Amaurobius

Physical description and identification

Amaurobius is the type genus of Amaurobiidae, the lace-web spiders of Europe and the hacklemesh weavers of North America. Around seventy species are recognised. All are cribellate: instead of glue-coated threads they comb out dry, woolly silk, and that feature explains how they hunt.

One correction is worth making early. Amaurobius ferox, the best known species, is not brownish-tan. It is near black with cream markings; the pale tan colouring often attached to it belongs to smaller relatives such as A. erberi.

Adults

• Size: from about 4 mm in the smallest species to roughly 16 mm in a large female black lace weaver. Females are bulkier; males are narrower and longer-legged.
• Colour: a glossy carapace from orange-brown through chestnut to almost black, darkest at the head. The softly haired abdomen carries a paler pattern — nested V-shaped chevrons in A. similis and A. fenestralis, cream blotches likened to a mask in A. ferox.
• Legs: stout, hairy and faintly ringed — the quickest way to separate one from the smooth, unbanded legs of a recluse.
• Eyes: eight small eyes of similar size in two short rows of four, none enlarged, which rules out jumping spiders and wolf spiders at a glance. See spider eyes.
• Diagnostic structures: a cribellum, a plate of hundreds of tiny spigots in front of the spinnerets, and a calamistrum, a comb of bristles on the hind legs that cards silk out of it. Naming a species needs an adult and a microscope.

Eggs and egg sacs

Females lay deep inside the retreat and wrap the eggs in a flattened white sac fastened to the roof of the tube, never out on the open web. Clutches run from a few dozen to well over a hundred in the larger species, and the female stays with them.

Spiderlings

Hatchlings are pale and darken with each moult. They stay in the maternal tube for a few days to several weeks — longest in A. ferox, where the mother feeds her brood on unfertilised eggs and is finally eaten by them — then build miniature webs a metre or two away. Maturity takes one to two years of molting.

The web

The spider lives in a tube of silk pushed into a crack — a mortar joint, a gap under bark, the rim of an air brick — and lays a rough collar of hackled silk around its mouth. Fresh silk has a bluish-white sheen and a woolly texture; within days it greys with dust. The threads are not sticky: they snag the claws and bristles of any insect that walks across them, and the vibration brings the spider out at speed. Repairs are made after dark, so a web brushed off in the evening is often back by morning. See our guide to spider webs.

Where Amaurobius spiders live

The genus is Holarctic. Its centre of diversity is southern and central Europe, with many species restricted to single mountain ranges, Mediterranean islands or caves in Greece, Italy, Spain, Croatia and Turkey. Others reach Scandinavia and temperate Asia, and a separate set occurs in North America. Popular lists placing the genus in Micronesia or Argentina are simply wrong; outside the northern temperate zone it is an introduction.

Three species account for most sightings. A. similis and A. ferox are established in North America and New Zealand as well as their native Europe, and A. fenestralis is widespread in European woodland; in the United Kingdom and Ireland all three are among the commonest spiders on old walls.

Habitat is consistent: a hard surface with a crevice behind it, and shade. In the countryside that means bark, rock, scree, stone walls and log piles; around buildings, mortar joints, window frames, air bricks, sheds and cellars. A. ferox is the most willing to come indoors. These are not living-room spiders in the way house spiders are — they want cool, damp and undisturbed.

Are Amaurobius spiders venomous?

Yes, in the ordinary sense that nearly all spiders are: Amaurobius species use venom to kill woodlice, beetles, earwigs and ants. No species in the genus is medically significant to people or pets, no antivenom exists because none is needed, and no systemic poisoning has ever been credibly attributed to the genus.

The claim that lace-web spiders are "infamous for painful bites" overstates the case badly. Verified bites are scarce, and the outcome is a short-lived sting rather than a wound. The genus is unrelated to the recluse spiders, which carry tissue-damaging enzymes, and to the widow spiders, whose venom targets nerve endings.

Can Amaurobius spiders bite?

They can, but almost never do. These are timid, web-bound spiders whose first reaction to disturbance is to vanish backwards into the tube. The bites on record involve a spider pressed against skin — a hand pushed into a meter box, or a wandering male trapped inside clothing. The larger species have fangs long enough to break human skin; the smaller ones often cannot.

A bite feels like a sharp pinprick or a mild bee sting, with redness and a little swelling that fade within a day or two. Wash the site with soap and water, apply a cold pack and leave it alone — no cutting and no home remedies. Seek medical advice only if pain keeps increasing, the area becomes hot and spreading, or symptoms appear away from the bite, which points to infection or an allergic reaction rather than the venom.

Ecological importance and behavior of Amaurobius spiders

An Amaurobius is a sit-and-wait predator with an unusually strong attachment to one place. It spends daylight hours in the tube, moves to the entrance after dark and rests its front legs on the collar threads. A woodlouse, ant, beetle, earwig or small moth crossing the silk sets off a vibration; the spider rushes out, bites and hauls the catch back inside. Woodlice are taken far more often than most spiders manage, because the hackled threads grip the plates of their armour. See what spiders eat.

Natural predators: wrens, robins and other insectivorous birds that pick along walls, plus shrews, lizards, centipedes and spider-hunting wasps that paralyse adults to provision their nests. Cellar spiders and the giant house spider take wandering males, and ichneumonid wasps parasitise egg sacs.

Prey-predator dynamics: the genus works a layer few other spiders exploit, the interface between a hard surface and the air just above it. Because the silk is dry rather than sticky, it keeps catching in rain, dew and cold, on nights when an orb web would be sagging and useless.

Seasonal behaviour: most European species mature in late summer and autumn, when males abandon their webs to search — making autumn the peak of both web building and indoor sightings. Adults overwinter in their retreats and are among the first spiders active again on warm late-winter days. Females of the larger species live two years or more in the same crevice.

Relationship with humans: harmless and quietly useful. Where the webs look untidy, brushing them away and pointing the crack behind them beats any spray, because it is the crevice, not the silk, that the spider needs.

Species in the genus and similar spiders

Quick facts

Did you know?

• The genus was erected in 1837 by Carl Ludwig Koch, one of the most prolific spider describers of the nineteenth century. The name comes from the Greek amauros, dark or dim — a fair description of both the spiders and the places they live.
• Cribellate silk is not one thread but thousands. The calamistrum combs out a mat of nanofibres only tens of nanometres across, and the fuzz holds prey by surface forces and entanglement — a dry adhesive that works when wet.
• Amaurobius ferox is one of the clearest cases of matriphagy known in spiders. The mother lays a batch of unfertilised eggs purely as food, then encourages her spiderlings to consume her, which measurably improves their survival.
• Fresh Amaurobius silk really is blue. The colour is structural, produced by light scattering from the nanofibre mat, and it fades as the web collects dust.

In summary, Amaurobius is a genus of quiet, dark, crevice-loving spiders whose woolly blue-white silk is far more familiar to most people than the animal that made it. They are harmless to humans, tenacious predators of the arthropods that share our walls, and — in the black lace weaver — the authors of one of the most extraordinary acts of parental investment known.