How to Identify Spiders You Find Indoors in Britain and North America

A long-bodied cellar spider has extremely fine legs and a small, narrow body. Look for it hanging upside down in a loose web in a cellar or ceiling corner. If disturbed, it can whirl in the web fast enough to blur. The web helps distinguish it from other long-legged animals around homes.

At a sunny window, look for a little black-and-white spider that pauses, turns, and hops. The light bands resemble zebra stripes and the two large front-facing eyes are conspicuous in a close photo. It stalks insects instead of catching them in a web.

The American common house spider has a mottled, rounded abdomen. Look for it in a loose tangle at a window or in a sheltered corner. Brown, papery egg sacs may hang in the web and are often easier to spot than the small spider beside them.

A big brown runner with long legs can look startling on the floor or in the bathtub. A sheet web with a funnel can point to the giant house spider group. Eratigena atrica, pictured here, occurs in only a few areas of Britain. Several related species look so much alike that an ordinary photo usually won’t settle which one you found.

The barn funnel weaver builds a flat sheet of silk with a tunnel at one edge, often along a wall or in another sheltered spot. This spider often has faint bands on its legs. That can help distinguish it from the larger giant house spider group, although a clear photo of the body has more value than size alone.

The cupboard spider is one of the false widows, with a rounded abdomen that may look nearly plain dark brown in an adult female. Younger spiders can show pale markings. It makes an irregular web in sheltered spots. A dark round abdomen alone isn’t enough to call a spider a black widow.

In Britain, the noble false widow often has a pale marbled pattern on a dark, bulbous abdomen. The pattern varies, so the outline and where you found it matter too. Leave the spider undisturbed when taking a photo. The British Arachnological Society describes bites as uncommon.

Look for a line of dark triangular marks on a lighter abdomen. The markings help separate this small indoor cobweb spider from a plain-colored adult cupboard spider. The web itself is an irregular tangle, so look at the spider’s pattern when both species seem plausible.

Check the silk around a window frame or crevice. British lace-weavers can leave a rather woolly sheet with a bluish cast when fresh. Amaurobius similis often lives on buildings, but it closely resembles the window lace-weaver Amaurobius fenestralis. Matching the web can get you to the group first.

This British mouse spider has a grayish, velvety abdomen. It hunts at night, so it can turn up running across a floor without an obvious capture web nearby. The scientific name matters here, since the same English name is also used for a very different group of Australian spiders.

On a dark abdomen, look for the pale lengthwise mark that gives this North American spider its name. A parson spider moves quickly over floors or walls while hunting at night. The patch helps separate a well-photographed individual from a uniformly brown runner.

A small, pale spider near the ceiling can be a yellow sac spider. Look nearby for a small silken retreat, often tucked where a wall and ceiling meet. This spider leaves the retreat to hunt. The pale body and retreat suggest the group, since related species also look alike.

The dark female pictured often stays near a crevice in a building. A slimmer, lighter male can be mistaken for a brown recluse in the southern United States. Its conspicuously long mouth appendages, called pedipalps, help an expert distinguish it in a photograph.

Start with geography. Established populations are mostly in the central and south-central United States. A dark mark that resembles a violin is easy to misread. In a clear photo an expert can check for its six eyes in three pairs. Leave a suspected recluse alone and seek professional identification.

In western North America, an adult female western black widow can be glossy black with a red hourglass underneath. She may build an irregular web in an undisturbed garage corner or under stored items. The mark can be hard to see from above. Leave a suspected widow undisturbed. If you think you were bitten, seek medical advice promptly.