The phenomenon of insects moving effortlessly across the surface of a pond is a sophisticated interaction between biology and physics. The most recognized of these creatures are the water striders (family Gerridae), often called pond skaters for their gliding motion. These slender, lightweight organisms inhabit the water-air boundary, where they hunt and live without breaking through the liquid’s surface. Their ability to remain suspended and propel themselves is a remarkable example of how their physical design harnesses a fundamental property of water.
Water’s Invisible Skin
The underlying physical principle that allows some bugs to walk on water is a phenomenon created by the attractive forces between water molecules. Water is a polar molecule, meaning it has a slight positive charge on the hydrogen atoms and a slight negative charge on the oxygen atom. This polarity enables the molecules to form strong connections known as hydrogen bonds, which cause them to stick together, a property called cohesion.
This strong cohesive force is particularly pronounced at the surface where the water meets the air. Here, water molecules are pulled inward and sideways by their neighbors, resulting in a net inward force that minimizes the surface area. This tension causes the water’s boundary to behave like a stretched, elastic membrane, sometimes described as an invisible skin. The resulting surface tension is strong enough to resist rupture from light objects that apply their weight over a sufficient area.
Any object with a density greater than water will sink if it breaks this cohesive layer. However, if an object is light enough, and its weight is distributed effectively, the surface tension force can be greater than the downward pull of gravity. The water surface will deform under the weight, creating a small dimple, or meniscus, but the tension around the perimeter of that dimple exerts an upward force that supports the object.
How Insects Minimize Their Footprint
Water striders and other surface-dwelling arthropods have evolved specific biological structures that maximize the effect of surface tension. The legs of a water strider are notably long and splayed out, a geometry that distributes the insect’s minimal body weight over a large area of the water’s surface. This long leg structure ensures the downward force exerted at any single point is too small to overcome the upward force from the water’s stretched membrane.
The legs are covered in a dense layer of microscopic hairs known as setae or microsetae. These needle-shaped hairs are scored with nanogrooves, creating an intricate, hierarchical structure responsible for the leg’s extreme water-repellency, a property scientists refer to as superhydrophobicity.
The physical arrangement of the setae and nanogrooves traps a cushion of air against the cuticle of the leg. This trapped air layer prevents the water from physically wetting the leg material. By not allowing the leg to touch the water directly, the insect rests on a composite surface of air and water, allowing the surface tension to support its full weight. Research has shown that this non-wetting structure is so effective that a single leg can support a load equivalent to about 15 times the insect’s body weight.
The Mechanics of Walking and Propulsion
Once supported by the water’s surface tension, the water strider uses a specialized rowing motion to move and accelerate. The propulsion is generated primarily by the middle pair of legs, which are longer and more powerful than the other two pairs. These legs execute a synchronized, sculling stroke that pushes against the curved water surface without penetrating the cohesive layer.
As the middle legs stroke backward, they create deep, hemispherical dimples, or menisci, in the water surface. The insect generates thrust by pushing horizontally against the rear wall of the dimple, where the water surface is curved upward. This action transfers momentum to the underlying fluid by generating a pair of asymmetrical vortices in the water.
These vortices are shed backward beneath the surface, effectively propelling the insect forward. The front pair of legs are used for grasping prey and steering, while the long hind legs act as rudders, helping to stabilize and guide the strider’s movement. By alternating the strokes and distributing its weight, the water strider is able to skate across the water at speeds up to 100 body lengths per second.