Spiders are active hunters and rely heavily on their bites to
paralyze and kill their prey before consuming it. They also bite in self
defense. While many spiders will never attack animals larger than
themselves, some exhibit a rather aggressive behavior and will stand
their ground when approached by larger animals, e.g., Atrax robustus.
Most spider bites, however, occur when humans unintentionally press up
against spiders and receive a defensive bite. On rare occasions, spiders
may make prey mistakes and bite a human finger or other body part as
though it were a caterpillar or other such insect.
Only spiders of fairly large species possess chelicera long enough to penetrate human skin. There is nothing smaller than members of the Cheiracanthium
genus (around 6 mm.) that gives a bite that is greatly troublesome to
humans. The effect of a bite on humans is dependent on both the toxicity
of the venom and the amount of venom. About 98% of the bites inflicted
by species that are large enough that their bites are noticed will have
no serious medical consequences.[1] Of those bites that humans notice, venoms can include necrotic agents, neurotoxins, and agents such as serotonin. Only some two hundred species in twenty genera (out of over 40,000 known species) are known to have serious, potentially lethal bites.[2]
In most cases of bites, the chief concern is the spider's venom,
although in some cases medically non-significant spiders can transmit
infectious diseases. Spiders regarded as dangerous possess venom that is
sufficiently toxic to humans that a single bite can deliver a medically
significant dose. Only three spider families are known to be
non-venomous, i.e. lacking venom glands. They are Uloboridae, Holarchaeidae and Liphistiidae.
The Liphistiidae, however, are large enough to deliver unpleasant
bites. In addition, their fangs can often inflict infections spread
through the skin, mostly due to their large size, which in theory could
represent more danger than the bite of a non-lethal venomous spider
species.