Barn (Araneus cavaticus)

Physical description and identification

The barn spider is a classic Araneus member of the orb weaver family: a bulky, hairy spider with a high-shouldered abdomen, short stout legs and a habit of hanging head-down in the centre of a big vertical web after dark. Most people meet it in late August or September, when the females have reached full size and their webs appear every evening around porch lights and outbuildings.

Adults

• Size: females are usually 13–20 mm in body length and males 8–13 mm, so the female is a substantial spider — roughly thumbnail-sized with legs folded. Some older references quote a smaller 12–16 mm for females; specimens vary a great deal with feeding.
• Colour: yellowish brown to grey-brown or reddish above, densely covered in short hairs. The abdomen bears a paler central band edged by darker markings, breaking into a row of pale chevrons toward the spinnerets. The underside is the most reliable feature: it is black with two bold white or cream comma-shaped marks.
• Legs: strongly banded in light and dark brown, thick, and armed with conspicuous dark spines.
• Shape: the front of the abdomen carries two low shoulder humps, and the whole body has a rough, slightly lumpy outline rather than the smooth globe of some relatives.
• Eyes: eight small eyes in two rows. Vision is poor; everything the spider knows about its web comes through vibration in the silk.

Eggs and egg sac

Mating takes place in late summer. In autumn the female spins a flattened, cocoon-like sac of pale silk in a sheltered crevice — under a board, inside a beam joint or behind loose bark — and lays several hundred eggs into it. She does not survive the winter. This is the single most misunderstood point about the species: the young hatch inside the sac and then stay put, overwintering there as tiny spiderlings rather than emerging in the autumn.

Spiderlings

The spiderlings leave the sac in spring as a dense cluster of pale, dark-legged juveniles, hang together for a day or two, then disperse on silk threads. Each builds a miniature orb immediately and starts feeding on gnats and other small flies. Growth runs through the summer via a series of molts, with maturity reached in late summer — which is why the spider seems absent all year and then suddenly enormous in September.

The web

Barn spiders build a vertical orb that can reach 45–60 cm across, typically slung in a doorway, a window frame, the gap between rafters, under an eave or across a rock overhang. Work starts at dusk and takes about an hour: the spider lays down frame and radial lines, then a temporary spiral, then the sticky capture spiral over the top of it. She waits head-down at the hub through the night, or just off to one side on a signal line leading back to a crack in the woodwork.

Before dawn most of the web is taken down again — and the silk is eaten, not discarded, so the protein is recycled into the following night's web. Only the strongest frame lines are usually left in place, which is why a web reappears in exactly the same doorway night after night. For how this nightly rebuilding compares with other designs, see our guide to spider webs.

Where the barn spider lives

Araneus cavaticus is a North American species. It is most abundant in the northeast — New York, New England, Pennsylvania and the Great Lakes states — and across southeastern Canada, but the range extends well beyond that, south along the Appalachians and west into the Great Plains.

The common name is accurate. This is a spider of built structures and rocky places: barns, sheds, stables, garages, boathouses, porches, decks, bridges, dock pilings, and natural equivalents such as cliff faces, rock ledges and cave mouths — the species name cavaticus means “living in hollows”. It needs an overhang to anchor a web under and a crevice to hide in by day. It does not colonise heated interiors, so finding one inside the living area of a house is unusual and almost always accidental.

Are barn spiders venomous?

Yes, in the sense that nearly every spider is: the barn spider uses venom to immobilise the insects it catches. It is not medically significant to people or pets. The venom acts on insect nervous systems, the fangs are short, and there are no documented cases of serious illness, necrosis or systemic reaction from this species. It is unrelated to the recluse and widow spiders that account for nearly all medically important bites in North America.

Can barn spiders bite?

They can, but it is rare. A barn spider disturbed in its web will either drop to the ground on a dragline or shake the web hard enough to blur its own outline; running at a person is not in its repertoire. Bites happen when the spider is pressed against skin — reaching blindly into a rafter joint, or pulling on gloves and clothing that have been hanging in a shed.

The result is a short-lived sting comparable to a pinprick or a mild bee sting, with a small red mark and perhaps slight swelling that clears within a day. Wash the area with soap and water, apply a cold pack and leave it alone. Seek medical advice if the pain increases after the first day, if redness spreads outwards, or if there are signs of a general allergic reaction — those indicate infection or individual sensitivity rather than the venom itself. Do not cut, suck or burn a spider bite.

Ecological importance and behavior of the barn spider

The barn spider is a nocturnal ambush predator whose web works as a floating net across a flyway. Prey is mostly night-flying insects — moths, midges, mosquitoes, crane flies, small beetles, leafhoppers and mayflies — which is why the species does so well next to a porch light. When something strikes the silk, the spider runs out along the radial lines, bites it, wraps it and either feeds on the spot or carries the bundle back to the hub.

Why the web moves: disturbed barn spiders bounce rapidly on their legs so that the whole orb oscillates. It is a defence, not an attack — a vibrating spider is far harder for a bird or wasp to seize.

Predators: mud-dauber wasps provision their nests with orb weavers of this size, and birds, bats, toads, centipedes and larger spiders take adults. Parasitic wasp larvae attach to juveniles and feed externally; ants and small wasps raid overwintering egg sacs.

Ecological role: a colony of barn spiders around a stable or dairy removes an impressive quantity of flies and mosquitoes through the summer, which makes them genuinely useful animals to keep around agricultural buildings.

Season and lifespan: the life cycle is annual. Spiderlings emerge in spring, mature in late summer, mate in August and September, and adults are dead by hard frost. Males stop building webs once mature and wander in search of females, approaching cautiously along a signal thread; they die within weeks of their final molt.

Similar spiders and how to tell them apart

Quick facts

Did you know?

• Charlotte in E. B. White's Charlotte's Web is named Charlotte A. Cavatica, after this spider's scientific name — and she lives, correctly, in a barn doorway.
• Recycling the web is not just tidiness. Spiders that eat their old silk recover most of its protein and can spin the next web within hours, which matters when a new one is needed every night.
• The species name cavaticus refers to hollows and caves, and barn spiders were living on rock overhangs long before there were barns to live on.
• Barn spiders have poor eyesight and read the world through vibration: the tension of each radius tells the spider where prey has landed and roughly how big it is.
• A female left undisturbed will use the same frame lines for weeks, so the same doorway can host a web every night through the autumn.

In summary, the barn spider is a harmless, hard-working night hunter that clears moths and mosquitoes from around outbuildings. Check the underside for two white commas on black, and remember the web will be gone by morning and back again at dusk.