How Are Water Striders Able to Walk on Water?

Water striders, also known as pond skaters, are insects that glide effortlessly across the surface of water. This unique locomotion has long fascinated observers. Understanding this phenomenon involves exploring specific properties of water and the specialized biological adaptations of the water strider itself.

Water’s Unique “Skin”

Water possesses surface tension, allowing it to behave as if it has a flexible, elastic “skin” on its uppermost layer. This phenomenon arises from the cohesive forces between water molecules. Water molecules are strongly attracted to one another due to hydrogen bonds, which form between the slightly positive hydrogen atoms and the slightly negative oxygen atoms of adjacent molecules.

Molecules within the main body of the water are surrounded by other water molecules, experiencing balanced attractive forces. At the surface, water molecules are exposed to air on one side and fewer water molecules above them. This imbalance causes surface molecules to form stronger bonds with their immediate neighbors below and to the sides, resulting in a net inward pull and resistance to external forces. This creates a taut, film-like layer at the water-air interface, strong enough to support light objects or small insects.

Water Strider Leg Design

The water strider’s ability to utilize surface tension is due to the specialized design of its legs. These insects are lightweight, allowing their mass to be efficiently distributed across the water’s surface. Their legs are long and slender, spreading their minimal weight over a larger area, which prevents them from breaking through the water’s surface film.

Thousands of microscopic, hair-like structures called microsetae, which are small (less than 3 micrometers in diameter) and etched with nanogrooves, cover their legs. This intricate structure makes the legs highly water-repellent, or hydrophobic, by trapping tiny air bubbles within the grooves. This trapped air layer creates an air cushion between the leg and the water, preventing the leg from getting wet and increasing buoyancy. The interaction between the hydrophobic legs and the water’s surface tension causes the water to bend inward, forming small dimples or menisci under each leg, which further aids in support.

Movement Across the Surface

Water striders propel themselves across the water’s surface. They primarily use their middle pair of legs for propulsion. By pushing these legs against the water’s surface tension, they create ripples or vortices, generating the thrust needed to move forward.

Their front legs, which are shorter, are adapted for capturing prey that falls onto the water’s surface. The hind legs are longer and serve multiple purposes, providing additional thrust, steering, and stability. The water strider’s movement requires applying enough force to propel themselves without breaking the surface tension that supports them. This allows them to glide swiftly, covering distances up to 100 body lengths per second, equivalent to a human swimming at high speeds.

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