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Redback spider

Species of spider

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A redback spider ( Latrodectus hasseltii ) female, found in suburban Sydney, NSW, Australia. (close crop)
Toby Hudson · CC BY-SA 3.0

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Through Species, Australia and Japan

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, and Japan.

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.

Taxonomy and naming

Common names

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.

History

Before DNA analysis, the taxonomy of the widow spider genus Latrodectus had been unclear—changes in the number of species reflect the difficulty of using morphology to determine subdivisions within the genus. Substantial interest in their systematics was most likely prompted by the medical importance of these venomous spiders. Swedish arachnologist Tamerlan Thorell described the redback spider in 1870 from specimens collected in Rockhampton and Bowen in central Queensland. He named it Latrodectus hasseltii in honour of colleague A.W.M. van Hasselt. In the same paper, he named a female from Cape York with an all-black abdomen L. scelio, now regarded as the same species. These specimens are in the Naturhistoriska Riksmuseet in Stockholm. Between 1872 and 1972, all subsequent authors spelled the species name as hasselti, and that altered spelling is maintained today.

German arachnologist Friedrich Dahl revised the genus in 1902 and named L. ancorifer from New Guinea, which was later regarded as a subspecies of the redback. Another subspecies, L. h. aruensis, was described by Norwegian entomologist Embrik Strand in 1911. Subspecies indica (of L. scelio) had been described by Eugène Simon in 1897, but its origin is unclear. Frederick Octavius Pickard-Cambridge questioned Dahl's separating species on what he considered minor anatomical details but Dahl dismissed Pickard-Cambridge as an "ignoramus". Pickard-Cambridge was unsure whether L. hasselti warranted species status, though he confirmed scelio and hasselti as a single species, other researchers such as Ludwig Carl Christian Koch noting the differences to be inconsistent. The redback was also considered by some to be conspecific with the katipō (L. katipo), which is native to New Zealand, though Koch regarded them as distinct.

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. Instead, he focused on differences in the morphology of the female sexual organs, and revised the number of recognised species from 22 to 6. 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. He did not consider the subspecies L. h. ancorifer, L. h. aruensis and L. h. indicus distinct enough to warrant recognition. Subsequently, more reliable genetic studies have split the genus into about 30 species, and the redback has no recognised subspecies in modern classifications.

Placement

A member of the genus Latrodectus in the family Theridiidae, the redback belongs in a clade with the black widow spider, with the katipō as its closest relative. A 2004 molecular study supports the redback's status as a distinct species, as does the unique abdomen-presenting behaviour of the male during mating. The close relationship between the two species is shown when mating: the male redback is able to successfully mate with a female katipō producing hybrid offspring. However, the male katipō is too heavy to mate with the female redback, as it triggers a predatory response in the female when it approaches the web, causing the female to eat it. There is evidence of interbreeding between female katipō and male redbacks in the wild.

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