The redback spider (Latrodectus hasselti), also known as the Australian black widow, is a species of highly venomous spider believed to originate in South Australia or adjacent Western Australian deserts, but now found throughout Australia, Southeast Asia and New Zealand, with colonies elsewhere outside Australia. It is a member of the cosmopolitan genus Latrodectus, the widow spiders. The adult female is easily recognized 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. Females usually have a body length of about 10 millimetres (0.4 in), while the male is much smaller, being only 3–4 mm (0.12–0.16 in) long.

Mainly nocturnal, the female redback lives in an untidy web in a warm sheltered location, commonly near or inside human residences. It preys on insects, spiders and small vertebrates that become ensnared in its web. It kills its prey by injecting a complex venom through its two fangs when it bites, before wrapping them in silk and sucking out the liquefied insides. Often, it first squirts its victim with what resembles 'superglue' from its spinnerets, immobilising the prey by sticking the victim's limbs and appendages to its own body. The redback spider then trusses the victim with silk. Once its prey is restrained, it is bitten repeatedly on the head, body and leg segments and is then hauled back to the redback spider's retreat. Sometimes a potentially dangerous victim can be left to struggle for hours until it is exhausted enough to approach safely. Male spiders and spiderlings often live on the periphery of the female spiders' web and steal leftovers. Other species of spider and parasitoid wasps prey on this species. The redback is one of a number of arachnids that usually display sexual cannibalism while mating.

After mating, sperm is stored in the spermathecae, organs of the female reproductive tract, and can be used up to two years later to fertilise several clutches of eggs. Each clutch averages 250 eggs and is housed in a round white silken egg sac. 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.

The redback is one of the few spider species that can be seriously harmful to humans, and its liking for habitats in built structures has led it to being responsible for a large number of serious spider bites in Australia. 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. Sweating in localised patches of skin occasionally occurs and is highly indicative of latrodectism. Generalised symptoms of nausea, vomiting, headache, and agitation may also occur and indicate severe envenomation. An antivenom has been available since 1956.

Animal name origin

The common name "redback" is derived from the distinctive red stripe along the dorsal aspect of its abdomen. Other common names include red-striped spider, red-spot spider, jockey spider, Murra-ngura spider, Kapara spider and the Kanna-jeri spider.

In Culture

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. Slim Newton drew popular attention to redbacks with his song "The Redback on the Toilet Seat", which won the Golden Guitar at the first Country Music Awards of Australia in 1973. Newton recalled an occasion when a friend used his outside toilet where the light globe had blown and reported he was lucky there was not a redback spider on the toilet seat. The phrase inspired him to write the song. A sculpture of an impossibly large redback, one of Australia's big things, was built in 1996 at Eight Mile Plains, Queensland. The Angels 1991 album Red Back Fever takes its name from the spider. Matilda Bay Brewing Company produces a wheat beer called Redback, with the distinctive red stripe as the logo. The redback appears in the name and emblem of the South Australia cricket team. The Airborne Redback, an Australian ultralight trike, was also named after the spider. Redback Boots is an Australian workboot manufacturing company, which uses the spider in its name and logo. In 2006 a redback spider postage stamp was designed as part of a "Dangerous Australians" stamp series, but was withheld from general circulation by Australia Post due to concerns that the realistic depiction would scare people opening their letter boxes. In 2012 an episode of the children's TV show Peppa Pig in which the title character picks up and plays with a spider was banned from Australian television due to fears that it would encourage children to pick up and play with redback spiders.

Distribution

Geography

Continents

Oceania, Asia

Countries

Australia, New Zealand

Introduced Countries

Japan

WWF Biomes

Desert and Xeric Shrublands, Tropical moist forests

The redback spider is widespread across Australia. The current distribution reported by the World Spider Catalogue includes Southeast Asia and New Zealand. Colonies and individuals have been found elsewhere, including Japan, England, Belgium, the United Arab Emirates and Iran. It was believed at one time that the redback may have been introduced to Australia, because when it was first formally described in 1870, it appeared to be concentrated around sea ports. However, an earlier informal description (1850) from the Adelaide Hills is now known, and names in Australian Aboriginal languages also show that it was present well before European settlement. Its original range is thought to be a relatively small arid part of South Australia and Western Australia. Its spread has been inadvertently aided by modern buildings, which often provide habitats conducive to redback populations. The close relationship between the redback and the New Zealand katipō also supports the native status of both in their respective countries.

