Trachelas pacificus

Physical description and identification

Trachelas pacificus is the western North American member of the genus Trachelas and the Pacific coast counterpart of the eastern broad-faced sac spider. It belongs to the family Trachelidae — the broad-faced or ground sac spiders — and not, as older accounts have it, to Clubionidae or Corinnidae. Trachelidae was raised from a corinnid subfamily to a family of its own in 2014, so any source printed before then files this spider somewhere else.

Adults

• Size: a small spider. Females average about 7.6 mm in body length and males about 6.6 mm, so roughly 6 to 8 mm covers most individuals. Females are slightly larger and heavier-bodied; males are slimmer with longer legs and club-tipped pedipalps.
• Colour: the carapace, chelicerae and legs are glossy orange-red to reddish brown, the front two leg pairs darker and stouter than the rear two. The abdomen is dull cream to pale tan. The word "magenta", used in some descriptions, overstates it — there is nothing pink or purple about this spider, and the colour is the warm brick-red of polished mahogany.
• Eyes: eight small dark eyes of roughly equal size in two short rows of four. The rows sit close together and are set well back from the steep, squared-off front edge of the carapace; they are not widely separated. No pair is enlarged, which immediately separates Trachelas from jumping and wolf spiders.
• Distinctive features: under magnification the carapace is covered in fine punctures, like hammered metal, and it is noticeably thicker and harder than in most spiders of this size. The legs carry no dorsal spines, but the front two pairs bear paired stout ventral spines used to clamp prey. Two claws per foot with adhesive hair tufts let the spider run on smooth bark, grape canes and glass.

Eggs and egg sac

A mature female lays a small clutch — around 26 eggs in the original description — inside a flattened, white, papery silk sac with a distinctly flat base, pressed against the underside of bark, a stone, a leaf or a fence rail. In vineyards the sacs are typically tucked under the loose bark of the vine trunk. The female stays with the sac and guards it. The eggs are pale and well under a millimetre across; a figure of about nine centimetres per egg, which appears in some accounts, is a unit error — that would be more than ten times the length of the adult spider.

Spiderlings

Newly emerged spiderlings do look like miniature adults, as the older accounts say, but they lack the crisp adult colour contrast and are paler and more translucent. They stay near the sac briefly, then disperse on foot, darkening and hardening with each moult. In California most individuals overwinter as part-grown juveniles inside silk sacs under bark and mature the following spring and summer.

Hunting instead of a web

This species builds no capture web, and it is correct to call it a wandering hunter. It patrols after dark over bark, canes, foliage and stonework, detecting prey by touch and by vibration through the surface rather than by sight, then seizes it with those spined front legs. Silk is still essential, but for other purposes: a dragline trailed as a safety line, the flat sac that serves as a daytime retreat and moulting chamber, and the egg sac. That retreat is the "sac" in the common name. For why a webless spider still spins constantly, see our guide to spider webs.

Where Trachelas pacificus lives

The name is accurate: this is a spider of the Pacific slope of North America. It occurs from Baja California north through California to Oregon and Washington, and inland into parts of Nevada and Arizona. "Mexico and the United States" is technically true but far too loose — the species does not occur in the eastern states, where its place is taken by the broad-faced sac spider, and in Mexico it is a northwestern species rather than a national one.

Its habitat is also broader than the vineyard label suggests. Vineyards are simply where it has been studied most, because California grape growers monitor their spiders closely; Trachelas pacificus is a general foliage and bark dweller of oak woodland, riparian corridors, chaparral, orchards and gardens. Within any of those it wants a dry, sheltered gap: loose bark, curled leaves, leaf litter, the undersides of stones or a wall crevice.

Like its eastern relative it climbs well and enters buildings through gaps around windows, doors and siding, particularly in autumn when adults are wandering, which places it among the common house spiders of the west coast. Individuals that stay outdoors overwinter in silk sacs under bark rather than dying back.

Is Trachelas pacificus venomous?

Yes, in the ordinary sense that applies to almost every spider: it has venom glands and uses venom to paralyse insects. It is not medically significant to people. There is no antivenom for any trachelid because none is required, and no death, systemic envenomation or confirmed necrotic wound has ever been attributed to this species or to the genus.

