Little thought experiment: A person swims in a calm lake, the sky is dark. And suddenly a huge raindrop falls down, heavy as a minibus. It hits the water next to the float, then another, and more.
This is what it must feel like for the thin-legged water striders when it rains normally. The only difference is that the insects, which are one to two centimeters in size, do not swim, but rather walk lightly over the water. All while surviving the impact of raindrops 40 times heavier than themselves.
Researchers led by engineer Andrew Dickerson from the University of Tennessee in Knoxville, USA, have now found out exactly how the water striders survive such a downpour. Like them in the trade magazine Proceedings of the National Academy of Sciences (PNAS) report that the water striders even survive when they are pushed underwater by a raindrop.
In some cases, the insects were even able to survive underwater for more than ten minutes
Water striders are amazing creatures. They are found all over the world, in ponds, lakes and rivers. Particularly popular in damp, rainy areas. Some species even spend their entire lives at sea. In order to be able to walk on water, the insects distribute their low body weight of just a few milligrams over a large area using their long, thin legs. The legs are covered with microscopic hairs that trap air and help the insects to be carried by the surface tension of the water.
For their experiment, Dickerson and his colleagues placed some water striders in an aquarium and sprinkled the animals with particularly thick drops. The researchers recorded the experiment with high-speed cameras. In the videos you can see how the water droplets create craters in the water surface. Some water striders are thrown away by the force, others are pulled under the water by the suction of the diving drop or they are sunk by a direct hit.
Impact of a water drop with water striders. Impact sequence of water striders struck by a drop. Slowed 107×
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(Video: Daren Watson and Andrew Dickerso
)
Dickerson and his team were now able to observe how a fine layer of air protects the insects’ bodies. Tiny hairs prevent the water from penetrating and prevent the animals from getting wet. The atmosphere also gives the water striders buoyancy. Once submerged, they fight their way back to the surface with a few powerful swimming movements of their legs.
In some cases, the insects were even able to survive underwater for more than ten minutes, according to the researchers. However, their chances worsened over time. Because the hairy shell gradually becomes saturated with water. As a result, the insects no longer have enough buoyancy to reach the surface.