Black Lace Weaver Spider (Amaurobius ferox)

Physical description and identification

The black lace weaver is the largest and darkest member of the genus Amaurobius, a cribellate spider in the family Amaurobiidae. It is heavily built for its length, with a low glossy carapace, thick legs and a broad, finely haired abdomen.

Two details commonly given for this species need fixing. It is not brownish-tan — that belongs to smaller relatives such as A. erberi. And the abdominal mark is not a simple pale heart: the pattern is a set of cream blotches on a near-black ground, broad at the front and breaking into paired patches behind, which many observers read as a skull or a mask. The clean nested V pattern belongs to A. similis and A. fenestralis.

Adults

• Size: females measure about 11–16 mm in body length, males 8–10 mm. A large female with legs spread covers roughly the area of a 50p coin.
• Colour: the carapace is dark chestnut to blackish and distinctly shiny; the abdomen is charcoal to black with the cream blotching described above. Old females darken further and the pattern can almost disappear.
• Legs: stout, dark and hairy, with faint pale rings at the joints. Males are longer-legged and much narrower, with swollen pedipalp tips.
• Eyes: eight small eyes of roughly equal size in two short rows of four, none enlarged, which rules out wolf spiders; see spider eyes.
• Diagnostic structures: a cribellum, the flat silk plate in front of the spinnerets, and a calamistrum, the comb of bristles on the hind legs that cards silk into a woolly ribbon.

Eggs and egg sacs

The female builds a flattened white silk sac inside her retreat, fastened to its roof or side wall, and stays beside it. Clutches are large, typically 60 to 180 eggs, and she may produce more than one sac in a season. The sac sits deep in the crevice, never on the open collar.

Spiderlings

This is where A. ferox becomes remarkable. The mother opens the sac to help the spiderlings out, then lays a batch of unfertilised trophic eggs whose only purpose is to feed them, and shares captured prey. A few days later she draws them onto her body by drumming and vibrating, and they kill and consume her — matriphagy. Broods fed this way grow faster, moult sooner and survive better. The spiderlings then hunt as a group in the natal web before dispersing nearby. Maturity takes one to two years; see spider molting and the spider life cycle.

The web

The black lace weaver does not build an orb. It occupies a tube of silk pushed into a crack — a mortar joint, a gap behind skirting, the underside of a loose stone — and lays a ragged collar of hackled cribellate threads around the entrance. Fresh silk has a bluish-white sheen and greys within days as it collects dust. The threads carry no glue at all: they snag claws and bristles, which is why this species takes armoured prey such as woodlice far more successfully than sticky-web spiders do. See our guide to spider webs.

Where black lace weaver spiders live

Amaurobius ferox is native across Europe and the western Mediterranean, from Portugal and Spain north to Britain, Ireland and southern Scandinavia, and east through Italy into western Asia. It travels well in cargo and building materials and is now established in eastern and Pacific North America — Massachusetts, New York, Oregon and Washington among them — plus parts of Canada and New Zealand.

It is the most strongly synanthropic member of the genus. Outdoors it lives under stones, bark, logs and in dry stone walls; around buildings it prefers cool, damp, undisturbed places — cellars, basements, crawl spaces, garages and shaded masonry. Unlike house spiders it does not colonise warm living rooms. A large dark spider in a British or Irish cellar with a collar of woolly white silk around a crack is almost certainly this species.

Are black lace weaver spiders venomous?

Yes, in the sense that nearly every spider is: Amaurobius ferox uses venom to kill woodlice, beetles, earwigs and other crawling prey. It is not medically significant to people or pets. No antivenom exists or is required, and there is no credible record of systemic poisoning or tissue damage from this species anywhere in its range.

It is unrelated to the groups that matter medically. The recluse spiders carry tissue-damaging enzymes; the widow spiders carry neurotoxins. The black lace weaver has neither. Its size and dark colour make it look more serious than it is, which is the main reason it gets misidentified and killed.

Can black lace weaver spiders bite?

It can, and unlike smaller amaurobiids it is large enough that its fangs will break human skin. Bites are still rare: the spider retreats into its tube at the first vibration, and almost every recorded bite involves the animal being pressed against skin — picked up bare-handed, trapped under a hand lifting a stone, or caught inside clothing by a wandering male.

The usual result is a sharp sting like a nettle, with two small punctures, local redness and slight swelling that fades over a day or two. Wash the site with soap and water, apply a cold pack and take an ordinary painkiller if needed. Do not cut, suck or apply any home remedy. Seek medical advice only if pain steadily worsens, the redness spreads and the area becomes hot, or symptoms appear away from the bite — those point to infection or an allergic reaction, not to the venom.

Ecological importance and behavior of the black lace weaver

The black lace weaver is a nocturnal ambush predator that stays with one retreat for its whole adult life. After dark it moves to the entrance and rests its front legs on the collar threads, and any vibration brings it out in a fast, short rush. Its prey is mostly ground arthropods: woodlice, ants, beetles, earwigs, millipedes, harvestmen and flies. Few spiders can handle the armoured plates of a woodlouse, but the hackled silk grips them and this species takes them routinely. See what spiders eat.

Natural predators: insectivorous birds such as wrens and robins that forage along walls, plus shrews, toads, centipedes and spider-hunting wasps that paralyse adults to stock their nests. Cellar spiders catch wandering males in basement corners and the giant house spider overpowers them in sheds; parasitoid wasps attack the egg sacs.

Prey-predator dynamics: because the silk is dry rather than glue-coated, it keeps working in damp, cold and rain, so the species hunts effectively in cellars and on wet stonework where sticky webs fail.

Seasonal behaviour: adults mature from mid-summer, and males abandon their webs from late summer into autumn to search for females — the source of most indoor sightings. Females guard their webs through winter and are among the first spiders active on mild late-winter nights. A female can live two years or more; a male rarely survives long after mating, and is frequently eaten by her.

Relationship with humans: entirely beneficial and no risk worth managing. Where a collar of silk is unwelcome, brushing it off and pointing the crack behind it works far better than any spray. Gloves when lifting stones remove the only realistic bite scenario.

Similar spiders and how to tell them apart

Quick facts

Did you know?

• The species name ferox means fierce — a reference to how readily a cornered female stands her ground, not to any danger she poses to people.
• Matriphagy here is not a weakened mother being scavenged. She actively signals to the brood by vibrating her body, drawing them onto her, and dies as a direct result.
• The spiderlings hunt as a temporary group after her death, tackling prey together that one could not subdue — a rare glimpse of cooperation in a species that is solitary as an adult.
• Fresh cribellate silk really is blue. The colour is structural, produced by light scattering from a mat of nanometre-scale fibres, and it fades to grey as the web collects dust.
• It is one of the very few spiders that eats woodlice regularly, making it an unusually effective predator in the damp corners where woodlice concentrate.

In summary, the black lace weaver is a big, dark, harmless spider with a woolly blue-white web and an extraordinary private life. It removes a steady stream of woodlice, ants and beetles from cellars and stonework, and it is the author of one of the most complete acts of maternal sacrifice known.