The important distinction is taxonomic. Trachelas is not related to the recluse spiders, whose venom contains the enzyme responsible for genuine dermonecrosis, nor to the widow spiders and their neurotoxins. Equally, this is not a spider whose bite you would fail to notice: the chelicerae are proportionally large and heavily built, and an adult female can deliver sharp pain well out of proportion to her 7–8 mm body.

Can Trachelas pacificus bite?

It can, and it does so a little more readily than most small hunting spiders — not out of aggression, but because it wanders at night across surfaces people touch and does not always flee when contacted. Nearly all recorded bites involve a spider pressed against skin: rolled inside bedding or clothing, caught under a waistband, or pinned when someone handles vine prunings, firewood or stored boxes.

The usual reaction is immediate sharp pain, then redness, modest swelling and a tender, itchy patch a few centimetres across, resolving in two to five days. A minority of reports describe a sore that heals slowly over one to two weeks. That is the honest limit of the documented picture: no verified case of tissue necrosis has been traced to a Trachelas bite. A wound that genuinely dies back needs a medical diagnosis, not a spider to blame for it.

Treat a bite as any minor puncture: wash with soap and water, apply a cold pack, keep it clean and covered, and use an over-the-counter painkiller if needed. Do not cut, burn or suck the wound. Seek medical care if pain worsens after the first day, if redness spreads, or if pus, fever or swollen glands appear. Keep the spider or a photograph — a confirmed identification is worth more than any description of symptoms.

Ecological importance and behavior of Trachelas pacificus

This is a nocturnal, free-roaming predator with a strong attachment to bark and foliage. The spider spends daylight inside its sealed silk sac and emerges at dusk to patrol, working along vine canes, branch bark and leaf surfaces. Prey is located by touch and substrate vibration and seized directly. The diet is dominated by small, soft-bodied insects: leafhoppers, aphids, thrips, small flies, ants, mites, caterpillars and the occasional small spider.

Why vineyards matter: in California grape-growing regions Trachelas pacificus is one of the most abundant spiders sampled from vines, and it is a recognised natural enemy of leafhoppers, the group that includes the main foliar pests of the crop. Because it hunts on foot at night, it reaches insects resting on leaf undersides that a web-building spider would never intercept. Retaining loose trunk bark and limiting broad-spectrum sprays are both recommended partly to conserve predators like this one.

Natural predators: the usual summary of "birds and larger insects" is only part of the picture. Foraging birds, lizards and shrews take exposed adults; ants, centipedes and other hunting spiders find them in their retreats; and spider wasps and ichneumonid parasitoids specialise in paralysing sac-dwelling spiders to stock their nests.

Seasonal behaviour: juveniles overwinter under bark and mature from late spring into autumn. Mature males abandon their retreats to search for females and rarely survive long afterwards; females live one to two years. Autumn wandering by males is when most people see the species.

Relationship with humans: beneficial outdoors, harmless indoors. Physical measures are all that is needed — seal gaps around windows and doors, shake out clothing left on the floor, and wear gloves when handling prunings or stacked wood. Catching an indoor individual in a glass is better than an insecticide that would also remove the leafhopper control.

Similar spiders and how to tell them apart

Quick facts

Did you know?

• The species was described in 1935 by the American arachnologists Ralph Vary Chamberlin and Wilton Ivie, a University of Utah partnership that named a large share of the spider fauna of the western United States.
• The family Trachelidae did not exist as such until 2014. Before that this spider was filed under Corinnidae, and earlier under Clubionidae, which is why older California field guides list it in three different places.
• Leafhopper monitoring in San Joaquin Valley vineyards is one of the few long-running datasets anywhere that tracks a trachelid season by season — which is why more is known about the ecology of this species than about most of its genus.
• The pitting on the carapace is not surface decoration. Those punctures are muscle attachment points showing through unusually thick cuticle: armour for a spider that grapples prey without a web to do the holding.

In summary, Trachelas pacificus is a small, armoured, web-free night hunter of the Pacific states that earns its keep in vineyards and gardens by eating leafhoppers and aphids. Its bite hurts more than its size suggests, but it has never been shown to damage tissue, and its orange-red front with a cream abdomen makes it straightforward to identify.