How pond skaters dodge raindrop drownings

Researchers in the United States have made a big splash in the science field this week by using ultrafast photography to watch what happens when pond skaters - also called water striders in America - are hit by falling raindrops, which massively outweigh the tiny insects. For them, it’s like one of us standing under Niagara Falls, says the study’s author at Florida Polytechnic University, Daren Watson…

Daren - We did this by first capturing the insects from our local ponds and we had to create a rainfall simulator. So we had a reservoir of water that we pumped through a nozzle that mimicked raindrops. Those raindrops struck the insect, and we observed the interaction using our high speed video cameras in the lab.

Chris - So you gave them a shower basically? <laugh>. How fast is fast? When you say high speed imaging, how many pictures a second are you taking of this?

Daren - We can capture up to 4,000 frames per second, so it's very fast. We see the droplets move in on the order of milliseconds.

Chris - Talk us through then, when you look at this footage, what does it show?

Daren - Before we talk about what occurs with the insects, when a raindrop hits a pool of water, what you're going to get is a splash. And we are all familiar with splash. We see this during rainfall, but that splash constitutes a couple different phases. So we see an underwater crater, we see a jet that goes back up above the water surface. And it was important for us to look at how the insect interacts with these different components of the splash. So when the insect is struck by a raindrop, we see that the raindrop pushes the insect into the water body and the insect. You'll find that along the inner surface of that particular raindrop as it creates a crater inside the body of the water. So we find that the insect is, you can say, attached to the water at that point in time. So then when the jet is formed, it is transported out of the water with the jet.

Chris - So the water gets pushed downwards and outwards and compressed by the incoming droplets. And what is a rebound of the water coming back underneath the insect that creates almost like a geyser underneath it that pushes it up in the air?

Daren - Yes. The rebound occurs as the dented surface of the water tries to go back to its original state. So raindrop pushes the insect beneath the water, and then there's a rebound and you have the jet coming upwards.

Chris - Does the action stop there or do you then get secondary effects? Because obviously what goes up must come down. If you've made a jet, do you then get secondary rain effectively off the back of having hit the insect the first time?

Daren - Yes. And that is where the danger lies for the insect because that jet then disintegrates to create what you would've termed secondary rain. And it then pushes the insect inside the body of the water again. And we find that the rebound is going to be so precipitous that the insect is going to be left beneath the waterline.

Chris - Does it have to swim up? And then how does it break through, assuming it does, the surface of the water? Because there's surface tension there between the air and the water isn't there. So how does the insect get back through there and end up on the air side rather than on the water side?

Daren - This is rather innate. So the insect, because the insects are generally born beneath the waterline and their youngs, what they do is they swim to the top to where that waterline is and they break the surface to get onto the airside. So the adults are also able to do that. And they do that through a series of what we call power strokes, applied at an acute angle. And that allows the insect to be able to break that waterline to get back onto the air side of things.

Chris - They obviously do it quite well because there's loads of them. And if I look at the pond near where I live, there's many, many to count. So they're obviously pretty good at this, especially with the terrible weather we have. But what are the applications of this? Because it's interesting and fascinating to understand how these insects have evolved to have this behaviour, but understanding this now as you do, can you apply it to any other aspects of what we see in the marine or aquatic realm?

Daren - Yes, we can. Our results here will allow us to better understand the transport of floating particles like microplastics on the Earth's water bodies. Now microplastics are similar in size to water striders or in your case pond skaters. And they would likely share a similar experience during rainfall. As a matter of fact, in some of our experiments, we replaced water striders with floating particles and observed these similar interactions. So that's the main real world application of the study at the moment. And we're going to be seeking to explore transport of microplastics on our world water bodies going forward.