Most venomous spiders

There are two main contenders for the title of most venomous spider, which varies depending on toxicity, number of envenomations and even the species being bitten. Wandering spider species within the genus Phoneutria of South and Central America are the spiders with the most toxic venom, and are also responsible for more severe envenomations annually than any other type of spider. In standard mouse assays, Phoneutria venoms have been the most potent to date for any spider. Extrapolating based on size, gives a human LD50 of as low as 7.5 micrograms of venom per kilogram of body weight.

Funnel-web spiders from Australia of the Atracidae family (aka atracids) are another group of extremely toxic spiders, with toxicity rivalling that of wandering spiders, although they bite fewer people; the most common culprit among this family are male Sydney funnel-web spiders (Atrax robustus). Based upon studies of the main toxin type against monkeys, which like humans are very sensitive to funnel-web spider venom, as little as 5 micrograms per kilogram of body weight produced significant effects and only 30 micrograms per kilogram of body weight was enough to be fatal.

In addition to producing lethality, Phoeneutria bites to human males can result in severe priapism (extremely painful penile erections lasting hours).

While the Sydney funnel-web spider (Atrax robustus) is the most medically important atracid, as it bites the most people due to its concentration in a heavily populated part of Australia, it is not the largest, which is the northern tree-dwelling funnel-web spider (Hadronyche formidabilis), nor the most toxic, which is the southern tree-dwelling funnel-web spider (H. cerberea).

The deadly element of Australian funnel-web venom are peptides known as δ-hexatoxins (δ-HXTXs), or delta-hexatoxins, that can be fatal as they affect sodium channels in the nervous system, causing nerves to keep firing.

While wandering spider females and males have similar venom, in the funnel-web spiders there is a unique sexual and age-related difference in the venom. Females of any age, as well as juvenile males, rarely venture far from the safety of their burrow, and largely use their venom for killing their arthropod invertebrate prey. In contrast, when males become sexually mature, they stop feeding and leave their burrows in search of females to mate with. At this time, they are now exposed to vertebrate predators. Evolution has selected the makeup of their venom to change at this point from being a prey-selective predatory venom, to a predator-selective defensive venom used against nocturnal mammals such as bandicoots. As an absolute fluke, humans are unusually sensitive to this venom compared to other mammals, which is what makes a bite from an atracid funnel-web particularly severe.

The term LD50 represents the dose of venom that proves lethal to 50% of a test population.