Some insects possess the remarkable ability to traverse the surface of water. This allows them to hunt, escape predators, and navigate their aquatic environments. The capacity to walk on water is a sophisticated interplay between the physical properties of water and specialized biological adaptations developed over millions of years.
The Science of Surface Tension
The primary principle enabling water walking is surface tension, where water’s surface resists external forces. Water molecules exhibit strong cohesive forces due to hydrogen bonding, attracting one another. At the water’s surface, molecules bond more strongly with neighbors below and to the sides, creating a tightened, elastic-like film or “skin.” This film supports lightweight objects that do not break through it.
This cohesive force minimizes the water’s surface area. If an object’s weight is sufficiently distributed and its surface does not disrupt these strong intermolecular attractions, it can remain atop the water. The downward force exerted by a light object must be less than the combined upward and sideways forces from water molecules at the surface. While humans are too heavy to be supported by this tension, small insects can deform the surface, creating dimples, and use the resulting upward force for support.
Bug Adaptations for Water Walking
Bugs that walk on water have evolved adaptations to leverage surface tension. Their legs are non-wetting or hydrophobic. Water repellency is achieved through waxy coatings and microscopic hair structures, known as setae. These specialized hairs often contain fine grooves that trap air, preventing water from clinging to the leg and increasing its buoyancy.
Their lightweight bodies and unique leg structure are also important. Long, slender legs allow the insect’s weight to be distributed over a larger surface area. This wide distribution reduces pressure on the water’s surface, preventing the insect from breaking through the surface film. The flexibility of their legs and arrangement of their joints help them adjust to the water’s meniscus, preventing their tarsi from piercing the surface.
Masters of the Water Surface
Several insect species exemplify water surface locomotion, each with distinct adaptations. Water striders, also known as pond skaters, are well-recognized water walkers, utilizing their long, splayed, hydrophobic legs to glide across ponds and streams. Their front legs are shorter and used for capturing prey, while their middle legs act as paddles for propulsion, and the longest rear legs provide steering and braking capabilities. Microsetae on their legs can support up to 15 times their body weight, allowing them to move at speeds equivalent to a human traveling at 400 miles per hour.
Fishing spiders, though arachnids, exhibit impressive water-walking abilities, often found near aquatic habitats. Their legs are covered in small, spiky hairs that repel water, allowing them to stand and “row” across the surface. These spiders can dive beneath the water, trapping an air bubble near their abdomen with waterproof hairs to breathe underwater for extended periods, aiding in hunting or predator evasion.
Whirligig beetles are known for their rapid, circular swimming patterns on the water’s surface. They possess flattened bodies and hydrophobic exoskeletons that enable them to skim across the surface. Their middle and hind legs are adapted into paddle-like structures for propulsion, while their front legs are elongated for grasping prey. These beetles also have uniquely divided eyes, allowing them to see simultaneously both above and below the waterline, providing situational awareness.