Broad-faced Sac Spider (Trachelas tranquillus)

Physical description and identification

The broad-faced sac spider is the best known member of the genus Trachelas and the species that gave the family Trachelidae its English name. It is recognisable once you know what to look for: a spider built in two contrasting halves, with a hard, polished, reddish front end and a soft, dull, pale grey rear end. Nothing else of this size in eastern North America combines those two colours so sharply.

Adults

• Size: females measure about 7–10 mm in body length and males about 5–6 mm. Legs included, an adult female covers roughly the width of a US dime — much of the alarm this spider causes comes from its colour, not its size.
• Colour: the carapace, chelicerae and legs are glossy chestnut to dark reddish brown, the front two leg pairs darker and heavier than the rear two. The abdomen is dull greyish white to pale grey-tan. That red-front, grey-rear contrast is the quickest field mark.
• Eyes: eight small dark eyes of roughly equal size in two short rows of four, set well back from the front edge of the carapace. No pair is enlarged; two big forward-facing eyes would mean a jumping spider instead.
• Distinctive features: under magnification the carapace is covered with fine punctures, like hammered metal — a family character and the most reliable way to confirm a trachelid. The legs have no dorsal spines, but the front two pairs carry paired stout ventral spines used to pin prey. Two claws per foot with adhesive hair tufts let the spider climb painted walls and glass. Mature males are slimmer and longer-legged.

Eggs and egg sac

After mating in late summer the female builds a flattened, papery white egg sac holding roughly 30 to 50 eggs and attaches it under a stone, loose bark, a curled leaf or, indoors, the top edge of a window frame. She sits on the sac and guards it, feeding little, and usually does not survive long after the young disperse. The eggs are well under a millimetre across.

Spiderlings

The spiderlings emerge pale and translucent, already shaped like miniature adults but without the adult colour contrast. They stay near the sac briefly, then scatter on foot to find their own crevices, darkening and hardening with every moult. In the north of the range they overwinter as part-grown juveniles inside silk sacs under bark and mature the following summer.

Hunting instead of a web

This species does not spin a capture web and never has. It hunts on foot after dark over bark, foliage, stonework and interior walls, grabbing prey directly with those spined front legs. Silk is used for three things only: a dragline safety thread, a flattened sac retreat where it spends daylight hours and moults, and the egg sac. That retreat — a closed silk envelope under a stone, in a bark crack or a window recess — is what the name "sac spider" refers to. Our guide to spider webs explains why a hunting spider still needs silk.

Where the broad-faced sac spider lives

Trachelas tranquillus is an eastern North American species. Its range runs from southern Ontario and Quebec and the New England states south to Georgia and Alabama, and west across the Midwest to roughly the Great Plains, taking in New York, Pennsylvania, Ohio and Ontario. It does not occur naturally on the Pacific coast; the western member of the genus there is Trachelas pacificus.

The habitat summary sometimes given for this species — "warm, dry areas" — is too narrow to be useful. It lives along woodland edges and hedgerows and in old fields, orchards, gardens and suburban yards, and within those it wants a sheltered gap: under stones and logs, beneath loose bark, in leaf litter, in dead leaves hanging on shrubs and in stone walls. It climbs well and is often beaten from shrub foliage in summer.

What makes it notorious is its autumn behaviour. From September through November adults and large juveniles move toward buildings and enter through gaps around windows, doors and siding. Indoors they turn up on ceilings, high on walls and in curtain folds, which puts them on lists of common house spiders. Those that stay outside overwinter in silk sacs under bark rather than dying off.

Are broad-faced sac spiders venomous?

Yes, in the ordinary sense: the broad-faced sac spider has venom glands and uses venom to subdue insects. It is not, however, medically significant. No antivenom exists for any trachelid because none is needed, and no death, systemic envenomation or confirmed necrotic wound has been attributed to this species.

It is worth being precise, because this spider sits in the middle of a long-running muddle. It is not related to the recluse spiders, whose venom contains the enzyme responsible for genuine dermonecrosis, nor to the widow spiders and their neurotoxins. At the same time it is not harmless to feel: the chelicerae are proportionally large and heavily built for a 9 mm spider, and a bite from an adult female produces sharp pain out of all proportion to her size. Both exaggeration and dismissal misrepresent it.