Outside urban areas, the redback is more often found in drier habitats ranging from sclerophyll forest to desert, even as harsh as the Simpson Desert. It became much more common in urban areas in the early decades of the 20th century, and is now found in all but the most inhospitable environments in Australia and its cities. It is particularly common in Brisbane, Perth and Alice Springs. 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. The redback spider is commonly found in close proximity to human residences. 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. Letterboxes and the undersurface of toilet seats are common sites. Populations can be controlled by clearing these habitats, squashing the spiders and their egg sacs, and using pesticide in outhouses. The CSIRO Division of Entomology recommends against the use of spider pesticides due to their toxicity, and because redbacks are rapid recolonists anyway.

Spiders in the French territory of New Caledonia in the Pacific were identified as L. hasselti in 1920, based on morphology. Their behaviour differs from Australian redbacks, as they do not engage in sexual cannibalism and are less prone to biting humans. The first recorded envenomation in New Caledonia was in 2007.

Biome

Climate zones

Tropical

Temperate

Arid

Venom

The redback and its relatives in the genus Latrodectus are considered dangerous, alongside funnel-web spiders (Atrax and Hadronyche), mouse spiders (Missulena), banana spiders (Phoneutria) and recluse spiders (Loxosceles). Venom is produced by holocrine glands in the spider's chelicerae (mouth parts). Venom accumulates in the lumen of the glands and passes through paired ducts into the spider's two hollow fangs. The venom of the redback spider is thought to be similar to that of the other Latrodectus spiders. It contains a complex mixture of cellular constituents, enzymes and a number of high-molecular-weight toxins, including insect toxins and a vertebrate neurotoxin called alpha-latrotoxin, which causes intense pain in humans.

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. Excess neurotransmitter activity leads to clinical manifestations of envenomation, although the precise mechanisms are not well understood. Acetylcholine release accounts for neuromuscular manifestations, and norepinephrine release accounts for the cardiovascular manifestations. Female redbacks have an average of around 0.08–0.10 mg of venom, and experiments indicate that the median lethal dose (LD50) for mice at room temperature is 10–20% of this quantity (0.27–0.91 mg/kg based on the mass of the mice used), but that it is considerably deadlier for mice kept at lower or higher temperatures. Pure alpha-latrotoxin has an LD50 in mice of 20–40 μg/kg.

The specific variant of the vertebrate toxin found in the redback was cloned and sequenced in 2012, and was found to be a sequence of 1180 amino acids, with a strong similarity to the equivalent molecule across the Latrodectus mactans clade. The syndromes caused by bites from any spiders of the genus Latrodectus have similarities; 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, and abdominal rigidity is a feature common with bites from the west coast button spider (Latrodectus indistinctus) of South Africa.

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; the venom of the redback, although little-studied, likely has similar agents.

Mating Habits

MATING BEHAVIOR

Polyandry

Spiderlings hatch from their eggs after about 8 days and can emerge from the egg sac as early as 11 days after being laid, although cooler temperatures can significantly slow their development so that emergence does not occur for months. After hatching they spend about a week inside the egg sac, feeding on the yolk and molting once. Baby spiders appear from September to January (spring to early summer). Male spiders mature through five instars in about 45–90 days. Females mature through seven–eight instars in about 75–120 days. Males live for up to six or seven months, while females may live between two and three years. Laboratory tests have shown that redbacks may survive for an average of 100 days, and sometimes over 300 days without any food, those starved at 10 °C (50 °F) faring better than those kept without food at 25 °C (77 °F). Spiders are known to reduce their metabolic rates in response to starvation, and can distend their abdomens to store large amounts of food. Redbacks can survive temperatures from below freezing point to 40 °C (104 °F), though they do need relatively warm summers, with temperatures of 15 to 25 °C (59 to 77 °F) for two to three months, to survive and breed.

