Spiders exhibit a remarkable ability to move across water surfaces. This phenomenon allows certain species to interact with their environment in ways that seem to defy typical arachnid locomotion. They harness principles of physics and specialized adaptations to glide effortlessly over ponds, streams, and other bodies of water. This capability enables them to thrive in semi-aquatic habitats, accessing food and evading threats.
The Physics of Water Walking
Spiders achieve water walking primarily through surface tension, a property where water molecules at the surface form a cohesive, skin-like layer. This creates a resilient film that supports light objects. Spiders distribute their weight across their eight legs, spreading the force over a larger area to prevent breaking this surface film. Each leg creates a slight dimple in the water, but the surface remains unbroken.
The spider’s legs and body are covered in microscopic setae, which are water-repelling hairs. These hydrophobic structures prevent water from wetting the spider, keeping it dry and buoyant. As the spider steps, these specialized hairs trap tiny air pockets, further aiding buoyancy. This combination of distributed weight and water-repellent surfaces allows the spider to “stand” on water.
Notable Water-Walking Spiders
Among the most recognized water-walking spiders are fishing spiders, genus Dolomedes. These large spiders, some with leg spans reaching up to three inches, are found near ponds, slow-moving streams, and swampy areas, often resting on floating vegetation. Many Dolomedes species have a pale stripe running down each side of their body.
A large fishing spider is the raft spider, Dolomedes plantarius. These spiders can grow to a considerable size, some reaching nearly the size of a human hand. They are often observed on the surface of water bodies in marshy areas and fens.
While not primarily aquatic, many species of wolf spiders (family Lycosidae) can run or float on water. Their bodies are covered in hydrophobic hairs, allowing them to move across the surface. Some wolf spiders can even submerge themselves briefly, using trapped air bubbles for respiration.
Life on the Water’s Surface
The ability to walk on water is important for the survival and foraging strategies of these spiders. Many species hunt prey on or just beneath the water’s surface. They detect subtle vibrations caused by insects, small fish, or tadpoles, then rapidly move across the water to capture their meal. Some fishing spiders can even dive underwater, carrying a silvery film of air around their bodies, to pursue and subdue submerged prey.
Water-walking also helps them evade predators. When threatened, these spiders can quickly dart across the surface to escape danger. Some species dive and remain submerged for significant periods, with Dolomedes triton staying underwater for up to 90 minutes. A tropical spider, Trechalea extensa, has been observed hiding underwater for as long as 30 minutes by maintaining a film of air over its body.
This locomotion also facilitates movement across semi-aquatic territories. By using their rear legs to anchor themselves to shoreline vegetation while their front legs rest on the water, they can effectively monitor their surroundings. This adaptation allows them to traverse diverse aquatic habitats, seamlessly moving between land and water.