water striderWhat an adventure we had today!  We started out in a sunny field full of wildflowers and grasshoppers and ended up soaked to the skin but oh-so-joyful!  Our Nature Study class met beside a meandering stream at a friend’s farm to learn about an amazing aquatic insect:  The Water Strider.

OBJECT LESSON:

Also known as “Jesus Bugs,” water striders  literally walk on water!  They use something called the “surface tension” of the water to their advantage.  Water molecules are attracted to each other and like to stay together, especially on the surface where there is only air above.  Since there are no water molecules above the surface for them to hold onto, the molecules at the surface cling extra tightly to the molecules beside them and under them — so tight, in fact, that a “skin” seems to form on the surface.  Water striders walk on this thin membrane.  Here’s a cool slow-motion video of them in action:  https://www.youtube.com/watch?v=RphuMEUY3Og

But water striders have another secret for walking on water — their legs!  The legs have tiny hairs that repel water and trap air.  By repelling water, the tiny insects stand on the water’s surface and the trapped air allows them to float and move easily — almost as if they were skating!

And finally, water striders are very, very lightweight.  So much so that they are able to float.  Lily pads and twigs float because the water is pushing up on them, but a rock dropped into the water sinks rapidly because it’s so heavy for its size that it overcomes the “push” of the water.

Using a large bowl filled with water, I asked the class to predict from their size, weight and shape whether these items would sink or float:  Paper clip, straight pin, bottle cap, coin, cotton ball, rubber band.  Only the bottle cap and cotton ball floated (and the cotton ball sank when it finally became saturated).

Using the surface of the water like a trampoline, water striders can jump into the air to avoid predators or catch prey!  Jumping requires a large amount of force on the starting surface.  Easy enough on terra firma, right?  But jumping on water is much more difficult because too much force will break the surface tension.  Recently mechanical engineers from Seoul National University have developed a robotic insect that can jump on water, too.  Researchers say this technology could someday be used in surveillance missions.  Watch! https://www.youtube.com/watch?v=Z83l347rh6E

Water striders can live for many months, and adults can overwinter by crawling inside a plant stem when it gets too cold.

FIELD WORK:

Like all insects, water striders have six legs, and each set has a different use.  I asked the students to observe water striders in the stream and try to determine to what use the insects puts each set of legs.  (They use their front legs to catch and hold prey, their middle legs to row, and their hind legs to steer.)

If there is time, observe what water striders eat and how they eat it.  (Water striders are carnivorous and use their piercing mouth parts to inject a chemical that liquifies the prey’s internal structure so they may then suck them dry.  They will eat whatever falls into the water (other insects, worms, honeybees) and use their front legs to find food by sensing ripples made by struggling prey, then grab and hold it while they drink its life juices.  Here’s a not-gross video of water striders catching and enjoying a meal:  https://www.youtube.com/watch?v=SVoG0Uy_uQ8

Try to determine what preys on water striders.  (fish, frogs, salamanders)

WRAP-UP:

A violent thunderstorm with dangerous lightning caused our class to be cut short, but some of the students recorded their observations at home.

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A few of us were able to wait out the storm and take a walk into the woods along the creek bank.  In addition to a perfectly-preserved, bleached-out cricket exoskeleton, a mystery nut (which turned out to be an immature beechnut)  and some nearly translucent, gelatinous, very slimy fungus (maybe snow fungus?), we found  the most amazing deep aquamarine bracket fungus growing on a fallen log.  What a treasure!  It looked just like this (only much more of it):

Bright blue bracket fungus.