Small bugs can “walk” on water because of a property called surface tension, combined with their own special adaptations.
It’s a team effort: the water creates a “skin,” and the bugs are built to stand on it without breaking it.
1. The Water’s “Skin” (Surface Tension) 💧
The main reason this is possible is surface tension. You can think of it as a stretchy, elastic film on the surface of the water.
This “skin” exists because water molecules ($H_2O$) are strongly attracted to each other. This attraction is called cohesion.
- Inside the water: A water molecule is pulled equally in every direction by all the other molecules surrounding it.
- At the surface: A molecule is only pulled by the molecules beside and below it. There are no water molecules above it to pull it up.
This creates a strong, net inward pull on the surface molecules, packing them tightly together. This tight packing is what forms the “skin” that can support very light objects.
2. The Bug’s “Trick” (Special Adaptations) 🐞
Surface tension alone isn’t enough; the bugs have to be specially adapted to use it. Water striders are a perfect example.
- They are extremely lightweight: Their body mass is so low that the force they exert (their weight) isn’t strong enough to break the cohesive forces of the water’s surface.
- They spread out their weight: Their long legs distribute their tiny weight over a wide surface area, so no single point pushes down very hard. You can see this by the little “dimples” or “dents” their feet make on the water’s surface.
- They have “non-stick” legs: This is the most important part. Their legs are hydrophobic, which means they repel water. They are covered in thousands of microscopic, waxy hairs (called microsetae). These tiny hairs trap a layer of air around the bug’s legs, creating an “air cushion” (called a plastron). This air cushion prevents the water from ever actually “wetting” the leg, allowing it to rest perfectly on top of the water’s “skin” instead of breaking through.