The water strider, an insect belonging to the family Gerridae, is a common sight on the surface of ponds and slow-moving streams. These semi-aquatic bugs, often called pond skaters or water skippers, appear to defy gravity as they glide effortlessly across the water’s surface. This remarkable feat is achieved through a perfect synergy of physics and specialized biological design. The water strider leverages the unique properties of water to create its habitat.
Surface Tension
The foundation of the water strider’s ability rests on surface tension, a property that makes liquid surfaces behave like a stretched elastic membrane. Water molecules at the surface are pulled inward, minimizing the surface area and creating a thin, resilient film capable of supporting lightweight objects.
Water has one of the highest surface tensions of any common liquid, allowing it to support objects denser than water, provided they are small enough. For the water strider, this surface acts as a physical barrier. When the insect rests on the surface, it creates a slight depression, or meniscus, without breaking the tension. This distortion generates the upward supporting force that counteracts the insect’s weight.
Biological Adaptations of the Water Strider
The water strider’s body structure is adapted to maximize interaction with the surface film. Its extremely low body weight is distributed across six long, slender legs, which function like snowshoes, spreading pressure over a broad area. The middle pair of legs is the longest and optimized for generating thrust, while the hind legs assist with steering and braking.
The legs are water-repellent due to thousands of microscopic, needle-shaped hairs called micro-setae. These hairs are coated in a waxy, superhydrophobic substance, which traps a layer of air around the leg, preventing the water from wetting the surface and increasing the strider’s buoyancy.
When the legs touch the water, they create a visible dimple without penetrating the surface. This maximizes the contact area and the resulting surface tension force. The strider can support up to 15 times its own body weight because its support is dependent on this curvature force. If the water’s surface tension is broken by contaminants like detergent, the hydrophobic barrier fails, and the strider sinks.
Mechanics of Propulsion
The process of movement involves the strider executing a rowing-like motion with its middle pair of legs. These legs push backward against the surface tension film, specifically pushing against the side of the dimples they create, rather than paddling through the water.
This pushing motion transfers momentum to the water, primarily generating a pair of hemispherical vortices beneath the surface. Modern studies show that these subsurface vortices are the main mechanism for forward thrust. The vortices act as temporary oars, allowing the strider to glide across the surface at speeds up to 100 body lengths per second. The rear pair of legs is used to control direction and slow the insect down, while the shorter front pair is reserved for grasping prey.