Charcoal Spider (Tegenaria ferruginea)

Physical description and identification

The charcoal spider is one of the larger European Tegenaria species: long-legged, hairy and built like a smaller version of the house spiders that alarm people every autumn. Despite the common name it is not black. Ferruginea means "rust-coloured", and a well-lit adult is a warm reddish brown; the "charcoal" impression comes from seeing the spider in the gloom of a cellar or cave mouth where it normally sits.

Adults

• Size: females reach about 11–16 mm in body length and males 9–12 mm, with a leg span of up to roughly 50 mm (2 in). It is smaller than the giant house spider but large enough to be startling indoors.
• Color: rust-brown to yellow-brown. The carapace has dark markings along each side and a paler central band, and the abdomen bears a dark chevron pattern flanked by paired pale spots. Legs are clearly annulated — marked with alternating dark and light rings — which is one of the most useful features for separating this species from the plain-legged hobo spider.
• Eyes: eight eyes of near-equal size in two rows. Vision is poor and used only to sense movement and light levels; hunting is done entirely by vibration.
• Distinctive features: a pair of long posterior spinnerets projects behind the abdomen, as in all funnel weavers. The whole animal is densely covered in fine hairs. Males have swollen pedipalps and longer, thinner legs; females are heavier bodied. Confident species-level identification within Tegenaria needs a microscope.

Eggs and egg sacs

The female builds rounded, off-white egg sacs of loosely spun silk, each holding several dozen eggs, and hides them deep inside the funnel retreat or in a crevice alongside it, frequently coating them with debris. She stays close to the sac and guards it, and in sheltered sites such as caves and cellars sacs may be produced over a long season rather than in a single autumn burst.

Spiderlings

Hatchlings are pale, almost translucent miniatures of the adults. They remain near the mother's retreat for a short period before dispersing, then darken and grow through repeated molts — many of them — over one to two years before reaching maturity. Because development is slow and the habitat is stable, all sizes can often be found in one cellar at the same time.

The web

The web is a broad, non-sticky sheet of tightly woven silk with a tubular funnel at one end. The spider sits inside the tube with its front legs on the sheet, feeling for vibration. Sheets are built into rock fissures, cave entrances, hollow trees, loose bark, dry stone walls, log piles, sheds and the undisturbed corners of cellars and stables. Because the silk is added to rather than replaced, an old web becomes a thick, dusty mat. Our guide to spider webs explains why this dry design catches prey without glue.

Where charcoal spiders live

Tegenaria ferruginea is a continental European species, recorded widely from France and Spain through central Europe and Italy to the Balkans, and eastward to Turkey and the Caucasus. It has been reported from a scattering of sites elsewhere as an accidental import, but it is not established in North America and it is not one of the Tegenaria species that colonised the British Isles.

Its preferred habitat is a dark, humid, undisturbed cavity. That covers caves and rock fissures, mine adits, hollow trunks, the space under loose bark, dry stone walls, and — where those are lacking — cellars, stables, barns, garages, woodpiles and the low outside corners of buildings. Compared with the barn funnel weaver, which thrives in dry heated rooms, this is a more outdoor spider that only borrows human structures when they resemble the rock crevice it really wants.

Are charcoal spiders venomous?

Yes, in the sense that virtually every spider is. Tegenaria ferruginea uses venom to paralyse the insects it catches, and that venom is made of peptides and enzymes tuned to invertebrate targets. It carries none of the neurotoxins or tissue-damaging enzymes that make widows and recluses medically important, and no serious human reaction has been documented from this species or from any European Tegenaria.

Its close relatives have been studied specifically because of bite scares. The verdict from that work applies here: funnel weavers of this group deliver a small dose of insect-specific venom through short fangs, and the outcome in people is a transient local sting at worst.

Can charcoal spiders bite?

A charcoal spider can bite, but doing so requires cornering it. Its instinct when a cellar light goes on is to disappear into the funnel, and it will only use its fangs if it is squeezed — inside a glove, under a hand on a woodpile, or against skin in bedding.

Verified bites from large Tegenaria produce a sharp momentary pain similar to a pinprick or a mild bee sting, sometimes with a small red mark and slight swelling that fades within hours. There is no venom-driven tissue damage, no systemic illness and no reason for antivenom or any other treatment.

Clean the area with soap and water, apply a cold pack if it is sore, and keep it covered. If redness spreads, the site becomes hot and painful over the following days, or a fever develops, see a doctor — that pattern indicates a bacterial skin infection, which is a much commoner cause of "spider bite" wounds than spiders are. Keeping the spider or a photograph helps anyone who has to identify it later.

Ecological importance and behavior of the charcoal spider

The charcoal spider is nocturnal and profoundly sedentary. A female may occupy the same crevice for her entire adult life, enlarging the sheet a little at a time and rushing out only to seize prey. Anything that walks or falls onto the silk is fair game: flies, moths, beetles, woodlice, crickets, earwigs, and in cellars a steady supply of the insects that gather around lights and drains.

Natural predators: spider-hunting wasps, centipedes, harvestmen, shrews, bats at cave entrances, and birds that forage along walls and woodpiles. Larger spiders sharing the same structures also take them.

Prey-predator dynamics: in caves and cellars it is often one of the few substantial predators present, so its webs do a disproportionate share of the insect control in an otherwise sparse food web.

Seasonal behavior: in sheltered, stable habitats adults can be found through much of the year rather than in a single autumn peak. Mature males still leave their webs to search for females, and they are the individuals most likely to be seen crossing a floor. Females are long-lived by spider standards: two to four years is realistic in a cool, damp cellar, and they may produce several egg sacs in that time.

Relationship with humans: harmless and quietly useful. The only nuisance is the dusty sheet web, and brushing it away costs the spider a home rather than a life — it will simply rebuild a few metres further into the dark.

Similar spiders and how to tell them apart

Quick facts

Did you know?

• The name is a contradiction: ferruginea is Latin for "rusty", while the English trade name "charcoal spider" describes how the animal looks in the dark. Neither is an officially recognised common name, so the scientific binomial is the only unambiguous label.
• It was described in 1804 by Georg Wolfgang Franz Panzer, a German physician and entomologist better known for his vast illustrated survey of the insects of Germany.
• Cave-dwelling populations sit at the edge of true cave fauna. It is not a specialised troglobite — it keeps its eyes and pigment — but it is one of the commonest large spiders in the twilight zone of European cave entrances.
• Because its web is expanded rather than rebuilt, the silk mat in a long-occupied cellar corner can represent years of work by a single spider.

In summary, the charcoal spider is a big, shy, rust-brown funnel weaver that takes over dark cavities and quietly eats the insects that share them. Learn the ringed legs and the sheet-plus-funnel web and it becomes easy to recognise — and easy to leave alone.