Water striders are known for their ability to walk on water. But when raindrops fall on the surface of the water, they create impacts with enormous force for the water striders. If the tiny insects are hit, they are swept under water. Researchers have now observed the strategies they use to fight their way back to the surface. Accordingly, their water-repellent shell helps water striders avoid drowning. The findings also help to understand how microplastics are distributed in the ocean.
As their name suggests, water striders (Trepobates subnitidus) spend most of their lives walking on water. This is made possible by fine, water-repellent hair on the animals’ legs and body armor. In their natural habitat, the insects, which are only around 15 millimeters in size, are exposed to all kinds of weather. When it rains, raindrops that are about 40 times heavier than the lightweight water striders hit the surface of the water. The force of the impact can knock the animals under water. How they survive has been a mystery until now.
Who will make the jump?
A team led by Daren Watson from Florida Polytechnic University has now investigated this question. To do this, the researchers created artificial water drops in the laboratory that correspond to particularly heavy raindrops and used high-speed cameras to film how water striders reacted to them. The analysis of the film footage revealed various processes: When a raindrop hits, a crater initially forms on the water surface, which pushes the water striders that were directly hit under the water. However, when the water crater subsequently collapses, an upward jet of water (Worthington jet) is created, which catapults the water striders back to the surface. In some cases, the short dive ends at this point and the insects jump off the briefly formed mound of water, as the researchers report. The more centrally the water striders are in the crater, the easier it is for them to jump off.
However, if the animals do not manage to jump off, they are thrown back onto the surface of the water by the water jet collapsing due to gravity. This creates a second crater. Under certain circumstances, the water striders can then be pulled under water again. However, whether this happens depends on the speed at which the second crater collapses and the exact location of the insects, the high-speed images revealed. It can happen that the insects are not automatically passively transported upwards again by the water movements, but are completely submerged.
With rowing movements and buoyancy aid
To prevent them from drowning, the tiny water striders then actively swim towards the surface, as the recordings showed. Using their four little legs, the animals row upwards with powerful and coordinated swimming movements until their heads are above the water again. “They are helped by their water-repellent exoskeleton, which forms an air bubble around their body from the chest plate and gives them buoyancy,” explains Watson. This air cushion in the exoskeleton shields the insects from the water and protects them from the forces caused by the impact of a raindrop. “The tank’s protection works for up to ten minutes, but only for a limited time,” emphasize Watson and his colleagues.
When repeatedly submerged within a short period of time or during longer dives, the water strider’s shell can become so saturated with water that the insects no longer have enough buoyancy to reach the surface. Only if the hairs on the exoskeleton have enough time to dry between two dives can they fully fulfill their task the next time the droplet hits.
Dynamics can be transferred to plastic particles
The results reveal how water striders can survive even pelting rain without drowning or even getting wet. In addition, the observations can be transferred to other particles floating passively in water, according to the researchers. The study therefore also provides insight into how microplastics move in the sea when it rains and under what circumstances they are thrown into the air.
Source: Daren Watson (Florida Polytechnic University) et al., Proceedings of the National Academy of Sciences, doi: 10.1073/pnas.2315667121