This is Why … Water Striders and I Are Kindred Spirits

(Shutterstock.com ID 1724117857)

A couple of summers ago I signed up with the Wellington Sculling Club for their Learn To Row* program. Coached by an amazing group of people, I successfully completed the course and joined as a full member, but I still have plenty to learn so I head down to the lake as often as I can.

The other morning, after putting the gear away, I stood on the dock watching a swarm of water striders skitching and sketching across the lake surface, occasionally leaping a significant height directly upwards. Their movements are so quick and directed, with sudden bursts of acceleration in one direction or another as they careen across the water. As soon as I got home, I started digging – how do they move around? How do they stay afloat? How can they leap straight up like that from a liquid surface? Imagine my surprise when I learned that these little creatures are nature’s skilled scullers!

An article in Nature in 2003, written by a team of mathematicians and engineers from MIT, studied water-strider motion through high-speed cameras and tracking software. If you look at the image at the top, there are three pairs of legs on a strider: two shorter ones at the front (under the antenna) and then two pairs of much longer legs, one at the midsection and one at the back. The pair at the midsection act as sculling oars; when the stroke is initiated, they push backwards and down, changing their angle to the bug’s body from 60⁰ to about 120⁰ before lifting out and returning to their initial positions. The bug’s legs push backwards on the water and the water pushes forward on the bug – Newton’s Action-Reaction law in action!

The group from MIT went further, showing that the leg motion creates spinning swirls of water (vortices) moving in the opposite direction to that of the bug. They even calculated that the momentum of the water moving in these vortices (approximately 1 g cm/s) nicely corresponds to the resulting momentum of the strider, but in the opposite direction. And, this is exactly what we do in our sculls (although at a much larger scale); the scoop-shaped blades are designed to create vortices of water that move opposite to the direction of the boat, as you can see in the overhead photo here. The rower is, as always, facing the back of the boat.

Vortices move in one direction, while the rower and his boat move in the opposite direction. Shutterstock ID 220619839

But water striders have six legs: what are the others doing during each stroke? An elegant study from 2018, reported in the Journal of Bionic Engineering by a group from China, coupled high-speed cameras with light sensors to analyse the forces applied by each leg. They used the “shadow method” ‒ by analysing the size and motion of the shadows cast by the strider’s legs, they could track position, velocity, acceleration, and applied force. From this they concluded that the two smaller front legs lift up during the stroke to minimize drag and then return to the water to help support the body weight as the sculling legs are lifted out after the stroke. The two rear legs act as rudders to help steer while also supporting the weight. All this revealed by watching some shadows dart across a screen in their laboratory!

The distinctive shadows cast by the water striders were used to analyse their motion. From Shutterstock.com ID 477082444

Could they really tell how hard the legs were pushing just by measuring the size of each shadow? It sounds crazy, but it holds up (no pun intended) …. when they added up the supporting forces for each of the six legs, the total matched the force of gravity acting on the insect based on the mass measured by putting their subjects on a teeny, tiny electronic balance.

These little creatures are optimally tuned to living life at the boundary between air and water: small enough to be supported by surface tension, able to apply the maximum amount of force in their sculling stroke to accelerate quickly without breaking the surface, and incredibly agile when leaping straight up into the air by rotating their legs just so to dynamically match the deformation of the water surface as they push off. Incredibly coordinated actions, all occurring within fractions of a second. So, if you’re looking for me around 6 am most days during the pandemic, I’ll be out taking a few lessons from my friends, the striders.

*Rowing is the general term for the sport, which consists of boats that have two oars at each seat (sculls) or one oar at each seat (sweep boats). Sweeps come in pairs, fours, and eights. Sculls exist as singles, doubles, and quads. Our club is strictly sculling; the Guelph Rowing Club at the other end of Guelph Lake has both sculls and sweep boats.

References:

The Hydrodynamics of Water Strider Locomotion, D. L. Hu, B. Chan, and J. W. M. Bush Nature 424 663 – 666 (2003) DOI 10.1038/nature01793

Jumping on water: Surface tension–dominated jumping of water striders and robotic insect Je-Sung Koh, Eunjin Yang, et al, Science 31 Jul 2015: Vol. 349, Issue 6247, pp. 517-521 DOI: 10.1126/science.aab1637 https://science.sciencemag.org/content/349/6247/517

Propulsion Principles of Water Striders in Sculling Forward through Shadow Method, Lu, H., Zheng, Y., Yin, W. et alJ Bionic Eng 15, 516–525 (2018). DOI 10.1007/s42235-018-0042-8

Published by joanneomeara

Professor, Department of Physics, University of Guelph

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