Some spiders can walk on water, an ability stemming from a combination of physical principles and biological adaptations. This allows them to navigate and hunt on aquatic surfaces.
The Physics of Water Walking
The primary physical phenomenon enabling water walking is surface tension. Water molecules exhibit strong cohesive forces, creating a thin, elastic “skin” on the surface. This “skin” forms because molecules at the surface are pulled inward and sideways, but not upward, by other water molecules, allowing it to resist external forces.
Water has a relatively high surface tension compared to many other liquids, largely due to the extensive network of hydrogen bonds between its molecules. For an object to walk on water, its weight must be distributed effectively across this “skin” without breaking through it. The buoyant force, which is the upward force exerted by the water, also plays a role in supporting the spider’s weight by displacing water and creating small depressions or dimples beneath its legs.
Spider Adaptations for Water Walking
Spiders capable of water walking possess specific biological features that exploit surface tension. Their legs are lightweight and designed to distribute their mass over a larger area. This ensures the pressure exerted on the water surface is insufficient to break the surface tension.
Their legs are covered in thousands of microscopic, water-repellent hairs called setae. These setae are superhydrophobic, strongly repelling water and preventing the legs from getting wet. This property, sometimes enhanced by a waxy coating, helps the spider maintain contact with the surface film. Tiny grooves in these hairs also trap air, increasing buoyancy and providing an air cushion.
Spiders That Master Water Walking
Several spider species have evolved to walk on water. Fishing spiders, belonging to the genus Dolomedes, are among the most well-known. These semi-aquatic spiders are commonly found near ponds, streams, and other wetlands.
Examples include the six-spotted fishing spider (Dolomedes triton) and the fen raft spider (Dolomedes plantarius). While many spiders can briefly skim across water, Dolomedes species have adaptations that make water walking a fundamental part of their lifestyle. Not all spiders can walk on water; it is an adaptation found in certain families, including Pisauridae (fishing spiders) and some Lycosidae (wolf spiders).
Life on the Water Surface
Spiders that walk on water engage in a variety of behaviors once on the surface. They move by “rowing” with their legs, pushing against the water’s surface to propel themselves forward. Some species, like fishing spiders, can even use wind to “sail” across the water by raising their front legs.
Their ability to sense vibrations on the water’s surface is crucial for hunting. Fishing spiders often position themselves with their front legs resting on the water, detecting ripples caused by struggling insects, tadpoles, or even small fish. Upon detecting prey, they can rapidly move across the surface to capture it. These spiders can also dive beneath the surface to escape predators or pursue aquatic prey, trapping a layer of air around their bodies with their hydrophobic hairs to breathe underwater for extended periods.