Redback spiderlings cohabit on the maternal web for several days to a week, during which time sibling cannibalism is often observed. They then leave by being carried on the wind. 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. The liquid silk is drawn out into a long gossamer thread that, when long enough, carries the spider away. This behaviour is known as ballooning or kiting. Eventually, the silken thread will adhere to an object where the young spider will establish its own web. They sometimes work cooperatively, climbing, releasing silk and being carried off in clusters. Juvenile spiders build webs, sometimes with other spiders.

Before a juvenile male leaves its mother's web, it builds a small sperm web on which it deposits its sperm from its gonads and then collects it back into each of its two palps (copulatory organs), because the gonads and palps are not internally connected. After it moults into its last instar, it sets off wandering to seek a female. The male spider does not eat during this period. How males find females is unclear, and it is possible they may balloon like juveniles. A Western Australian field study found that most males took 6 to 8 weeks to travel around 3 to 3.5 metres (9.8 to 11.5 ft) with occasional journeys of over 8 m (26 ft), but that only around 11–13% successfully found a mate. They are attracted by pheromones, which are secreted by unmated sexually mature female redback spiders onto their webs and include a serine derivative (N-3-methylbutyryl-O-(S)-2-methylbutyryl-L-serine). This is thought to be the sole method by which males assess a female's reproductive status, and their courtship dismantles much of the pheremone-marked web.

During mating, the male redback attempts to copulate by inserting one of its palps into the one of the female's two spermathecae, each of which has its own insemination orifice. It then tries and often succeeds in inserting the other palp into the female's second orifice. The redback spider is one of only two animals known where the male has been found to actively assist the female in sexual cannibalism. In the process of mating, the much smaller male somersaults to place his abdomen over the female's mouthparts. In about two of three cases, the female fully consumes the male while mating continues. Males which are not eaten die of their injuries soon after mating. Sacrifice during mating is thought to confer two advantages to the species. The first is the eating process allows for a longer period of copulation and thus fertilisation of more eggs. The second is females which have eaten a male are more likely to reject subsequent males. Although this prohibits future mating for the males, this is not a serious disadvantage, because the spiders are sufficiently sparse that less than 20% of males ever find a potential mate during their lifetimes, and in any case, the male is functionally sterile if he has used the contents of both of his palps in the first mating.

Some redback males have been observed using an alternative tactic that also ensures more of their genetic material is passed on. Juvenile female redbacks nearing their final moulting and adulthood have fully formed reproductive organs, but lack openings in the exoskeleton that allow access to the organs. Males will bite through the exoskeleton and deliver sperm without performing the somersault seen in males mating with adult females. The females then moult within a few days and deliver a clutch of fertilised eggs.

Once the female has mated, the sperm is stored in one or both of her spermathecae. 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), but typically restarts the female's pheromone production advertising her sexual availability about three months after mating. A female spider may lay four to ten egg sacs, each of which is around 1 cm (0.39 in) in diameter and contains on average around 250 eggs, though can be as few as 40 or as many as 500. She prepares a shallow concave disc around.mw-parser-output.frac{white-space:nowrap}.mw-parser-output.frac.num,.mw-parser-output.frac.den{font-size:80%;line-height:0;vertical-align:super}.mw-parser-output.frac.den{vertical-align:sub}.mw-parser-output.sr-only{border:0;clip:rect(0,0,0,0);clip-path:polygon(0px 0px,0px 0px,0px 0px);height:1px;margin:-1px;overflow:hidden;padding:0;position:absolute;width:1px}3 mm (1⁄8 in) in diameter before laying eggs into it over a period of around five minutes before laying more silk to complete the sac, which becomes spherical, the whole process taking around one and a quarter hours. She can produce a new egg sac as early as one to three weeks after her last.