Belonging to the family Dipluridae, subfamily Macrothelinae, funnel-web spiders are mygalomorphs (the most ancient of all spider groups). This grouping is characterised by having two pairs of lungs and by parallel hinged fangs which move vertically downwards so that the spiders rear up before striking. They are generally ground-dwellers, living in burrows or silk-lined tubes under rocks or logs.

Members of the genus Atrax, of which there are at least nine species, funnel-web spiders are found only in Australia. They generally inhabit coastal rainforests, although some species have adapted to drier inland conditions. The best-known species is the Sydney funnel-web, Atrax robustus, the deadliest of all Australian spiders, occurring from north of Sydney to the north-east of Victoria. This is the only member of the Dipluridae family whose venom is known to be fatal to humans. Usually black, although sometimes a reddish brown in colour, A. robustus has glossy front parts and long, sharp fangs. The male generally grows to about 2.5 cm. The female may grow to 4 cm, although its venom is far less toxic than that of the male.

An effective antivenom was developed by the Commonwealth Serum Laboratories in Melbourne in 1981.