute of Technology campus, he passed by Caltech's Millikan Pond, which was still because the fountain had been turned off. Roh saw a honeybee stranded in the water, and since it was midday, the sun cast shadows of the bee directly onto the bottom of the pool. What really caught his eye, though, were the shadows of the waves created by the bee's wings.
As the bee buzzed in the water, Roh realized the shadows showed the amplitude of the waves kicked up by its wings, along with the interference pattern created as waves from one wing collided with waves from the other.
"I was very excited to see this behavior," Roh says in a statement about the research, "and so I brought the honeybee back to the lab to take a look at it more closely."
Back in the lab, Roh recreated the conditions he'd seen in Millikan Pond. With his advisor, Caltech aeronautic and bioengineering professor Morteza Gharib, he placed a single bee in a pan of still water, then shone filtered light on it from above, casting shadows on the bottom of the pan. They did this with 33 individual bees, but only for a few minutes at a time, and then gave each bee time to recover afterward.
The results of this experiment were recently published in the Proceedings of the National Academy of Sciences, but you can also see a glimpse in the video above.
While water prevents a bee from flying by clinging to her wings, that same phenomenon apparently provides another way to escape. It lets the bee drag water with her wings, creating waves that can propel her forward. This wave pattern is symmetrical from left to right, the researchers found, while the water behind the bee develops a strong, large-amplitude wave with an interference pattern. There is no big wave or interference in front of the bee, and that asymmetry nudges her forward with a tiny amount of force, totaling about 20 millionths of a newton.
To put that in perspective, an average-sized apple exerts about one newton of force due to Earth's gravity, which we experience as the apple's weight. The honeybee's waves only generate about 0.00002 of that force, which might sound too weak to be useful, but apparently it's enough to help the insect "surf" to safety.
"The motion of the bee's wings creates a wave that its body is able to ride forward," Gharib says. "It hydrofoils, or surfs, toward safety."
Surfing to survive
Instead of flapping flatly, honeybee wings curve downward as they push into water, then curve upward as they pull back to the surface. The pulling motion generates thrust, the researchers explain, while the pushing motion is a recovery stroke.
The bees also beat their wings more slowly in the water, based on a metric known as "stroke amplitude," which measures how far the wings move while flapping. The stroke amplitude of a honeybee's wings is about 90 to 120 degrees while flying, the researchers note, but in the water it drops to less than 10 degrees. This lets the top of the wing stay dry, while water clings to the underside, pushing the bee forward.
"Water is three orders of magnitude heavier than air, which is why it traps bees," Roh explains. "But that weight is what also makes it useful for propulsion."
There are some limitations to this technique, since the bees apparently can't generate enough force to lift their bodies out of the water. It can propel them forward instead of just flailing in place, though, which might be enough to reach the water's edge, where they can then crawl out and fly away. But the behavior is more tiring for bees than flying, and Roh estimates they can only keep it up for about 10 minutes before wearing out, so the opportunity to escape may be limited.
This behavior has never been documented in other insects, Roh adds, and it might be a unique adaptation in bees. This study focused on honeybees, but future research could investigate whether it's also used by other bee species, or possibly even other winged insects. Anything that helps us better understand bees is likely worth the effort, given the ecological importance of bees and their widespread declines in recent years — a problem plaguing many wild species as well as honeybees.
As engineers, Roh and Gharib also see this discovery as an opportunity for biomimicry, and they've already begun applying it to their robotics research, according to a news release from Caltech. They're developing a small robot that can move on the surface of water like a stranded honeybee, and they envision the technique eventually being used by robots that can fly and swim.