Parasteatoda

Physical description and identification

Parasteatoda is a genus of small to medium tangle-web spiders in the family Theridiidae, erected by the American arachnologist Allan Frost Archer in 1946. Around fifty species are currently recognised, the majority of them Old World, and one of them — the common house spider, Parasteatoda tepidariorum — has been carried by trade to almost every inhabited part of the planet.

Adults

• Size: females run about 4–8 mm in body length depending on species, males 3–5 mm and noticeably slimmer. Leg span in a well-fed female house spider reaches 20–25 mm.
• Colour: the genus is built on greys and browns — blackish-brown, grey-brown, sandy tan, sometimes with an orange or yellowish cast. The abdomen is mottled and streaked rather than plain, usually with a paler band or chevrons down the middle and dark side marks, and the legs are faintly banded with darker rings. Nothing in the genus is glossy black.
• Body shape: females carry a tall, domed, almost spherical abdomen; in males it is smaller and more elongate, and the legs look proportionally longer. Males also have swollen pedipalp tips.
• Distinctive features: like all cobweb spiders they have a comb of curved, serrated bristles on the fourth leg, used to cast sticky silk over prey. Separating Parasteatoda species from each other, and from similar Steatoda and Theridion spiders, requires a microscope and adult genitalia.

Eggs and egg sacs

Females build pear-shaped or teardrop egg sacs of brownish, papery silk, roughly 6–9 mm across, and hang them in the densest part of the web where they can guard them. A sac may hold anywhere from about 100 to 400 eggs, and a female produces several across a season, so a long-occupied corner often shows three or four sacs of different ages at once.

Spiderlings

Spiderlings hatch inside the sac in roughly one to two weeks and emerge a few days later, pale and almost translucent. They stay briefly in the mother's web, then disperse on foot along silk lines or by ballooning. Growth takes several moults, and in heated buildings development can continue through winter, which is why house spiders of every size are present indoors all year.

The web

The web is a messy, asymmetrical three-dimensional tangle with no pattern — the archetypal cobweb — built in a sheltered corner and anchored by taut vertical lines running down to the floor or sill. Each of those gumfoot lines carries glue at the bottom, so an ant or beetle walking underneath sticks, the line detaches under tension and hoists the insect off its feet, and the spider comes down to it. Old webs collect dust and are abandoned rather than cleaned, which is why a neglected garage accumulates grey drapes of silk. Our guide to spider webs explains how this differs from an orb.

Where Parasteatoda spiders live

The genus is close to worldwide. Most described species are Old World — concentrated in eastern and southern Asia, with others in Europe, Africa, New Guinea and the Pacific — but there are New World species as well, and P. tepidariorum itself is now genuinely cosmopolitan, found on every continent except Antarctica. Descriptions that restrict the genus to New Guinea, Asia and Europe understate it considerably.

Habitat across the genus is consistent: a sheltered overhang with a solid surface above and a floor below. In buildings that means ceiling corners, window and door frames, eaves, porches, garages, sheds, barns, stairwells and cellars. Outdoors it means rock overhangs, tree bark, fence rails, culverts, the underside of bridges and dense shrubbery. Several species are strongly synanthropic — that is, they now live almost entirely alongside people — which is exactly why one of them became a global traveller.

Are Parasteatoda spiders venomous?

Yes, in the same trivial sense as almost every spider: they use venom to subdue flies and other small insects. None of them is medically significant. There is no antivenom because none is needed, no Parasteatoda species has caused a serious envenomation, and the venom lacks the alpha-latrotoxin that makes the widows in the same family dangerous.

This is worth stating plainly because the family connection alarms people. Theridiidae contains both the black widows and a great many harmless spiders, and sharing a family says nothing about risk. A house spider in a ceiling corner is closer in consequence to a fly than to a widow.

Can Parasteatoda spiders bite?

They can, but they almost never do. These are timid, web-bound spiders whose response to disturbance is to drop on a line, retreat into the tangle or curl up and play dead. Bites essentially require the spider to be pressed against skin — caught in clothing, or squashed against a hand during cleaning.

Where a bite does occur it produces brief, localised pain often compared to a bee sting, sometimes with mild redness or a small itchy welt that clears within a day or two. There are no systemic effects, no tissue damage and no lasting consequences. Wash the area with soap and water, use a cold pack if it stings, and watch for the spreading redness or unusual swelling that would indicate infection or an allergy rather than venom. No further treatment is needed and no home remedy is appropriate.

Ecological importance and behavior of Parasteatoda spiders

Parasteatoda spiders are sedentary ambush predators that can hold a single web for months. The diet is broad — flies, mosquitoes, midges, ants, small beetles, wasps and other spiders — and the gumfoot design lets a 6 mm spider subdue insects considerably larger than itself, because the prey is hauled off the ground and wrapped in sticky silk before it is ever bitten. In houses this makes them genuinely useful; a single well-placed web near a porch light removes a steady stream of insects all summer.

Natural predators: mud dauber wasps are the most consistent enemy and stock their nests with house spiders; spider-hunting wasps, parasitoid wasps that attack egg sacs, birds, geckos and larger spiders such as cellar spiders also take them. Cellar spiders sharing a basement will invade a Parasteatoda web and eat the resident.

A laboratory animal: P. tepidariorum has become one of the standard model organisms in evolutionary developmental biology. It breeds readily in culture, its embryos are easy to manipulate, and its genome — published in 2017 — provided the key evidence for an ancient whole-genome duplication early in arachnid evolution. Much of what is now known about how spiders build a body plan comes from this unremarkable house spider.

Social behaviour: most species are solitary, but Parasteatoda wau of Papua New Guinea is subsocial, with females and offspring cooperating in a shared web and hunting prey together — one of a handful of spiders anywhere that live in groups.

Seasonal behaviour and lifespan: outdoors, adults peak in late summer and autumn; indoors, heated buildings blur the seasons and all life stages occur year round. Females typically live about a year and sometimes two, producing egg sacs throughout; males live a few months after maturing and often remain in a female's web after mating.

Relationship with humans: harmless, beneficial and easily managed. Where a web is unwelcome, a vacuum or a brush removes it in seconds; sealing the gaps insects come through does more good than insecticide, since the spiders follow the food.

Species in the genus and similar spiders

Quick facts

Did you know?

• The name combines Greek para, "beside", with Steatoda: Archer created the genus for spiders that sit next to Steatoda without belonging to it. Many of its species were previously placed in Achaearanea or Theridion.
• The Japanese name for the genus is ōhimegumo. Himegumo, literally "princess spider", is the Japanese word for cobweb spiders in general, so the name means roughly "great princess spiders" — not "silky thread spider", a translation that circulates widely and is simply incorrect.
• The species name tepidariorum means "of the hothouses": the common house spider was first described from heated European greenhouses, where the tropical warmth let a spider of mild climates thrive long before central heating existed.

In summary, Parasteatoda is the genus of the everyday cobweb: harmless, useful, quietly cosmopolitan and, in one species, one of the most scientifically important spiders alive. Learn the mottled grey-brown body and the faintly banded legs and you will stop mistaking the spider in the garage corner for something dangerous.