How Water Striders Play With Light

It may have been the angle of light on the water, at that hour and in that season, but it was also the magic of multiple minds that caused me to pause long enough to see what I might otherwise have passed by. The minds were those of Mariah and her graduate student friends from the Princeton School of Public and International Affairs, who came three weeks ago to help at Herrontown Woods. After a midday workday, pulling some last remnants of stiltgrass along Barden pathways before their seeds dropped, we walked down to the stream. I wanted to tell them the story of the improbably large fish I had seen there back in April, but we soon focused in on something much more diminutive.

After a long late-summer drought that continued into fall, there was just enough water to host a gathering of water striders calmly zigzagging about on the waters's surface in the rocky stream. I pointed them out, thinking we'd note their novel buoyancy and move on. But the students took an interest, bent down to look more closely and, because they enjoy each other's company, began talking about what they were seeing. One observation led to another, and before I knew it, we began seeing aspects of the water striders we hadn't noticed before. What were those bright points of light where the striders' feet touch the water? And why did their tiny feet make such comparatively big shadows on the streambed?

It occurred to me that their feet make dimples in the water's surface that serve as tiny curved mirrors, reflecting focused light up towards us while casting shadows on the streambed. 

As with any natural phenomenon, the internet is packed with information to amaze and edify. AnimalScienceTV has an excellent video that lauds the water striders' quick reflexes and appetite for mosquito larvae. Water-phobic oily hairs help keep them high and dry. Water striders can be preyed on by an aquatic bug called a backswimmer, which looks like a water boatman. 

All this enlightenment began with an auspicious angle of light on the water, and a synergy of minds.