The adult female is easily recognised by her spherical black body with a prominent red stripe on the upper side of her abdomen and an hourglass-shaped red/orange streak on the underside.

Mainly nocturnal, the female redback lives in an untidy web in a warm sheltered location, commonly near or inside human residences.

The redback spider has a widespread distribution in Australia, and inadvertent introductions have led to established colonies in New Zealand, the United Arab Emirates, Japan and greenhouses in Belgium.

Predominantly neurotoxic to vertebrates, the venom gives rise to the syndrome of latrodectism in humans; this starts with pain around the bite site, which typically becomes severe and progresses up the bitten limb and persists for over 24 hours.

[7] Reviewing the genus Latrodectus in 1959, arachnologist Herbert Walter Levi concluded that the colour variations were largely continuous across the world and were not suitable for distinguishing the individual species.

This included reclassifying the redback and several other species as subspecies of the best-known member of the group, the black widow spider (Latrodectus mactans), found in North America and other regions.

[10][17] A member of the genus Latrodectus in the family Theridiidae, the redback belongs in a clade with the black widow spider,[10] with the katipō as its closest relative.

[21] The round abdomen is a deep black (occasionally brownish), with a red (sometimes orange) longitudinal stripe on the upper surface and an hourglass-shaped scarlet streak on the underside.

[22] Another species in Australia with a similar physique, Steatoda capensis, has been termed the "false redback spider", but it is uniformly black (or plum), and does not display the red stripe.

Redbacks usually prey on insects, but can capture larger animals that become entangled in the web, including trapdoor spiders, small lizards,[22][36] and even on rare occasion snakes.

[23] Once alerted to a creature becoming ensnared in a trap line, the redback advances to around a leg's length from its target, touching it and squirting a liquid glutinous silk over it to immobilise it.

They follow light and climb to the top of nearby logs or rocks before extending their abdomens high in the air and producing a droplet of silk.

[50] The sperm can be used to fertilise several batches of eggs, over a period of up to two years (estimated from observations of closely related species),[31][51] but typically restarts the female's pheromone production advertising her sexual availability about three months after mating.

[61] It is wide spread throughout urban Australia, with most suburban backyards in the city of Canberra (for instance) having one or more nesting females in such places as firewood piles, stored brick stacks and around unused or restoring motor vehicles as well as generally behind the shed - as observed since at least the 1970s and probably earlier.

Webs are usually built in dry, dark, sheltered sites, such as among rocks, in logs, tree hollows, shrubs, old tyres, sheds, outhouses, empty tins and boxes, children's toys or under rubbish or litter.

This is concerning due to the risks to people being bitten who are unaware of its venomous nature, and also to the conservation of local threatened insect species that the redback might prey upon.

[43] Some redbacks were found in Preston, Lancashire, England, after a container of parts arrived from Australia; some may have escaped into the countryside before pest controllers could destroy them.

[82][83] As the female redback is slow-moving and rarely leaves her web, bites generally occur as a result of placing a hand or other body part too close to the spider, such as when reaching into dark holes or wall cavities.

Bites often also occur when a hidden spider is disturbed in items such as clothes, shoes, gloves, building materials, garden tools or children's outdoor toys.

[94][95] In vertebrates, alpha-latrotoxin produces its effect through destabilisation of cell membranes and degranulation of nerve terminals, resulting in excessive release of neurotransmitters, namely acetylcholine, norepinephrine and GABA.

[101] The syndromes caused by bites from any spiders of the genus Latrodectus have similarities;[102] there is some evidence there is a higher incidence of sweating, and local and radiating pain with the redback, while black widow envenomation results in more back and abdominal pain,[88] and abdominal rigidity is a feature common with bites from the west coast button spider (Latrodectus indistinctus) of South Africa.

[103] One crustacean-specific and two insect-specific neurotoxins have been recovered from the Mediterranean black widow (L. tredecimguttatus), as have small peptides that inhibit angiotensin-1-converting enzyme;[a] the venom of the redback, although little-studied, likely has similar agents.

[103] It appears clinically active against arachnidism caused by Steatoda spiders;[80][115][116][117] however, as these cases are often mild and the evidence of its effectiveness is limited, this treatment is not recommended.

[80] Animal studies also support its use against envenomation from other widow spiders, having successfully been tested against venom from L. mactans, L. hesperus, and L. tredecimguttatus (synonym L. lugubris).

[88] Some subjects with delayed symptoms may present with a characteristic sweating and pain in the lower limbs,[121] generally below the knees,[103] or a burning sensation in the soles of the feet.

[112] Past guidelines indicated two vials, with a further two vials recommended if symptoms did not resolve within two hours, however recent guidelines state "antivenom is sometimes given if there is a history, symptoms and signs consistent with systemic envenoming, and severe pain unresponsive to oral analgesics ... however recent trials show antivenom has a low response rate little better than placebo, and any effect is less than might be achieved with optimal use of standard analgesics.

[103] Nevertheless, it is recommended that an injection of adrenaline be ready and available in case it is needed to treat a severe anaphylactic reaction,[134] and also that the antivenom from the vial be administered diluted in a 100 ml bag of intravenous solution for infusion over 30 minutes.

[83] According to NSW Health, redback spider bites were considered not life-threatening but capable of causing severe pain and systemic symptoms that could continue for hours to days.

Dogs may also suffer vomiting and diarrhea, muscle tremors or clonic contractions, and abdominal wall rigidity, while cats may salivate excessively, protrude their tongue or be overexcitable.

[103] Indigenous Australians in New South Wales mixed the spiders' bodies with the venom of snakes and pine tree gum to form a broth used to coat spear tips.