Fishing Spiders: The Very Large Ones Near Water

A dark fishing spider on a wall has a leg span that can reach 75–90 mm, which makes it among the largest spiders most people in eastern North America will ever see. It is also completely harmless, and it is in the nursery web family rather than being a tarantula or anything related to one.

Recognising one

• Very large. Body 15–26 mm in females, leg span up to about 90 mm.
• Mottled brown and grey, cryptic against bark and wet wood.
• Distinct dark W-shaped or chevron markings across the abdomen.
• Banded legs, with dark rings.
• Eight eyes in two rows of four, all similar in size — no enormous pair.

That last point is what separates it from a wolf spider, which has two very large forward-facing eyes and reflects torchlight brilliantly. Fishing spiders do not light up the same way.

What they do

Dolomedes species hunt at the water's edge, and they are genuinely semi-aquatic. They:

• Walk on the water surface, supported by surface tension and water-repellent leg hairs.
• Detect prey by vibration transmitted across the surface film, resting their front legs on the water like a fisherman's line.
• Dive and stay under, sometimes for many minutes, carrying a film of air trapped in body hairs. They can climb down a plant stem to submerge.
• Take insects mostly, and occasionally tadpoles and very small fish — which is where the name comes from, though the fish part is uncommon.

Watching one hunt from a dock is worth the time if the opportunity comes up.

Why one is in your basement

The dark fishing spider (Dolomedes tenebrosus) is the least aquatic of the group and is regularly found far from water — on tree trunks, in woodpiles, in garages and basements, and inside houses.

It does not establish indoors and does not build a web. It wandered in, and it will wander out. Like most large spiders found in the open in late summer, an adult male searching for a mate is the usual explanation — the seasonal pattern covered in why there are suddenly spiders everywhere in autumn.

Its size makes it one of the most alarming finds in the eastern United States and one of the most frequently misreported as something dangerous.

Not a wolf spider, not a tarantula

Both are in the running-hunter category and both get reported as recluses, which they are not — a recluse is small, smooth, uniformly tan and has six eyes. The full separation is in wolf spider or nursery web spider and, for the recluse question, brown recluse or wolf spider, where size alone settles it.

The nursery

Females carry the egg sac beneath the body, held in the jaws and palps, which forces them to walk with the front of the body raised. Close to hatching, she builds a tent of silk in vegetation, puts the sac inside, and stands guard on the outside until the spiderlings disperse.

A large spider standing motionless beside a ball of silk in a shrub is doing exactly that, and it is worth leaving alone rather than worth worrying about.

Bites

Very rare. They retreat, and a large one on a dock will run or dive rather than approach.

A bite would produce local pain and swelling comparable to a bee sting, resolving in a day or two. No tissue damage and no systemic effect.

The size is the whole of the alarm, and size is not correlated with medical significance in spiders — the two medically significant groups in North America are both small, and the largest spiders on the continent are all harmless.

Moving one

A large glass and a piece of stiff card, unhurriedly. They are fast, but they are not trying to reach you, and one of this size is easier to catch cleanly with a wide container than a small one.

Getting a spider out of the house without killing it covers the method, and a fishing spider is a genuine outdoor animal — this one can go straight into the garden rather than the garage.