A number of spiders can cause spider bites that are medically important. Almost all spiders produce venom but only a few are classified as "venomous" and able to cause significant harm to humans. Two medically important spider genera have a worldwide distribution—Latrodectus and Loxosceles. Others have a limited distribution.
Medical reports have been criticized for poor evidence. In the last century, both white tailed and wolf spiders were considered medically significant, only to be recanted. Only eight genera (Phoneutria, Atrax, Latrodectus, Loxosceles, Sicarius, Hexophthalma, Hadronyche and Missulena) are considered medically significant. Bites of these spiders have a range of severity, with only a minority having severe symptoms. Deaths by verified spider bites are exceedingly rare (e.g. one in Australia in 37 years).
Brazilian wandering spiders
The Brazilian wandering spider (a ctenid spider) is a large brown spider similar to North American wolf spiders in appearance, although somewhat larger. It has a highly toxic venom and is regarded (along with the Australian funnel-web spiders) as among the most dangerous spiders in the world. Based on one of the few pharmacological studies performed in the 1970s, Phoneutria'''s venom toxicity was more virulent than both Atrax and Latrodectus.
As their name suggests, Brazilian wandering spiders are active ground hunters. If the spider has a reason to be alarmed, it will bite in order to protect itself, but unless startled or provoked, most bites will be without venom. Venom bites will occur if the spider is pressed against something and hurt. In this case, the high levels of serotonin contained in the venom, plus at minimum-strong chelicera, will contribute to deliver a very painful bite.
Children are more sensitive to the venom of wandering spiders. The spiders often make threatening gestures, such as raising up their legs, or hopping sideways on the ground, which might amuse a child to the point of reaching towards the spider. In humans, bites of this spider may also result in prolonged painful penile erections (priapism). Scientists are attempting to create an erectile dysfunction treatment that can be combined with other medicines out of the peptide that causes this reaction.
Australian funnel-web spiders
The Australian funnel-web spiders (families Hexathelidae, Atracidae, Macrothelidae and Porrhothelidae), such as the Sydney funnel-web spider Atrax robustus (a mygalomorph spider not to be confused with the araneomorph funnel-weaver or grass spiders) are regarded as among the most venomous in the world. They react vigorously to threats and, reputedly, will more often attempt to bite than run away. A. robustus, a large black spider, is found within a radius of about 100 km from Sydney. Its venom contains a compound known as δ-atracotoxin which is highly toxic to primates. Approximately 10% of bites lead to serious symptoms for a total of 3–4 severe envenomations annually.Funnel Web Spider Envenomation Treatment & Management, Retrieved December 1, 2013A. robustus is one of three designated species of the genus Atrax.Platnick N I, Merrett P and Brignoli P M Advances in spider taxonomy, 1981–1987 p. 75 The related genus Hadronyche is represented by about 40 other dangerous species in eastern Australia, including Queensland and Tasmania. The males in this case have somewhat more potent venom than females and they also wander, making them more likely to be encountered in summer. Bites by males of two large species, the Sydney funnel-web and northern tree funnel-web, have resulted in death. One specific case denoted an individual being bitten on the heel through his leather footwear after provoking the spider, indicating the great strength of the spider's chelicerae.
One other genus in the family Hexathelidae has been reported to cause severe symptoms in humans. Severe bites have been attributed to members of the genus Macrothele in Taiwan, but no fatalities. In other mammals, such as rodents, for example, the effects of funnel web spider venom are much less severe.
Funnel web spiders are related to other mygalomorphs such as tarantulas and trapdoor spiders. These spiders are similar to funnel-webs in size and general appearance and also have huge chitinous fangs that can deliver painful bites, but their venom is usually extremely mild or harmless to humans.
Tangle-web spiders
One genus of the tangle web spiders has venom which is known to be medically significant. This genus, the widow spiders of genus Latrodectus, has caused human fatalities. The other genus, Steatoda, the false widow spiders, has yet to be proven to possess a venom of medical significance.