Research

Surviving Rainfall

What do water striders do when falling rain disrupts the water surface?

Water striders spend their lives at the air-water boundary, skittering along this interfacial world. But what happens when falling rain destroys their flat existence? That’s the question that motivated today’s research study, which looks water striders subjected to artificial rain.

Although the water drops themselves are far heavier than the insects, the water doesn’t strike hard enough to injure the insects. Neither a direct impact nor the forces from a neighboring impact, the researchers found, were enough to pose a problem for the water strider’s exoskeleton. Instead, they’re more likely to get flung or submerged, as follows:

The initial impact of a raindrop creates a large crater. Depending on the position of the insect relative to the point of impact, this may fling the insect away or pull it down into the cavity.
The initial impact of a raindrop creates a large crater. Depending on the position of the insect relative to the point of impact, this may fling the insect away or pull it down into the cavity.

When the drop hits, it creates a big crater in the water’s surface. Insects to the outside of the splash get flung outward, while those closer to the point of impact ride the crater wall downward. As the crater collapses, it forms a thick jet that pushes nearby water striders up with it.

As the initial cavity collapses, it creates a large jet that can push the strider into the air.
As the initial cavity collapses, it creates a large jet that can push the strider into the air.

As that initial jet collapses, it forms a second crater, which — being smaller and narrower — collapses much faster than the first one. That action, researchers found, often submerges a water strider caught in the crater.

The first jet's collapse creates a second crater, and it's this one that tends to trap and submerge the water striders underwater.
The first jet’s collapse creates a second crater, and it’s this one that tends to trap and submerge the water strider underwater.

Fortunately for the insect, their water-repellent nature means they’re covered in a thin bubble of air that lets them survive several minutes underwater. That’s time enough for the water strider to rescue itself. (Image credit: top – H. Wang, animations – D. Watson et al.; research credit: D. Watson et al.; via APS Physics)

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