Water Striders: Survival Adaptations

Background Information for Teachers

Water striders are small insects that can walk on the surface of water without falling in or sinking (Figure 1). Their long, skinny legs spread out horizontally and are supported by the water's surface tension. This is in contrast to other insects with shorter (and sometimes thicker) legs that point downward, as shown in this slideshow that you can show your students.

Image Credit: Wikimedia, user Cory / Public Domain
Figure 1. Water striders on the surface of water (image credit: Wikimedia Commons user Cory).

As shown in the video here, this adaptation to walking on water gives water striders a survival advantage over other insects in their habitat. Other insects that fall into the water get stuck and are unable to escape. This allows water striders, which can easily move around on the water, to prey on them.

In this lesson, your students will build their own insects by twisting together pieces of wire. Depending on the size and shape of the insects' legs, they might float on top of the water like a water strider, or sink into the surface like a different insect (Figure 2).

Image Credit: Ben Finio, Science Buddies / Science Buddies
Figure 2. A wire "insect" with long, skinny legs that point outward can float on top of the water like a water strider. The insect with shorter legs that point downward sinks—making it easy prey for the water strider!

Additional Background Links

• This is Why Water Striders Make Terrible Lifeguards | Deep Look, Deep Look
• Surface Tension, Hyperphysics

Prep Work (5 minutes)

• Print copies of the student worksheet.
• Fill the plates/trays with water and place them at your students' desks.
• Cut the wire into sections roughly 2 feet long.

Engage (5 minutes)

Show your students this video:

https://www.youtube.com/watch?v=E2unnSK7WTE

You can also show your students this slideshow, or let them look up pictures of different types of insects online.

How do water striders look different from many of the other insects in the video or slideshow?

The water striders have very long, skinny legs relative to their bodies.

Why do you think water striders can walk on water, while the other insects cannot? How does this help them survive?

Let students discuss their ideas with their partners. Explain that they will build their own insects using pieces of wire, and find out what allows them to walk on water.

Explore (20 minutes)

1. Let students use the student worksheet to draw an idea for one insect that can walk on water, and one that cannot walk on water.
2. Show the students how to make an insect by cutting three pieces of wire and twisting them together in the middle, as shown in the following video. They can make different "types" of insects by changing the length and shape of the legs.
   

   https://www.youtube.com/watch?v=8h7UZ8pE_Q0
3. Allow students to experiment with building different insects and placing them on the surface of the water. Can they make one that floats like a water strider? What about one that sinks like the water strider's prey?
4. Optional: allow students to decorate their insects by adding googly eyes using double-sided foam tape. Be careful, though—adding weight means you might have to adjust the legs.

Reflect (5 minutes)

Discuss the results as a class.

What types of insects floated the best on water? What types sank? Why? Did you notice similarities between the insects that floated and those that sank? How does this relate to what we saw in the video about water striders?

Water striders have long, skinny, spread-out legs that allow them to walk along the surface of the water without sinking. Many other insects have shorter legs that point downward. These legs are better for walking on land, but bad for walking on water. This allows water striders to prey on other insects that get "stuck" in the water. If you built an insect that looked like a water strider using wire (with long, horizontally spread out legs) you could probably get it to float. Smaller wire insects with short, downward-pointing legs did not float.

Assess

Use the student worksheet for a before and after assessment of student understanding. Can the student explain why water striders float but other insects do not?

Make Career Connections

Discussing or reading about these careers can help students make important connections between the in-class lesson and STEM job opportunities in the real world.

Zoologist and Wildlife Biologist

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Career Profile

Zoologists and wildlife biologists study all kinds of wild animals. An entomologist is a type of scientist who specifically studies insects like water striders. Read more

Lesson Plan Variations

• How heavy can your students make their water striders? Try adding weight to the "body" a little bit at a time using something waterproof, like pieces of tape (not paper). Can they keep the body above the water? Is there a limit to how heavy they can make it before it sinks?
• Experiment with changing the shape and size of the legs. Do the water striders float better with even longer legs? What about gradually curved legs vs. sharply curved legs, or legs that curve sideways instead of upward?
• Try the activity with different gauges of wire. Challenge your students to make a water strider using much thicker wire. Eventually, the wire becomes too thick and heavy to be held up by surface tension. This is why very small animals, like water striders, can float on the surface of the water, but big animals (like people) cannot!