Science Scoops: Walk This Way!
by Stephen James O'Meara
One of the lingering mysteries of the insect world—how insects (such as water striders) seemingly skate across the surface of ponds, rivers, and oceans — has come to an end. The secret is out. John Bush, a mathematician at the Massachusetts Institute of Technology (MIT) in Cambridge, MA, and his colleagues know the answer.
Water striders come in hundreds of different species. All can walk on, or skim across, water. They range in size from half an inch long to 20 times bigger than that. For years, researchers believed that these watery lightweights move about by creating tiny waves. The thinking was that the insect could push backward against the face of a wave and move forward.
Lots of animals push. Humans push their feet against the ground. Birds push against the air. But the truth about the water strider isn't that simple.
Bush and his research team used sophisticated tracking and a high-speed video camera to uncover the truth. The real secret to the bug's motion, Bush says, is in its three sets of hairy legs. The water strider propels itself by using the hairs of its central pair of legs like oars. This action, like the oars of a rowboat, creates rearward swirling vortices that propel the insect forward at speeds of up to 5 feet per second. The vortices, by the way, are not spirals, but instead are an unusual “U” shape. Although tiny waves are created in the spirals, they are not the main driving force, Bush says.
To further prove their results, the Bush team created a mechanical water strider, called “Robostrider,” based on the real thing. The robot—a soda can with stainless steel wire legs—has an elastic band and pulley at its middle (driving) legs. The results were the same.
- A spiral motion of fluid within a limited area, especially a whirling mass of water or air that sucks everything near it toward its center.
- Draw three diagrams that show the progression of a water strider across the water. In the first diagram, show the water strider at rest. In the second diagram, show the water strider as it is in mid-stride. How are its legs positioned? What is happening to the water around the water strider? In the last diagram, show the water strider at rest again after it has moved across the surface of the water. What does the water that the strider has moved through look like?
[anno: Students should draw three diagrams. The first diagram should show a water strider sitting on the surface of a still area of water. The second diagram should show movement in the center pair of legs. Students should indicate that the water strider is using this set of legs to propel the body forward. The third diagram should show a series of U-shaped vortices in the area of the water that the strider has crossed.]
- Why do you think scientists are interested in learning how water striders move across the water? What kinds of things could be developed that mimic the motion of a water strider? Write a few sentences to explain why you think scientists are interested in water striders and what might be developed that mimics a water strider's movement.
[anno: Answers will vary. Students may suggest that scientists might be interested in developing a water vessel that mimics the motion of a water strider because the strider can move very quickly over the surface of the water.]