In the United States, people worry most about the fangs of the brown recluse and black widow. A bite from a brown recluse can leave you with significant scarring from tissue breakdown, but odds are strongly in favor of your survival. You can identify brown recluses by the violin-shaped marking on their abdomens. Statistically, black widows, with their red hourglass calling card, pose more of a threat to humans. Before doctors discovered antivenin for the widow family of spiders, the mortality rate for bite victims was around 5 percent [source: Vetter and Visscher].
Although there isn't a definitive answer to what the world's deadliest spider is, experts often place the Sydney Funnel-web spider, or Atrax robustus, at the top of the list of poisonous spiders. This arachnid species bears responsibility for the highest number of bites and human fatalities among all spiders [source: Alcock]. With fangs strong enough to penetrate finger and toenails, in extreme cases, a Funnel-web bite can kill a person in 15 minutes.
As you can infer from its name, the Sydney Funnel-web spider is native to Australia, found across a 99-mile (160-kilometer) radius around Sydney [source: Australian Venom Compendium]. Humans encounter dark-colored Funnel-web spiders mostly during mating seasons in summer and fall, when the males leave their burrows in search of females [source: Australia Museum]. However, since the discovery of a Funnel-web antivenin in 1980, no one has died from its bite. Behaviorally, Funnel-web spiders are more dangerous than black widows or brown recluses because they become aggressive when threatened. If you tried to shoo one away, it's less likely to skitter off. The male Funnel-webs are more potentially harmful than females and juvenile spiders -- and they're the most likely to attack.
The active ingredient in Funnel-web venom is delta-atracotoxin. This amino acid chain is a neurotoxin that causes our nerve cells to continually fire once it enters the bloodstream. That rapid firing sparks intense pain at the bite site that will start within 28 minutes of the actual bite [source: Alcock]. In general, spider venom affects humans by interacting with ion receptors in our cells [source: Escoubas, Diochot and Corzo]. When that happens, it unnaturally excites the cells, which can lead to pain and cell dysfunction.
On the other side of the world, the Brazilian wandering spider, or Phoneutria nigriven, often ranks second, below the Sydney Funnel-web, as the world's deadliest spider. The 2007 Guinness Book of World Records actually named it the most venomous spider in the world because it's capable of injecting 1 to 2 milligrams of venom in its victims [source: Richards]. You can spot these by the red hairs on their necks. Like the Funnel-web, the Brazilian wandering spider has aggressive tendencies and is known as a hunting spider. Like other spider venoms, the Brazilian wandering spider's affects sodium ion receptors, particularly in muscle and sensory cells [source: Meier and White]. Bites from these spiders account for around half of spider-related hospitalizations in South America [source: Meier and White].
Getting bitten by any of these spiders would probably be an unpleasant experience. But today, even the most venomous arachnids with the largest, sharpest fangs don't pose much of a health threat to humans thanks to antivenins. As a rule of thumb, be careful when you're working in a dark corner of your basement or storage shed. Spiders often don't want to attack, but they'll defend their territory when necessary.