Is This Spider Dangerous? How to Tell Venomous Spiders From Harmless Ones

Almost every spider is venomous in the technical sense: that is how they eat. Almost none of them can hurt you. In most countries fewer than five species are medically significant, and even those bite rarely and only when pressed against skin.

So the practical question is not "is it venomous" but "is it one of the few that matter". Here is how to tell.

The short list of spiders that matter

Black widow (Latrodectus). Glossy black, round abdomen, red hourglass underneath. Found in sheds, woodpiles, outdoor furniture, and meter boxes across the Americas. The bite causes muscle cramps and pain that spreads from the bite; it is rarely fatal but needs medical care. The Australian redback is a close relative with a red stripe on top.

Brown recluse (Loxosceles). Uniform tan to brown, about the size of a coin with legs, with a dark violin shape on the back and only six eyes in three pairs. Mostly the central and southern United States. The bite can cause a slow-healing wound. If you suspect one, see a doctor within a day.

Funnel-web (Atrax, Hadronyche). Large, glossy black, aggressive when cornered, eastern Australia only. The Sydney funnel-web is the one spider for which a bite is a genuine emergency. Antivenom works; get to a hospital.

Hobo spider, yellow sac spider, wolf spider. These are often listed as dangerous. The evidence is weak. Bites are painful and may swell but rarely cause lasting harm.

Everything else, including the big hairy ones, is in the "harmless to humans" category.

Four questions to rule out danger fast

1. Where are you? Geography does most of the work. No brown recluse in Europe, no funnel-web outside Australia, no redback in North America. If the dangerous species for your region doesn't match, you can relax.

2. Is it in a web, and what kind? Widows build messy, irregular tangle webs close to the ground. Recluses build small, flat, untidy webs in hidden corners. Orb-weavers, which build the beautiful circular webs on porches, are harmless. Jumping spiders and wolf spiders build no web at all and are harmless.

3. What are the markings? Red hourglass on the underside of a glossy black abdomen: widow. Dark violin on a plain brown body with no stripes or bands on the legs: possibly a recluse. Banded or striped legs, patterns on the abdomen, or any hair: almost certainly not a recluse.

4. How big and how fast? Recluses are small and shy. Widows are slow and hang upside down in the web. Big, fast, hairy spiders that run at you are usually wolf spiders or huntsmen, alarming but harmless.

Common lookalikes that are harmless

• False widow (Steatoda). Brown or dark with cream markings, no red hourglass. Common in Europe. A bite is like a bee sting.
• Grass spider and funnel weaver. Build sheet webs with a funnel in lawns and shrubs. Fast, striped, harmless. Not related to the Australian funnel-web.
• Wolf spider. Large, hairy, striped, carries its egg sac and spiderlings. Painful bite, no lasting effect.
• Huntsman. Huge, flat, fast, found on walls in warm climates. Looks terrifying, is a useful roach hunter.
• Jumping spider. Small, fuzzy, big front eyes, moves in hops. Completely harmless and often curious about people.
• Cellar spider (daddy longlegs). Thin legs, shaky web in corners. The myth that it is the most venomous spider is false.

What to do if you've been bitten

1. Wash the bite with soap and water.
2. Apply a cold pack to limit swelling.
3. If you can, catch or photograph the spider. Identification changes treatment.
4. Watch the bite for 24 hours.

Seek medical help right away if you have muscle cramps, sweating, nausea, a spreading blister or ulcer, difficulty breathing, or if the spider might be a funnel-web, widow, or recluse.

Identify it before you decide

Descriptions only go so far, and the dangerous species have convincing lookalikes. The Bug Identifier app recognizes spiders from a single photo, flags the handful that are medically significant, and shows you what to do next. Point, shoot, know.

Download Bug Identifier on the App Store and settle the "is it dangerous" question in seconds.

Frequently asked questions

Are all spiders venomous?

Nearly all have venom to subdue prey, but very few have fangs that can pierce human skin or venom that affects people. "Poisonous" is the wrong word; spiders are venomous, and almost never dangerously so.

How can I tell a brown recluse from other brown spiders?

Look for three things together: a uniform brown body with no stripes or bands, a darker violin shape with the neck pointing toward the abdomen, and six eyes in three pairs. Missing any one of these, it is probably not a recluse.

Should I kill a spider I find in the house?

Most house spiders are beneficial and reduce flies, mosquitoes, and roaches. Relocating it with a cup and card is usually better. The exception is a confirmed widow or recluse in a place where children or pets could reach it.

Can a spider bite go unnoticed?

Yes. Many "spider bites" are actually other insect bites or skin infections. Unless you saw the spider, keep an open mind and treat it as a general bite.