The ability of certain spiders to traverse the water’s surface is a fascinating adaptation. These creatures navigate ponds and streams with ease. This feat is not magic, but a sophisticated interplay of biological design and the physical properties of water.
Spiders That Master Water Walking
The spiders known for their water-walking capabilities belong to the genus Dolomedes, commonly referred to as fishing spiders or raft spiders. They are a group of spiders sharing this unique trait. Fishing spiders, such as Dolomedes triton, are found across various wetland habitats, including areas around lakes, ponds, and slow-moving streams. They are robust spiders, typically grey to brown, often displaying light spots or stripes on their abdomen. Females can range in size from about 17 to over 30 millimeters in body length, with legs extending their overall span up to 80 millimeters.
Raft spiders, including species like Dolomedes fimbriatus and Dolomedes plantarius, are large, chunky spiders often seen near ponds, ditches, and bogs in Europe. They typically have a chocolate-brown body with a distinct pale yellow or cream stripe along each side. These spiders inhabit semi-aquatic environments.
The Science Behind Their Stride
The ability of these spiders to walk on water is due to surface tension, specialized leg structures, and effective weight distribution. Water molecules’ strong cohesive forces create surface tension, acting like an elastic “skin” on the water’s surface. This “skin” is strong enough to support light objects. When a spider places its leg on the water, it creates a small depression or dimple in this “skin.”
The spider’s legs are covered in thousands of microscopic, water-repellent hairs called setae. These setae are hydrophobic, meaning they repel water, preventing the legs from breaking through the surface tension. This coating ensures the spider’s weight is distributed across the water’s surface without penetrating it.
The spider’s light body weight and eight widely spread legs distribute its mass over a larger area. This reduces pressure on the water’s surface, allowing surface tension to support the spider. For movement, the spider propels itself by pushing its legs against the water’s surface tension. Forward motion comes from the drag created as the leg and its dimple move backward through the water, acting like tiny oars.
Life on the Water’s Surface
The water-walking ability of fishing and raft spiders is important for their semi-aquatic lifestyle and hunting strategies. These spiders reside in wetlands, near ponds, lakes, and slow-moving streams. They are ambush predators and do not construct webs for hunting prey. Instead, they position their front legs on the water’s surface, using sensitive legs to detect vibrations. These vibrations signal potential prey, such as insects, small fish, or tadpoles, moving on or in the water.
Upon sensing prey, the spider rapidly runs across the water to capture it. They can also dive underwater to escape predators or pursue aquatic prey, remaining submerged for up to 45 minutes or more. While submerged, a silvery film of air surrounds their bodies, trapped by specialized hydrophobic hairs, allowing them to breathe. This adaptation allows them to thrive in watery environments, transitioning between surface and underwater life.
Interaction with Humans
Despite their large size and rapid movements across water, fishing spiders and raft spiders are not considered a threat to humans. They are shy and prefer to avoid contact with people, fleeing quickly if disturbed. Bites are rare and occur as a defensive response if the spider feels threatened, such as when accidentally trapped or handled.
When a bite occurs, effects are mild, comparable to a bee or wasp sting. The venom is not medically significant to humans, meaning it does not cause serious health issues beyond temporary discomfort, localized pain, or minor irritation. Serious allergic reactions are uncommon. These spiders play a beneficial role in their ecosystems by helping to control insect populations.