False Black Widow (Steatoda grossa)

Physical description and identification

Seen in a torch beam, Steatoda grossa reads as a small, hard, shiny bead with legs: a high globular abdomen, a low polished carapace, and a body so dark that its markings vanish unless light hits them directly. That is exactly why it is reported as a black widow every autumn.

Adults

• Size: mature females measure roughly 6.5–10.5 mm in body length and males about 4–6 mm. Older sources quoting 15 mm are describing leg span, not body length; a female's body is closer to the size of a shirt button.
• Colour: deep purplish brown to almost black with a varnished sheen. Most females carry a pale cream crescent across the front edge of the abdomen, sometimes with faint chevrons behind it. Males and immature females are often paler and more strongly patterned than mature females, which can look plain black.
• Never a red hourglass: the single most useful rule. Steatoda grossa has no red or orange hourglass beneath the abdomen; its pale markings sit on the upper surface and are cream or off-white, never crimson.
• Eyes: eight small eyes of similar size in two rows of four, set close together at the front of the carapace.
• Comb feet: like all Theridiidae, it has a comb of curved, serrated bristles on the fourth leg, used to flick liquid silk over struggling prey from a safe distance — the trait behind the name dark comb-footed spider.
• Males: slimmer, longer-legged and with visibly swollen pedipalp tips. They wander in search of females, so a specimen crossing a bathroom floor at night is usually male.

Eggs and egg sacs

A female produces several egg sacs in a season — commonly three or more — each an off-white, loosely woven sphere about 8–10 mm across, suspended in the densest part of the tangle. Roughly 40 to 100 eggs per sac is typical, so one well-fed female can put out several hundred young in a year. She stays with the sacs during incubation, which takes a few weeks and runs faster in a warm house than in an unheated shed.

Spiderlings

Newly emerged spiderlings are pale, almost translucent and finely speckled; they are not miniature black adults, and they darken gradually with each moult. They linger briefly in the mother's web, then disperse to build their own nearby — there is no long period of active feeding by the mother. Maturity normally takes a year or more, through the moults described in our guide to the spider life cycle.

The web

The web is a tangle of threads built into a corner, under a shelf or behind a cistern, always with a silk retreat where the spider sits by day. Running down from it are gumfoot lines: taut vertical threads glued to the floor and loaded with adhesive droplets at the bottom. A woodlouse that blunders into one sticks, snaps the thread free and is hoisted into the air, where the spider combs more silk over it before biting — the same trap-line design true widows use. See our overview of spider webs.

Where the false black widow lives

Steatoda grossa is native to Europe and the Mediterranean basin and has been carried by trade to almost every temperate and subtropical region on Earth. It is now established across the United States — especially the Pacific coast — and in Canada, Australia, New Zealand, South Africa and parts of South America. In the United Kingdom it is one of several Steatoda species lumped together in the press as "false widows", and it has been resident there for well over a century.

Its habitat preference is simple: dry, dark, undisturbed and sheltered. Outdoors it uses wall cavities, window ledges, log piles, rock crevices, sheds and rubbish heaps. Indoors it takes garages, cellars, cupboards, wardrobes, the space behind a toilet and the corners of rarely opened rooms. Because it tolerates central heating and low humidity, it is one of the few spiders that can complete its whole life cycle inside a building, alongside the common house spider.

Are false black widow spiders venomous?

Yes — like almost all spiders it uses venom to subdue prey. The important point is how its venom compares with that of a true widow spider. Latrodectus venom contains alpha-latrotoxin, a neurotoxin that triggers massive neurotransmitter release and can cause the severe, whole-body syndrome called latrodectism. Steatoda venom produces a much milder, mostly local reaction that clinicians sometimes call steatodism. Serious outcomes are rare, and there is no record of a death attributed to Steatoda grossa.

One clinical detail links the two genera: in Australia, redback (Latrodectus hasselti) antivenom has been reported to relieve symptoms in a confirmed Steatoda grossa envenomation, evidence that the venoms share related components even though the severity is not comparable. That decision belongs to a treating doctor, not to a first-aid kit.

Can false black widow spiders bite?

They can, but only in defence — typically when a spider is pressed against skin inside clothing, bedding, a glove or a boot, or when a hand is pushed into a web in a dark cupboard. The spider's first reaction to a person is to drop on a dragline and hide.

A bite feels like a wasp sting at the moment it happens. Reported symptoms include sharp local pain, a red mark with swelling, occasional blistering, and sometimes malaise, sweating or headache. The older claim that everything settles within twelve hours is optimistic: pain and tenderness commonly last one to three days. Infection of a scratched bite site is a more frequent complication than the venom itself.

Ecological importance and behaviour of the false black widow

This is a sedentary, nocturnal, solitary spider that spends daylight in a silk retreat and comes out to the web after dark. Its eyesight is poor and it reads the world through vibration: the rhythm of a plucked thread tells it whether it holds prey, a rival or a courting male, and males drum a coded signal on a female's web before approaching.

Diet: the gumfoot design catches ground-level crawling prey far more often than flying insects. Woodlice, beetles, ants, earwigs, silverfish and other spiders make up most of its diet; in a damp cellar, woodlice alone can support a dense population.

A predator of widows: Steatoda grossa is a documented spider-eater and readily kills other web-builders, including true widows — in Australia it takes redbacks, and in North America it will kill western black widows sharing the same garage.

Natural enemies: parasitoid wasps, other spiders, centipedes, house geckos in warm regions and ants that raid egg sacs. Birds of prey and large lizards, sometimes listed as predators, have essentially nothing to do with a spider living behind a cupboard.

Season and lifespan: females are long-lived, surviving several years, overwintering as adults and breeding across more than one season. Males mature in a few months to a year and usually die within weeks of mating. Sightings peak in late summer and autumn, when wandering males are on the move.

Living with them: vacuuming webs, reducing clutter, sealing gaps around pipes and thresholds and drying out damp harbourage for woodlice removes both the spider and its food supply. Chemical treatment is almost never justified for a harmless species that eats household pests.

Similar spiders and how to tell them apart

Quick facts

Did you know?

• The resemblance to a true widow is often described online as protective mimicry. There is no evidence for that: Steatoda and Latrodectus are close relatives in the same family, and a dark, glossy, globular body is simply the shared build of a cobweb spider that hangs in the open.
• The species was described in 1838 by Carl Ludwig Koch, making it one of the earliest named members of the genus Steatoda.
• Live Steatoda grossa are widely reported to have stood in for the fictional "super-spider" filmed in the 2002 Spider-Man movie, precisely because they look dangerous and are not.
• A female can store sperm from one mating and produce fertile egg sacs for months afterwards — which is how a single spider carried indoors in a box founds a whole cellar population.

In summary, the false black widow is a dark, glossy, long-lived cobweb spider with a painful but rarely serious bite. Check the underside for a red hourglass, look for the pale crescent on the upper abdomen, and you can tell in seconds whether the spider in the cupboard needs respect or simply a glass and a piece of card.