Woodlouse (Dysdera crocata)

Physical description and identification

Few spiders in a temperate garden are as easy to name on sight. Nothing else of this size combines a deep blood-red front half, a smooth pale abdomen with no markings at all, and jaws that look several sizes too large for the animal carrying them.

Adults

• Size: females measure about 11–15 mm in body length (0.43–0.6 in) and males about 9–10 mm (0.35–0.4 in), legs excluded. Legspan reaches roughly 25–30 mm.
• Colour: the carapace, jaws and legs are dark orange-red to deep mahogany and faintly polished; the abdomen is plain glossy cream, buff or pinkish grey with no stripes, chevrons or folium. That abrupt two-tone split is the most reliable field mark.
• Eyes: six, not eight, grouped into a small oval at the front of the head. Dysderidae have lost the anterior median pair, a condition they share with the recluse spiders — which is exactly why the two are confused. Our guide to spider eyes explains why eye arrangement is the quickest way to place an unfamiliar spider.
• Distinctive features: the chelicerae point forward instead of hanging down and end in long curved fangs, which in a mature female approach a third of the carapace length — among the largest fangs relative to body size of any spider in the region. The body is cylindrical, smooth and slightly flattened, a build that suits life in narrow crevices.

Eggs and egg sac

Mating peaks in spring and early summer. The female then seals herself into a flattened silk chamber beneath a stone, log or loose bark and lays roughly 30 to 70 pearly eggs, bound in a thin untidy sheet of silk rather than a tough papery purse, so individual eggs stay visible. She guards the chamber, feeding little, until the young emerge.

Spiderlings

Hatchlings are pale and almost colourless, gaining the adult red over their first few molts. Two claims repeated in older accounts are wrong and worth correcting: the young do not stay with their mother until maturity, and there is no evidence that she feeds them. They leave after the second or third molt and hunt alone. Maturity takes about one to one and a half years, and because Dysderidae cannot balloon on silk, dispersal is on foot — which is why a garden population creeps along a wall rather than appearing everywhere at once.

Hunting instead of a web

The woodlouse spider builds no capture web. What it spins is a retreat: a closed, flattened cell of tough white silk about the size of a small coin, tucked under a stone, into mortar or beneath bark, where it rests through daylight, molts, mates and broods its eggs before leaving at dusk to hunt on foot. See our guide to spider webs for how this compares with silk traps.

The attack itself is precision work. A woodlouse is armoured on top and soft underneath, so the spider drives one fang over the crustacean's back and the other beneath its body, pinning it like a pair of callipers and injecting venom into unprotected tissue. Pill woodlice are gripped before they can roll up. The oversized jaws that make the species look menacing are, in short, a specialised can opener.

Where the woodlouse spider lives

Dysdera crocata originated around the Mediterranean Basin and spread north and west through Europe to Spain, Italy, France and eventually the United Kingdom and Ireland, where it is common in gardens as far north as central Scotland. That native range is worth stating, because popular summaries of the species usually list only the countries it was introduced to and leave its home out altogether.

From the nineteenth century onward it travelled in ballast, potted plants, timber and garden soil, and it is now one of the most widely introduced spiders on Earth, with established populations across North America — including California — and in New Zealand, Australia, South Africa, parts of South America and Japan. A single fertilised female in a shipment of nursery stock is enough to found a colony.

The habitat requirement is the same everywhere: cool, humid ground cover where isopods are abundant — leaf litter, compost heaps, mulch beds, the underside of logs, stones, bricks, paving slabs and old boards, and the cavities of dry-stone walls. Indoors it stays at floor level in cellars, garages and crawl spaces rather than climbing.

Are woodlouse spiders venomous?

Yes, in the same way that nearly every spider is venomous: Dysdera crocata has venom glands and uses them to subdue prey. Whether that venom is medically significant is the question that matters, and the answer is no. There is no antivenom for this species because none has ever been needed, and no confirmed case of serious systemic illness has been attributed to it.

One correction is due. For decades the woodlouse spider appeared on lists of spiders supposedly causing necrotic skin lesions, largely because it is red-brown, six-eyed and lives around houses — the superficial profile of a brown recluse. Dysderidae venom contains no sphingomyelinase D, the enzyme that makes Sicariidae venom dangerous, and verified-bite studies have never reproduced a necrotic wound from it. What it really has is mechanical power: fangs long enough to pierce human skin easily, unusual in a spider this small.

Can woodlouse spiders bite?

It can, and among small garden spiders it manages it more readily than most, because those long fangs penetrate skin without difficulty. Bites are still uncommon. The spider's first response to being uncovered is to run for the nearest dark gap, and almost every recorded bite involves a spider pressed against skin — picked up in a handful of mulch, trapped inside a glove, or caught under a bare knee.

The usual outcome is a sharp prick, like a deep pinprick, followed by localised redness, mild swelling and itching that fade within a few hours. Wash the site with soap and water, apply a cold pack and take a simple painkiller if needed. Keep the puncture clean and seek medical advice if redness spreads, pus forms or a fever develops — signs of infection, not of venom. Do not apply heat, cuts, suction or folk remedies. Keep the spider or a photograph if you can, because identification settles the question quickly.

Ecological importance and behavior of the woodlouse spider

This is a nocturnal, solitary, ground-level hunter with an unusually narrow job description. Woodlice are abundant, slow, and protected by a mineralised cuticle that most predators cannot breach, which leaves a rich food supply barely touched. Dysdera crocata evolved the jaws to exploit it, and in doing so occupies a niche almost nothing else in the leaf litter competes for.

Diet: terrestrial isopods dominate — Oniscus, Porcellio and Armadillidium in particular — but the species is not a strict specialist. Feeding trials and gut-content work on Dysdera show that it readily takes silverfish, earwigs, small millipedes, beetle larvae and other spiders when isopods are scarce. For context on spider feeding generally, see what spiders eat.

Natural predators: centipedes, ground beetles, shrews, toads and ground-foraging birds all take woodlouse spiders, larger nocturnal hunters such as ground spiders kill them, and parasitoid wasps attack individuals sealed into their retreats. Wandering males are the most exposed, which is why they are the ones that end up in a bathtub.

Seasonal behaviour: adults occur through most of the year in mild climates, peaking in spring and early summer when males search for mates. In colder regions they move into compost, wall cavities and cellars for the winter and stay quietly active rather than dormant. Lifespan is commonly two years, sometimes three, the first year or more spent as a juvenile.

Relationship with humans: almost entirely positive. Woodlice can damage seedlings and soft fruit in damp greenhouses and beds, and a resident population of woodlouse spiders suppresses them with no chemicals. The spider eats no plants and builds no webs across doorways. A glass and a card returns an indoor wanderer outside; sealing gaps at floor level prevents repeats better than any spray.

Similar spiders and how to tell them apart

Quick facts

Did you know?

• The species was described in 1838 by the German entomologist Carl Ludwig Koch. Crocata means saffron-coloured, after the orange-red front of the body.
• The genus Dysdera has radiated spectacularly on the Canary Islands, where dozens of endemic species differ mainly in jaw shape according to how they tackle woodlice.

In summary, the woodlouse spider is a harmless, useful and unmistakable animal: red in front, pale behind, six-eyed and armed with jaws built to open crustaceans. It spins no trap, it will not chase you, and the worst it can do is a sharp pinch that fades before the day is out.