Water striders exhibit a remarkable ability to move without breaking through the water. This ability comes from a combination of water’s physical properties and unique biological adaptations these insects possess.
The Power of Water’s Surface Tension
Water has a property called surface tension, which acts like a delicate skin at its surface. Water molecules are strongly attracted to each other, a phenomenon called cohesion. At the surface, these molecules are pulled inward, creating a tightly packed layer that can support small, lightweight objects.
Water striders capitalize on this natural phenomenon, effectively standing on this molecular “skin.” If this tension is broken, for example by adding detergents, the strider would immediately sink.
Specialized Water-Repellent Legs
Water strider legs are long, slender, and have features that prevent them from getting wet. Their legs are covered in thousands of microscopic, hair-like structures called microsetae. These tiny hairs, which can be around 50 micrometers long, are etched with even finer nanogrooves.
This intricate structure makes the legs highly hydrophobic, meaning they strongly repel water. The microsetae trap a layer of air around the legs, creating an air cushion that prevents direct contact between the water and the leg’s surface. This trapped air helps them remain atop the water without penetrating the surface tension. The contact angle between the water strider’s leg and water is approximately 168 degrees, indicating its superhydrophobic nature.
Leveraging Weight and Movement
The water strider’s light body, combined with its specialized legs, allows it to distribute its weight across the water’s surface. The long, widely spaced legs spread the insect’s mass over a large area, ensuring that the pressure exerted on the water’s surface does not exceed the capacity of the surface tension. This weight distribution creates small indentations, or dimples, in the water’s surface beneath each leg, without breaking through the surface.
To move, water striders use their middle pair of legs in a synchronized rowing motion, pushing against the back wall of these dimples. The hind legs act as rudders, providing steering and braking capabilities. This coordinated movement allows them to propel themselves across the water at considerable speeds, sometimes reaching over 100 body lengths per second, all while maintaining their position on the water’s delicate surface.