By our employee
Water striders, the small slender insects that run at lightning speed across the surface of the water, are remarkably resistant to the impact of raindrops. American and Turkish scientists write this this week PNAS. They analyzed film images they made with high-speed cameras and applied all kinds of physics to the forces between raindrop and insect.
The chance that a water strider will be hit by a raindrop is considerable. During a heavy shower, an average water strider gets a drop of water on his head no less than every seven seconds, the researchers calculated. The droplet is five to forty times as heavy as the insect and can travel as fast as 30 kilometers per hour. That has a significant impact. The researchers describe it plastically: they write about the ‘fate’ of the insect, which is ‘knocked down’ by the ‘attack’.
But the effect seems to be not too bad: each animal happily continues on its way. How is that possible? The researchers measured in the lab the force required to crush a water strider between two surfaces. Or yes, to injure, they write. That happens at about one and a half newtons. The force that a falling raindrop exerts on the water strider is less than half that, partly due to the deformation of the surfaces. So far no problem.
But what if the water strider ends up underwater? That doesn’t matter either, as the video footage showed. First the water strider is sucked down into the crater created by the impact of the raindrop. If that crater then collapses, the insect can spin around underwater. But this collapse creates a jet of water upwards, which the water strider can spit out again. Sometimes it lands safely on the water, sometimes in a second crater, which is created by the fall of that water jet. Even then, it can be launched again – or end up underwater, depending on its initial position.
The water strider also experiences no problems under water. He is then surrounded by a very thin layer of air that is held by the paper-thin hairs of the armor. This coat is therefore extremely water-repellent. The layer of air helps water striders float back up and break through the water surface while swimming. All in all, survival during the study was 100 percent – except for the crush tests.