Dark Fishing Spider, Swamp Spider
Dolomedes tenebrosus
Pisauridae (Nursery Web Spiders), Order Araneae. Characterized by three claws and a specific eye arrangement where the posterior row is slightly curved.
Physical Description
A large, robust spider with a flattened body profile optimized for resting against vertical surfaces. The cephalothorax is broad, and the abdomen is elongated and tapered. The entire body is covered in fine, velvety hairs (setae) and appears somewhat bristly.
Size Estimate
Large; body length typically 15-26mm (females) or 7-13mm (males). Together with the legs, large females can reach a span of 75-100mm (3-4 inches). This specimen appears to be an adult female.
Coloration & Markings
Features a mottled pattern of grey, brown, and black providing excellent camouflage. The legs are distinctly banded with dark and light rings. The abdomen often displays several light-colored, chevron-like 'W' markings or spots.
Web Type
This is a hunting spider that does not build a web for prey capture. Females build a 'nursery web' only after the eggs are close to hatching to protect the spiderlings.
Eye Arrangement
Eight eyes in two rows. The posterior row is slightly recurved (curving backward). This arrangement provides excellent vision for spotting movement from a distance.
Leg Characteristics
Long, thick legs equipped with noticeable spines. The legs are held in a wide, sprawling posture. They are adapted for running across water surfaces (surface tension) and quick sprints on bark.
Venom & Safety Information
Venom Information
Possesses venom used to immobilize prey. The venom is not considered dangerous to humans; it is primarily neurotoxic to insects and minor vertebrates.
Medical Significance
Low medical significance. The bite is comparable to a bee sting in terms of pain and localized swelling. No systemic effects are expected. Consult a doctor only if signs of allergic reaction or secondary infection appear.
Safety Recommendations
If found indoors, relocate the spider using a large cup and a piece of cardboard. They are beneficial and harmless to the home. Avoid touching them directly to prevent a defensive nip.
Habitat & Behavior
Habitat & Range
Common throughout North America, particularly the eastern United States and southeastern Canada. While many fishing spiders stay near water, this species is frequently found far from it, in wooded areas, on tree trunks, and occasionally inside buildings or basements.
Behavior & Temperament
An ambush predator that relies on speed and camouflage. It is nocturnal, active mostly at night. While they are large and look intimidating, they are generally shy and will run away if disturbed, only biting if handled roughly.
Diet & Prey
Generalist predators eating a wide variety of insects, including crickets, moths, and flies. Larger individuals may occasionally capture small vertebrates like minnows or tadpoles if near water.
Seasonal Activity
Most active from late spring through late summer. They are frequently spotted in fall as they seek sheltered spots to overwinter.
Description
The Dark Fishing Spider is one of the most impressively large spiders found in North American woodlands. Despite its name, this species is a true woodsman, mastering the art of blending into tree bark to ambush prey. Its intimidating appearance belies a timid nature, as it would much rather flee into a crevice than stand its ground against a human.
Key Features
Large size, distinctly banded legs, 'W' shaped markings on the abdomen, and a sprawling posture against flat surfaces.
Similar Species
Often confused with Wolf Spiders (Lycosidae), but can be distinguished by the eye arrangement and the lack of a prominent single 'union jack' pattern on the carapace. Also similar to other Dolomedes species like D. triton, which usually has white spots.
Life Cycle
Females carry their large, spherical egg sacs in their chelicerae until the spiderlings are ready to emerge. At that point, she builds a nursery web in foliage. Lifespan is approximately two years; they often overwinter as sub-adults.
Ecological Importance
Acts as a major natural pest control agent in forests and around homes, consuming large quantities of insects. They are also a food source for birds and small mammals.
Conservation Status
Not evaluated by IUCN, but considered common and widespread with no immediate threats to the population.
Interesting Facts
They can walk on water by distributing their weight across surface tension. They use their feet to sense vibrations in the air and water, allowing them to locate prey without seeing it.
Photography Tips
Focus on the eyes and the banding on the legs. Using a flash can help pop the mottled colors out against dark wood backgrounds, but avoid overexposure as the grey hairs can reflect light strongly.
Identified: 2026-07-10T22:15:12.987294