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Water Walkers

By Robert B. Suter

Have you ever seen animals walking on top of the water? A few small animals do that very well, and I find that amazing.

Water striders spend much of their lives standing on the water surface, supported by surface tension. Their weight pushes their legs or feet down and stretches the water surface. This makes a dimple in the surface, just the way your finger makes a dimple in the surface of a balloon when you push on it.

Water striders catch insects that land on the water. Sometimes, though, these water-walking animals are themselves eaten by fish. I've been trying to learn more about how water striders avoid being caught by fish.

I know from my investigations that water striders can jump straight up from the water surface, sometimes as high as 3 or 4 centimeters. I suspect that this jumping is their way of protecting themselves against being eaten.

I have seen lots of jumping, but I've never observed one of these creatures jumping to avoid a predator that I could see. So I think that actually being attacked must be a very rare event, but it must be common enough so that these animals still jump for the slightest reason--just in case.

The Challenge: Help me figure out how often water striders jump "for no visible reason," and how often they jump in response to something you can see, like a fish attack or bumping against another water strider. Just fill out the data sheet below. Thanks for your help!

What You Do

1. Find a pond that has water striders near the edge, so that you can see them easily.

2. Make yourself comfortable: Remember you'll have to sit very still for 15 minutes so you don't frighten the water striders.

3. Watch the striders for 15 minutes. Don't watch more than four striders at a time, because you don't want to miss a jump. Every time you see a strider jump, put a check mark on the data sheet. If you could see why it jumped, write down the reason it jumped next to the check mark. If you could not see why it jumped, use only a check mark.

4. Add up the total number of check marks, and put the number in the first box.

5. Add up the number of check marks with "known causes," and put the total in the second box.

6. Add up the number of check marks with "no known causes," and put the total in the third box.

If you have time to do this study more than once, that would be great! Please visit this page separately for each 15 minutes of strider observations.

Water Strider Data Sheet

Fill out the data sheet below for as many jumps as you see. Remember only to fill out the "cause" if you actually saw something that caused the water strider to jump straight up into the air.

See if you can tell what caused the jump you see (but only report something as the cause of the jump if you actually saw the cause).

Total number of jumps observed in the 15 minute observation period.

Number of jumps observed in the 15 minute observation period with known causes.

Number of jumps with observed in the 15 minute observation period with no known causes.

If you saw any jumps with known causes, enter the causes you observed here. Then click on the submit button.






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Robert as a child.

Dr. Robert B. Suter is a professor of biology at Vassar College in Poughkeepsie, New York. Robert's favorite book as a kid was Charlotte's Web. Now he has pet fish, three cats, six chickens, and an iguana. His hobby is photography.

Some thoughts from Robert:

My mother says that by the time I was 2 years old, I was already turning over every log and rock. I spent lots of time at the edges of ponds and streams, watching and catching everything that moved. I continue to be fascinated by the way things work--not only the ways of insects and spiders but also how disk drives and electronic watches work. So designing and building the instruments I use in my research also excites me.

While conducting my research, one of the neatest things I came to realize was that very small animals live in a world we cannot experience--where things like gravity and momentum don't make much difference. But surface forces, like surface tension, do matter.

I've had countless disasters and goof-ups in my research! One time a large fishing spider, which is also a water-walker, climbed up my arm. This caused me to scream and dance! (I'm fearful of the big spiders.)

Robert's advice to young investigators?: Follow what intrigues you--as a hobby or your life's work.

Robert today.

Insect photos by Robert B. Suter. Additional photos courtesy of Suter.

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