Dolomedes vittatus

Physical description and identification

Dolomedes vittatus is a large, long-legged hunting spider with a low, flattened body that lets it squeeze into bark crevices and press itself against wet stone. Among the eastern American fishing spiders it is the dark one: where its relatives are sandy or olive, this species runs from deep chocolate brown to very nearly black, relieved only by thin pale lines along the flanks and a scatter of small white dots on the back.

Adults

• Size: females measure about 15–25 mm in body length, males roughly 9–14 mm. Legs extended, a large female spans about 75 mm (3 in). The figure of 7.6 cm often quoted for this species is that leg span, not the length of the body — a common source of confusion when people try to match a photograph to a description.
• Color: medium to very dark brown, sometimes appearing black when wet. A narrow cream or white trim line edges each side of the carapace and continues along the abdomen in many individuals, and the abdomen carries small white spots, usually in paired rows. The front of the carapace generally shows a pair of darker, wedge-shaped marks converging toward the eye group.
• Sexes: the pale marginal stripes are usually sharpest and most complete on males, which also have noticeably longer legs relative to the body and swollen, club-shaped pedipalps. Females are bulkier, especially when carrying eggs, and are the spiders people normally photograph.
• Legs and hairs: the legs are faintly ringed and, like the body, are covered with short hydrophobic hairs. Those hairs are why the spider can stand on the water film without sinking and why a diving individual looks as if it has been dipped in mercury.
• Eyes: eight eyes in two rows of four, the rear row curved backwards, none of them enlarged. That pattern separates Dolomedes from the wolf spiders, whose two huge forward eyes reflect torchlight, and from the jumping spiders.

Eggs

The female produces one large, round egg sac of tough white silk holding several hundred eggs. She carries it under her body, gripped in her chelicerae and steadied by her palps, and does not feed while she does so. This fang-carrying habit is the diagnostic behaviour of the family Pisauridae; wolf spiders carry theirs attached to the spinnerets instead.

Spiderlings

Shortly before hatching the female anchors the sac in low vegetation and spins a loose silk tent — the nursery web — around it, then stands guard on the outside. The spiderlings emerge as pale miniatures of the adults, stay in the tent through their first molt, and disperse a few days later. Juveniles overwinter part-grown and reach maturity the following late spring or summer, a cycle that gives the species a working lifespan of roughly one to two years.

The web

This spider builds no capture web at all. Older descriptions that credit it with an "irregular" web are mistaken — the untidy silk sometimes seen near one is the remains of a nursery tent, not a trap. Hunting is done on foot: the spider anchors its hind legs on a root, log or stone and lays its front legs on the surface film, which transmits the vibrations of struggling prey exactly the way a web transmits them to an orb weaver. Our guide to spider webs explains why some families abandoned silk traps for this kind of ambush hunting.

Where Dolomedes vittatus lives

Dolomedes vittatus is an eastern North American species. Records run from southern New England and the Great Lakes states south through the Appalachians and the Ozarks to Florida and the Gulf Coast, and west into Arkansas, Oklahoma and eastern Texas, with scattered reports from just across the Canadian border. It is genuinely less common than the dark fishing spider or the six-spotted fishing spider, and in most states it is considered a local rather than an abundant spider.

Habitat is the key to finding it: shaded, wooded banks of creeks, small rivers and spring runs, particularly where there are undercut banks, mossy boulders, fallen logs and deep leaf litter right at the waterline. It is also found around cave mouths, sinkholes and rock shelters in karst country, where cool air and permanent moisture mimic a streamside. Unlike the dark fishing spider it seldom strays far from water and almost never turns up inside buildings.

Are Dolomedes vittatus spiders venomous?

Yes, in the sense that every hunting spider is venomous: Dolomedes vittatus injects venom to subdue the insects and other small animals it catches. That venom is not medically significant to humans or pets. It contains none of the tissue-damaging enzymes found in the recluse spiders and none of the potent neurotoxins of the widow spiders, and no confirmed case of serious envenomation by any North American fishing spider has been published in the medical literature.

Can Dolomedes vittatus spiders bite?

It can, though almost nobody is ever bitten. Faced with a person the spider flattens, then runs, then dives and hides underwater; biting is a last resort that generally requires it to be grabbed, pinned inside clothing or squeezed while being moved. Because a mature female's fangs are long, the bite is mechanically painful in the moment — a sharp sting, a small red mark, a little swelling, occasionally some itching — and it typically settles within a few hours to two days.

Treatment is simple: wash with soap and water, apply a cold pack and keep the area clean. Get medical advice if the pain worsens rather than eases, if redness spreads outward over the following day, which suggests infection rather than venom, or if there are signs of an allergic reaction such as widespread hives or breathing difficulty. Photograph the spider rather than crushing it — accurate identification saves a lot of unnecessary worry.

Ecological importance and behavior of Dolomedes vittatus

This is a nocturnal ambush predator. By day it wedges itself under bark, in a rock crack or beneath an overhanging bank; after dark it takes a position at the water's edge with the front legs on the surface film and waits. Prey is mostly aquatic and shoreline insects — mayflies, stoneflies, caddisflies, midges, water striders and beetles — along with crickets, moths, millipedes and other arthropods that wander down to drink. Large females can and occasionally do take tadpoles, small crustaceans and even minnows, but reports that this species lives on fish and prawns exaggerate a rare event; insects are the bulk of the diet.

Natural predators: herons, kingfishers and other waterside birds, frogs, larger fish, snakes, shrews, centipedes and spider-hunting wasps of the family Pompilidae, which paralyse adults and seal them in a burrow with a single egg. Spiderlings are heavily predated by ants, dragonfly larvae and other spiders.

Prey–predator dynamics: occupying the boundary between stream and forest floor, Dolomedes vittatus transfers energy across that boundary. It intercepts the insects that emerge from the water and then feeds the birds and wasps that patrol the bank, linking aquatic and terrestrial food webs in the same few metres of shoreline.

Seasonal behavior: part-grown juveniles overwinter under bark and leaf litter, mature in late spring, and females carry egg sacs and build nursery webs through summer. A well-fed female may produce two sacs in a season. Most adults die before the following spring.

Relationship with humans: harmless and useful. It does not colonise houses, does no damage, and its presence indicates a shaded, undisturbed stream bank — the kind of habitat that is disappearing as creeks are channelised and cleared. Leaving logs, leaf litter and bankside vegetation in place is the single best way to keep it around.

Similar spiders and how to tell them apart

Quick facts

Did you know?

• The species was described in 1837 by Baron Charles Athanase Walckenaer, the French naturalist whose enormous survey of the world's spiders named hundreds of species he never saw alive.
• A female carrying an egg sac in her fangs cannot eat for one to two weeks. She fasts for the whole period rather than put the sac down, and often loses noticeable weight before the nursery web goes up.
• The silvery sheen on a submerged fishing spider is a layer of air held by its water-repellent hairs. It acts as a physical gill, drawing dissolved oxygen out of the water, which is why the spider can stay under for far longer than a lungful of air would allow.
• Fishing spiders read the water the way an orb weaver reads its threads. The surface film carries vibration, so a spider with its front legs on the water is effectively sitting in a web it did not have to build.
• All of North America's Dolomedes were only sorted out properly in a single taxonomic revision published in 1973, which remains the reference arachnologists still use to separate this species from its relatives.

In summary, Dolomedes vittatus is a big, dark, harmless hunter of shaded stream banks in the eastern United States. It spins no trap, carries its eggs in its fangs, dives to escape danger, and is far more likely to vanish under a log than to bother anyone who finds it.