Red House (Nesticodes rufipes)

Physical description and identification

A red house spider is small, glossy and unmistakably rusty. It hangs upside down in a loose tangle of silk, and the colour reads as bright orange-red in daylight but much darker in a shaded corner, which is part of why the same spider gets described so differently.

Adults

• Size: females are about 5–8 mm in body length (roughly 0.20–0.33 in) excluding the legs; males are smaller and slimmer at around 3–4 mm, with proportionally longer legs.
• Colour: the carapace and all eight legs are rusty red to orange-brown, sometimes with darker shading at the leg joints. The abdomen is high and globular and ranges from the same red through chocolate to a dark purplish brown, occasionally with faint darker chevrons on top. The whole animal looks uniformly coloured rather than patterned.
• The marking that is not there: the red house spider has no hourglass and no discrete red spot. Older accounts describe a red marking on the female's abdomen "like a black widow's", which is a misreading — what people are seeing is the spider's overall red body, not a ventral hourglass. The absence of a contrasting mark on a dark background is a useful way to rule the widows out.
• Other features: eight small eyes in two rows, and the comb of curved serrated bristles on the fourth legs that the whole family of comb-footed spiders uses to throw silk over prey. Mature males have swollen pedipalps and are often seen wandering rather than in a web.

Eggs and egg sacs

The female builds a rounded, loosely woven, creamy-white sac a few millimetres across and suspends it in the densest part of the tangle beside her retreat. She stays with it and defends it, and a well-established female in a sheltered indoor corner will produce sac after sac through the warm months, so several generations can occupy one window frame.

Spiderlings

The claim that the juvenile stage has never been observed is out of date. Spiderlings emerge pale and almost colourless, take on the family red after their first moults, and stay in or beside the mother's web for a short period before spreading out to build their own small tangles nearby — which is why these spiders turn up in clusters along a single ceiling line rather than singly.

The web

The web is a typical theridiid gumfoot tangle, not a shapeless mess. An irregular three-dimensional scaffold occupies the corner, with a retreat where the spider sits, and a set of taut vertical lines runs down from it to the floor, sill or shelf below, each tipped with glue. A walking insect that touches one snaps the anchor and is hoisted off its feet. It is built in sheltered, undisturbed places: ceiling corners, door and window frames, behind cisterns and cupboards, under outdoor furniture, in sheds and on porch ceilings. See our guide to spider webs.

Where the red house spider lives

Nesticodes rufipes is a pantropical species — it occurs in warm regions around the world rather than in one continent. It is abundant across the Caribbean, Central and South America, tropical Africa, southern and south-east Asia, northern Australia and most of the Pacific islands. In the United States it is established in Hawaii, Florida, southern Texas and coastal California, and it survives farther north only inside heated buildings and glasshouses.

That distribution is not natural in the strict sense. Like several of the most successful house spiders, this one has travelled with cargo for centuries and now lives mainly where people do. It is strongly synanthropic: buildings, verandas, sheds, greenhouses, animal housing and garden walls, with a preference for a dry, shaded, undisturbed corner out of direct sun and wind. Outdoors in the tropics it also uses rock overhangs, tree hollows and the underside of dense foliage.

Are red house spiders venomous?

Yes, in the ordinary sense that virtually all spiders are: Nesticodes rufipes uses venom to paralyse insects. It is not medically significant to people or pets. Despite belonging to the same family as the widows, it does not produce the α-latrotoxin that makes black widow venom dangerous, and there is no record of latrodectism, tissue damage or any serious systemic illness following a bite from this species. No antivenom exists because none is needed.

Can red house spiders bite?

They can, and among indoor cobweb spiders this one is comparatively quick to defend its web — a red house spider that is pinned against skin or has its retreat pressed will bite rather than drop. Even so, bites are uncommon, because the spider spends its life in a web well above head height or tucked out of reach.

The effect is local and short-lived: a sharp pinprick, then an hour or two of stinging or burning, a small red weal and sometimes mild itching or swelling that fades within a day or two. There is no spreading pain, cramping or sweating — the symptoms that define a widow bite. Wash the site with soap and water, apply a cold pack, and keep it clean; the usual reason a spider bite worsens is a secondary skin infection from scratching, not the venom. Seek medical advice if redness expands over the following days, if pus appears, or if there are signs of an allergic reaction such as widespread hives or breathing difficulty.

Ecological importance and behavior of the red house spider

The red house spider is a sit-and-wait predator that reads its web by vibration. It hangs inverted in the scaffold, and when a gumfoot line breaks it drops on the prey, throws sticky silk over it with the comb on its hind legs, bites, and hauls the parcel back up. Its usual catch is exactly the insect list people complain about indoors — house flies, fruit flies, mosquitoes, ants, small cockroaches, larder beetles and moths.

An unusual appetite: this species takes prey that other small theridiids will not, including insects much larger than itself and, notably, other spiders. Where it becomes abundant in a building it can displace the resident population of smaller web spiders, which is one reason it so often ends up as the only spider in a room.

Natural predators: house geckos are probably the most significant in the tropics, along with jumping spiders, pirate spiders that invade theridiid webs and eat the owner, mud-dauber and spider-hunting wasps that stock their nests with paralysed females, assassin bugs, ants and small birds.

Seasonal pattern: in the tropics the species breeds year-round, and indoors in cooler regions the heating does the same job, so all stages can be present at any time. Males mature, stop feeding and wander in search of females, which is when they are most often seen crossing a wall or a ceiling.

Relationship with people: beneficial and low-risk. A tangle in a corner is unsightly, but each web removes a steady stream of flies and mosquitoes. If a web is unwanted, a brush and a vacuum are enough; sealing gaps and reducing outdoor lighting near doorways cuts the insect supply that brings the spiders in the first place, and works better than spraying.

Similar spiders and how to tell them apart

Quick facts

Did you know?

• The species was described in 1846 as Theridion rufipes by Pierre Hippolyte Lucas, from material collected in Algeria; rufipes simply means "red-footed".
• Nesticodes is monotypic — this one species is the entire genus, which is unusual in a family of more than two and a half thousand species.
• Despite the name, it is not closely related to the cave-dwelling Nesticus spiders; the similarity is in the name only.
• In Hawaii it is one of the most frequently reported spiders in homes, and it has taken over indoor corners that smaller native cobweb spiders once held.
• Its willingness to tackle prey far larger than itself, including other spiders, is documented and is unusual for a theridiid of this size.

In summary, the red house spider is a harmless, hard-working tropical tenant that is repeatedly mistaken for something dangerous purely because of its colour. Uniform red all over, including the legs, with no contrasting hourglass, is the whole identification.