Walking on Caltech's campus, research engineer Chris Roh (MS '13, PhD '17) happened to see a bee stuck in the water of Millikan Pond. Although it was a common-enough sight, it led Roh and his advisor, Mory Gharib (PhD '83), to a discovery about the potentially unique way that bees navigate the interface between water and air.

Roh spied the bee during California's years-long drought, when the pond's fountain was turned off and the water was still. The incident occurred around noon, so the overhead sun cast the shadows of the bee—and, more importantly, the waves churned by the flailing bee's efforts—directly onto the bottom of the pool.

As the bee struggled to make its way to the edge of the pond, Roh noticed that the shadows on the pool's bottom showed the amplitude of the waves generated by the bee's wings, as well as the interference pattern created as the waves from each individual wing crashed into each other.

"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 says.

Working with Gharib, Caltech's Hans W. Liepmann Professor of Aeronautics and Bioinspired Engineering, Roh recreated the conditions of Millikan Pond. They 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. Roh and Gharib 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.

A paper describing what they found was published in the Proceedings of the National Academy of Sciences on November 18.