Dock spiders, members of the Dolomedes genus, are large, semi-aquatic spiders known as fishing spiders or raft spiders. These arachnids are notable for their size and adaptations to aquatic environments. They are widely distributed in areas with abundant freshwater sources like lakes, rivers, and ponds. Understanding their characteristics and behaviors helps clarify their role in local ecosystems.
Physical Appearance
Dock spiders are characterized by their considerable size. Female dock spiders can reach a body length of 15 to 26 millimeters, with a leg span exceeding 100 millimeters (4 inches), making them among the largest native spiders in some regions, such as Canada. Males are typically smaller, averaging 8 to 13 millimeters in body length. Their coloration ranges from brownish-grey to dark brown or black, often featuring black and light brown markings on their abdomens and striped legs. Some species display a pale yellow or cream stripe along each side of their body.
These spiders possess eight long, widely spaced legs, often covered with water-repelling hairs. This specialized leg structure allows them to move across water surfaces without sinking. Their eyes are arranged in two rows of four, distinguishing them from spiders like wolf spiders, which have three rows of eyes. Dock spiders do not construct traditional webs for prey; instead, they use silk for egg sacs and protective nursery webs for their young.
Habitat and Behaviors
Dock spiders primarily inhabit areas adjacent to freshwater sources, including lakes, ponds, streams, rivers, marshes, damp basements, or boathouses. They are commonly found on docks, shorelines with dense vegetation, under rocks, or in logs, which provide shelter and hunting grounds. Their activity is often nocturnal, and they navigate both on land and water surfaces.
Their hunting strategy does not involve spinning a web. Instead, they are ambush predators, utilizing the water’s surface as a sensing platform. A dock spider rests with its front legs extended onto the water, detecting vibrations from potential prey. Upon sensing movement, they swiftly dart across the water to capture their meal, moving with impressive speed and agility.
Their diet includes aquatic insects, tadpoles, small fish, and even small frogs, with some species capable of preying on creatures up to five times their own size. Dock spiders can also dive underwater, remaining submerged for several minutes by trapping air bubbles on their bodies for oxygen.
Bites and Venom
While dock spiders possess venom, it is generally not dangerous to humans. Their venom is primarily used to immobilize and digest prey like insects and small aquatic animals. Bites to humans are infrequent, typically occurring only when the spider feels threatened, is accidentally handled, sat upon, or trapped against the skin.
Symptoms from a dock spider bite are usually mild, often compared to a bee or wasp sting. Individuals may experience localized pain, redness, and some swelling at the bite site. These symptoms usually resolve on their own within a few days without requiring specific medical intervention. Severe allergic reactions are rare, but seeking medical attention is advisable if unusual or severe symptoms develop, such as difficulty breathing. For typical bites, cleaning the area thoroughly and applying a cold compress can help reduce swelling and discomfort.