Spider bites: what they look like and when to worry
Last updated September 3, 2026.
Almost all spiders are harmless to humans, and most suspected spider bites turn out to be something else: a staph skin infection, another insect's bite, or an inflamed follicle. Genuine spider bites happen when a spider is trapped against skin, and in most of the world they cause nothing worse than a localized red, itchy, or painful bump that settles within days.
What does a real bite look like?
A typical harmless bite: one or two small puncture marks close together, local redness, mild swelling, itching or a stinging pain, settling over hours to days. The medically important spiders are few. Black-widow-type bites cause severe muscle pain and cramps, sweating, and abdominal rigidity starting within hours. Brown-recluse-type bites can develop a blister then a dark center with surrounding redness over days, occasionally ulcerating. In Australia, funnel-web and redback bites are the dangerous ones. Outside the ranges of these species, serious spider bites are essentially unheard of.
Why are most suspected bites not spiders?
Spiders bite rarely and only in self-defense, usually when pressed against skin in clothing, bedding, or gloves. They do not bite repeatedly, hunt humans, or bite in lines. A cluster of itchy bumps is far more likely to be fleas, bed bugs, or midges; a single red hot painful lump that comes to a head is usually a staph boil, and doctors regularly see MRSA infections confidently described as spider bites. If you did not see the spider, assume it was something else until proven otherwise.
What actually helps?
- Basic first aid: wash with soap and water, apply a cold pack 10 minutes at a time, elevate the limb if swollen, and use an antihistamine or hydrocortisone cream for itch.
- Pain relief: paracetamol or ibuprofen for the ache; most bites need nothing more.
- Catch the culprit: if you saw the spider and can trap it safely in a container, identification helps the medical team; do not risk another bite to do it.
- Watch the wound: a spreading dark area, an enlarging ulcer, or increasing pain over days needs review.
- Tetanus: any skin break is a prompt to check your tetanus vaccination is current.
When is it an emergency?
Call emergency services for severe whole-body symptoms after a bite: spreading muscle cramps or spasms, severe abdominal pain and rigidity, heavy sweating, difficulty breathing or swallowing, or facial or tongue swelling (anaphylaxis). A bite in Australia from a large black spider is a funnel-web until proven otherwise: pressure-immobilize the limb and call an ambulance. A bite that develops an expanding ulcer or dark necrotic center over the following days needs same-week review even if you feel well. Pymander's escalation routing is built and tested specifically for this class of decision; see the safety architecture working paper.
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Common questions
How do I know if it was actually a spider?
Mostly, you cannot, and that is the point doctors emphasize: without seeing the spider bite you, the odds favor other causes. Spiders bite once, in self-defense, and rarely. Multiple bumps, bumps in lines or clusters, or bumps appearing days apart point to fleas, bed bugs, or midges. A single hot, painful lump that comes to a pus head is usually a bacterial infection, not a bite at all. Genuine spider bites usually come with a story: putting on a shoe or glove and feeling the bite happen.
Which spiders are actually dangerous?
Far fewer than their reputation. In North America: the black widow (severe muscle cramps and pain, rarely fatal) and the brown recluse (can cause skin ulceration). In Australia: the funnel-web (genuinely life-threatening, needs immediate pressure-immobilization and ambulance) and the redback (severe pain and sweating, antivenom available). In the UK and most of Europe: no spider causes serious harm; the false widow generates headlines and gives a painful bite that settles like a wasp sting. Everywhere else: local species matter, so local advice applies.
What does a brown recluse bite look like?
Classically: a mild sting at first, then over hours a blister surrounded by redness, then over days the center can turn dark and sink as tissue dies, sometimes forming an ulcer that takes weeks to heal. Severe cases bring fever, rash, and joint pain. The honest complication: brown recluse bites are massively over-diagnosed, including in regions where the spider does not live; studies show most suspected recluse bites are actually staph infections, other bites, or skin conditions. The diagnosis needs the spider, the geography, or a very characteristic evolving lesion.
Should I try to catch or photograph the spider?
If you can do it safely, yes: a photo or the spider itself trapped in a container gives the medical team a firm identification, which decides whether antivenom or observation is needed. Do not risk a second bite to catch it, and do not handle it with bare hands even if it looks dead; spiders can bite reflexively. If it got away, note what you can: size, color, markings, where you were, and what you were doing.
What should I do for a suspected funnel-web or redback bite?
For a suspected funnel-web (a large black spider in eastern Australia): call an ambulance immediately, apply a pressure-immobilization bandage over the bite and up the limb, splint the limb still, and keep the person calm and motionless; antivenom is highly effective when given promptly. For a redback: do not bandage (the venom moves slowly and pressure increases pain); apply a cold pack, take pain relief, and seek medical care, since severe cases get antivenom in hospital. Do not cut, suck, or tourniquet any bite.
Why does my bite keep getting redder and more swollen?
Two possibilities, and the difference matters. Itch-driven scratching commonly introduces bacteria, converting a simple bite into cellulitis: spreading redness, warmth, and pain, sometimes with fever, needing antibiotics. The alternative is a normal large local reaction, where redness and swelling peak around 48 hours then recede, without increasing pain or fever. The dividing line: spreading redness with increasing pain, heat, or feeling unwell means infection and needs same-day review; stable, itchy, slowly-fading swelling just needs time and antihistamines.