Can broad-faced sac spiders bite?

They can, and they are among the more frequent biters of small house-entering spiders in North America. That is not aggression but behaviour: the spider wanders at night across the exact surfaces people sleep on, does not flee readily when contacted, and defends itself when pressed. Most bites happen when it is rolled against skin in bedding or clothing, or pinned when firewood, stones or boxes are handled.

What a bite feels like. The first sensation is immediate, sharp pain, much worse than expected from a spider this small. Over the next hour the site reddens and swells modestly and a tender, itchy patch of a few centimetres develops, settling in most cases within two to five days. A minority of reports describe a sore that heals slowly over one to two weeks. That is the outer limit of the documented picture — there is no verified case of tissue necrosis caused by Trachelas tranquillus, and any wound that genuinely dies back needs a medical diagnosis rather than a spider to blame.

The frequently repeated claim that bites become infected because this spider scavenges dead insects is a hypothesis, not a finding. It has never been demonstrated and should not be presented as an established risk.

What to do: wash the site with soap and water, apply a cold pack, keep it clean and covered, and take an over-the-counter painkiller if needed. Do not cut, burn, suck or apply household remedies. Seek medical care if pain increases after the first day, if redness spreads, if pus, fever or swollen lymph nodes develop, or if there is any sign of an allergic reaction. Photograph the spider if you can — a confirmed identification saves a great deal of worry.

Ecological importance and behavior of the broad-faced sac spider

This is a nocturnal, free-ranging predator. By day it lies sealed inside its silk sac; at dusk it emerges and patrols, locating prey by touch and by vibrations carried through the surface it stands on rather than by sight. Prey is seized with the spined front legs and bitten at once. The diet is small-bodied: aphids, leafhoppers, small flies, ants, caterpillars, mites, small beetles and other spiders. It also takes freshly dead insects, the origin of the "scavenger" label — but scavenging is a supplement, not the main strategy, and calling it a scavenger that feasts on dead insects understates what it is: an active hunter.

Natural predators: ground-foraging birds, shrews, mice, toads and lizards take exposed adults; centipedes, ants and ground spiders hunt them around their retreats; and spider wasps and ichneumonid parasitoids paralyse sac-dwelling spiders to provision their nests.

Prey-predator dynamics: hunting on surfaces rather than in the air, it works a layer that orb weavers and sheet-web builders never reach, which makes it a useful generalist predator of soft-bodied plant pests in orchards and garden shrubbery.

Seasonal behaviour: the species matures in late summer, with mating through August and September. Males abandon their retreats to search, which is precisely why indoor encounters peak in autumn, and rarely survive the winter. Females live around a year and guard a single egg sac.

Relationship with humans: beneficial outdoors, a nuisance rather than a danger indoors. The sensible measures are physical: seal gaps around window frames, doors and siding before autumn, fit door sweeps, shake out clothing and bedding left on the floor, move firewood and leaf piles away from walls, and lift stones with gloves. Catching an indoor individual in a glass works better than any spray.

Similar spiders and how to tell them apart

Quick facts

Did you know?

• The species was described in 1847 by Nicholas Marcellus Hentz, the French-born American naturalist who produced the first systematic account of the spiders of the United States. He placed it in Clubiona, which is why the authority is written in parentheses today.
• The family name Trachelidae only came into use after a 2014 revision of the two-clawed spiders, so books printed before then file this species under Corinnidae or Clubionidae.
• "Broad-faced" is a real anatomical description: the carapace rises steeply and is cut off square at the front, so the spider looks blunt-nosed head-on.
• The pitting on the carapace is not decoration. The punctures are muscle attachment points visible through an unusually thick cuticle — body armour for a spider that wrestles its prey without a web.

In summary, the broad-faced sac spider is a small, armoured, web-free night hunter, useful outdoors and merely inconvenient indoors. Its bite hurts more than its size suggests and can be slow to settle, but it has never been shown to destroy tissue — and the red-front, grey-back colour scheme makes it easy to identify with confidence.