Triangulate Cobweb (Steatoda triangulosa)

Physical description and identification

The triangulate cobweb spider is small, round-bodied and, unusually for a house spider, genuinely pretty when you look closely. The abdomen is pale cream to yellowish and carries two rows of dark purple-brown blotches shaped like blunt triangles; where the two rows meet along the midline they produce a zigzag that runs from front to back. That pattern is the fastest way to name the species and it is present even in half-grown juveniles.

Adults

• Size: adult females measure about 3–6 mm (one-eighth to one-quarter of an inch) in body length, and males are a little smaller with proportionally longer, thinner legs. Even a large female is dwarfed by a house spider or a cellar spider hanging nearby.
• Colour: the carapace and head region are glossy orange-brown to purplish brown, the legs are pale yellow to light orange and often faintly darkened at the joints, and the globular abdomen is cream, buff or faintly pink with the purple-brown triangular markings described above. Well-fed individuals look paler because the stretched abdomen spreads the pattern out.
• Eyes: eight eyes in two rows of four, small and set close together. Like other Steatoda species it has weak vision and hunts entirely by feeling vibrations in its silk; the arrangement of the eyes is compared across families in our guide to spider eyes.
• Distinctive features: the hind legs carry a comb of curved serrated bristles, the tool every cobweb spider uses to throw sheets of gluey silk over prey it does not want to grapple with. The body is soft and the spider hangs upside down; it moves slowly and jerkily when disturbed rather than sprinting like a house spider.

Eggs and egg sacs

The egg sac is a small, off-white sphere of loosely spun silk through which the eggs can sometimes be seen, hung in the densest part of the web near the retreat. Each sac contains roughly thirty round eggs, and a female typically produces about half a dozen sacs across her adult life, spaced over weeks. She stays with them, repairing the web and driving off intruders, but does not carry them.

Spiderlings

Spiderlings emerge after a few weeks and remain in and around the mother's web for a short time, sharing space with surprisingly little aggression before they disperse to build small webs of their own nearby. Because indoor temperatures stay stable, all stages can be found in a basement in any month of the year. Juveniles already show the triangular pattern and darken as they pass through successive moults, a process described in our page on spider molting.

The web

The web is a small, scruffy three-dimensional tangle with no visible design, spun into a corner, behind a pipe, under a stair tread, along a window frame or in the fold of a stored cardboard box. It has the same gum-footed construction as a widow's web on a miniature scale: a dry retreat above and a few taut vertical lines running down to the floor or shelf, each carrying glue only at the very bottom. When an ant walks into one, the line releases and lifts the insect off its feet, and the spider comes down to throw silk over it from above. This is also why the species can safely tackle prey that would otherwise be dangerous to a soft-bodied spider. Our overview of spider webs explains how tangle webs differ from orb and funnel webs.

Where the triangulate cobweb spider lives

Steatoda triangulosa originated in Europe and around the Mediterranean and has been carried by trade to nearly every temperate region on earth. It is now found across the whole of the continental United States and southern Canada, throughout Europe, across Russia and central Asia, and in New Zealand and parts of South America. Few spiders have travelled so successfully with so little notice.

It is strongly synanthropic, meaning it lives with people. Typical places are basements and crawl spaces, garages and sheds, the corners of window frames, behind and beneath furniture, in undisturbed cupboards, around outbuilding eaves, and in the clutter of stored boxes. Outdoors it uses rock piles, the underside of bridges and culverts, fence posts and tree hollows, but in cool climates the great majority of individuals are indoors. Its habit of settling in the least-visited corners is why people usually meet it during a clear-out rather than day to day, alongside other common house spiders.

Are triangulate cobweb spiders venomous?

Yes, in the sense that every spider uses venom to subdue prey — and this one is an efficient killer of insects and other spiders relative to its size. What matters for people is whether the venom is medically significant, and for Steatoda triangulosa the answer is no. It belongs to the same family as the widow spiders, and other members of its genus such as the noble false widow can deliver a painful bite, but this species is far smaller, its fangs are short, and it has no record of causing significant envenomation.

Saying that the venom is harmless to humans is more accurate than saying it is non-toxic: the chemistry is functional, there is simply very little of it and the spider is unable to deliver a meaningful dose through human skin. No antivenom, medical monitoring or special precaution is required for this species.

Can triangulate cobweb spiders bite?

They can bite, but confirmed cases are genuinely rare. The spider is slow, shy and reluctant to leave its web, and its instinct when disturbed is to drop on a dragline or curl its legs and play dead. A bite realistically requires the spider to be pressed against skin — most often when someone reaches into a stored box, moves a cluttered shelf or puts on a garment that has hung unused in a garage.

Where bites have been reported the effect is minor: a brief pinprick, a small red mark, some local itching or soreness, and occasionally mild swelling. Symptoms usually fade within hours and almost always within a day. There is no tissue destruction, no spreading pain and no systemic illness of the kind associated with true widows.

Ecological importance and behaviour of the triangulate cobweb spider

This is a nocturnal ambush predator that rarely leaves its web. It takes a wide range of small household arthropods: ants, including fire ants in the southern United States, beetles, flies, springtails, silverfish, woodlice and pillbugs, and ticks that wander indoors on pets. For its size it is remarkably willing to attack hard, well-armoured prey, because the gum-footed lines immobilise the victim before the spider ever has to touch it.

A predator of dangerous spiders: the trait that makes this species worth keeping is its habit of preying on other spiders, including the brown recluse and the hobo spider. Extension entomologists in North America have repeatedly recorded triangulate cobweb spiders killing recluses that wander into their webs, and in buildings with established populations the two species tend not to coexist comfortably. Sweeping every small web out of a basement can therefore remove one of the better natural checks on more troublesome spiders.

Natural predators: larger spiders, notably cellar spiders, take triangulate cobweb spiders when their webs meet. Parasitoid wasps target both adults and egg sacs, and outdoors birds, lizards and centipedes take exposed individuals. The wider picture is set out in our page on what eats spiders.

Seasonal behaviour: indoors, where temperature barely changes, the species breeds and remains active all year. Outdoor populations in cool regions slow down in winter and shelter in crevices and leaf litter. Adults are longer-lived than most small spiders, commonly surviving a year or more, with females outliving males by a wide margin.

Relationship with humans: entirely benign and quietly useful. The webs collect dust and look untidy, which is the only genuine complaint against the species. Vacuuming the webs away and reducing clutter controls numbers without insecticide, and leaving a few in a garage or basement is a cheap form of pest control.

Similar spiders and how to tell them apart

Quick facts

Did you know?

• The species was described in 1802 by the French naturalist Charles Athanase Walckenaer, making it one of the earliest named spiders still carrying its original species epithet.
• Its reputation as a recluse killer is not folklore: North American extension entomologists have documented triangulate cobweb spiders repeatedly subduing brown recluses that blundered into their webs.
• The gum-footed web lets a 4 mm spider kill armoured prey such as beetles and ticks without ever wrestling with them — the silk does the dangerous part of the job.
• Because the pattern is visible in young spiderlings, this is one of the few house spiders that can be identified confidently at almost any age with nothing but a hand lens.
• Indoors the species has effectively abolished its own seasons: in a heated basement it breeds, hunts and matures at the same rate in January as in July.

In summary, the triangulate cobweb spider is a tiny, harmless and rather useful neighbour. Learn the zigzag of purple triangles and the pale yellow legs, leave a few webs in the basement to keep the recluses and ants down, and reserve the vacuum for the corners you actually use.