Crossing a puddle may pose no challenge to you, but for a tiny insect, it is akin to scaling a slippery, steep mountain. Two mathematicians have accurately explained how these minuscule creatures achieve this remarkable feat.
They exploit surface tension to their advantage, “gliding” across the water’s surface with almost no effort, according to John Bush from the Massachusetts Institute of Technology in Cambridge, Massachusetts, who co-authored the study with David Hu.
The puddle rises at the edges due to capillary action, causing the liquid at the point where it meets the surrounding land to elevate. This characteristic (known as the meniscus of a liquid) is also commonly observed at the rim of water glasses.
Larger insects, such as water striders, are fast and big enough to run across the meniscus of a puddle. However, smaller species cannot do this. Instead, these insects, which are only a few millimeters long, deform the surface of the water with their legs, creating a force that “launches” them to the top of the meniscus.
Bush and Hu studied three species of insects: Mesovelia and Microvelia (which walk on water in their adult stage) and the larvae of the beetle Pyrrhalta. They used high-speed video to capture their movements and analyze them.
All three groups of organisms used their legs to pull the water surface up at the front while pressing down the middle section of their bodies. The water that rises beneath their feet creates an area with exceptionally high surface tension; in other words, the meniscus of the puddle at this point is very steep. Similar to the bubbles on the surface of a glass of champagne, the two regions of high tension above attract each other to reduce the overall surface tension. As a result, this attraction pulls the insect up the slope.
“They climb up the meniscus and slide down under the influence of gravity, then find a new position and continue that journey,” Bush explained.
According to Bush, the front legs of the insect play a key role in this effect. The middle legs push down to counteract gravity and prevent it from sinking, while the hind legs pull the insect up, maintaining balance with the front legs; otherwise, it risks slipping and tumbling backward.
This technique turns it into an agile acrobat. Insects can glide across the liquid surface at speeds of 30 times their body length per second, whereas the human champion can manage only about 5 times their height.
T. An (according to Nature)