What Spider Is This? Identifying Common US Spiders

In the United States only two groups of house-entering spiders are treated as medically important: recluse spiders and widow spiders. The University of Kentucky's urban spider chart lists the rest of the usual indoor cast — wolf spider, jumping spider, cellar spider, grass spider, orb weaver, American house spider — as harmless except to allergic individuals.

Work in this order and you will usually land on an answer:

• Eyes — six eyes in three pairs is the recluse signature
• Legs — uniform color, or striped, ringed, spotted or spiny
• Body — glossy black with red underneath points to a widow
• Web — circular, funnel-shaped, a messy cobweb, or none at all
• Where it was — crossing the floor, in a ceiling corner, under a woodpile

Photograph it from above and from the side with something for scale before it moves. That is also what the app works from: photograph it and the app identifies it.

Brown recluse. University of California IPM gives the native range as the south central Midwest, from southeastern Nebraska to southwestern Ohio and south through Texas to north Georgia, with a body not more than 3/8 inch long. The reliable feature is the eyes: six equal-sized eyes arranged in three pairs. UC IPM states plainly that the presence or absence of the violin marking is not a reliable way to identify this spider, because it varies in intensity and can fade. The second good feature is the legs: uniformly colored and covered in fine hairs, without the stout spines, stripes, rings or spots that many other brown spiders carry.

Black widow. Kentucky's chart puts the adult at about 1/2 inch long, glossy black with a variable number of red markings, spinning a cobweb in concealed outdoor places such as rock piles and firewood piles. The Illinois Department of Public Health describes the female as up to half an inch with bright red or red-orange markings on the underside of the abdomen, in an irregular web with a short funnel-shaped section, about a foot across in all directions.

Most "brown recluse" reports turn out to be one of these, all listed as harmless except to allergic individuals by the University of Kentucky:

• Wolf spider — Missouri Department of Conservation gives a body length of 1/4 inch to 1 inch, gray, brown, black or tan with dark markings and especially stripes. It builds no web and runs prey down; the female carries her egg sac on her spinnerets and then her spiderlings on her back for up to two weeks. Hold a flashlight near your head at night and the eyes shine back like a cat's.
• Jumping spider — about dime-sized with legs out, large forward-facing eyes, quick jerky movements, no web.
• Cellar spider — very long thin legs, about half-dollar size with legs outstretched, messy cobwebs in basements and corners.
• Grass spider — brown with lengthwise gray or tan stripes; builds the big funnel webs that show up on lawns in late summer.
• American house spider — nickel-sized with legs out, brown and tan, compact messy cobweb.

The leg pattern is the practical tiebreaker: stripes, rings, spots or spines mean it is not a recluse.

Spider bites are diagnosed far more often than they happen. UC IPM notes that most brown recluse bite diagnoses in California are incorrect, and that multiple lesions point to a bacterial infection rather than a spider, because a spider bites once.

What the sources say about real bites: the Illinois Department of Public Health describes a brown recluse bite as redness, swelling and a burning sensation developing within an hour, sometimes blistering and forming a scab within 24 hours; a black widow bite typically brings chest pain, muscle tightness and cramping, with symptoms usually declining after 48 hours.

This page cannot diagnose a bite. If you have a painful, spreading or blistering wound, muscle cramps, chest pain or trouble breathing, get medical care now. In the US, Poison Control answers free at 1-800-222-1222, and the webPOISONCONTROL online tool gives guidance too. If you safely captured the spider, bring it or a clear photo — what bit you changes what the clinician looks for.

Both medically important groups bite mainly when pressed against skin. The Illinois Department of Public Health notes that recluse spiders favor wooden surfaces — inside furniture, cardboard boxes, wall voids and the framing of crawlspaces, basements and attics. Kentucky's chart puts widows in concealed outdoor spots such as rock piles and firewood piles.

Habits that follow from that:

• Shake out stored clothing, gloves and boots before putting them on
• Wear gloves when moving firewood, rocks or boxes that have sat undisturbed
• Store seasonal clothing in sealed plastic rather than open cardboard
• Move woodpiles off the house wall and keep them off the ground
• Use a flashlight in crawlspaces and attics instead of reaching in blind

None of this calls for killing the harmless species: Kentucky lists their bites as harmless except to allergic individuals.