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. 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. That's one dangerous spider! However, since the discovery of a funnel-web antivenin in 1980, no one has died from its bite.

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].

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.