Redback Spider

The **redback spider** (*Latrodectus hasseltii*) is a species of highly venomous spider believed to originate in the South Australian 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 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. 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 few 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 including New Zealand, the United Arab Emirates, the United Kingdom, Japan and in 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. There have been no deaths directly due to redback bites since its introduction, however Isbister et al. have suggested patients for whom antivenom is considered should be fully informed "there is considerable weight of evidence to suggest it is no better than placebo", and in light of a risk of anaphylaxis and serum sickness, "routine use of the antivenom is therefore not recommended". As of the 2013 (updated 2014) edition of the Snakebite & Spiderbite Clinical Management Guidelines from NSW HEALTH (latest available in 2017), Red-back spider bites were considered not life-threatening but capable of causing severe pain and systemic symptoms that could continue for hours to days.

## Description

The adult female redback has a body around 1 centimetre (0.4 in) long, with slender legs, the first pair of which are longer than the rest. 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. Females with incomplete markings or all-black abdomens occasionally occur. The cephalothorax is much smaller than the abdomen, and is black. Redback spiderlings are grey with dark spots, and become darker with each moult. Juvenile females have additional white markings on the abdomen. The bright scarletred colours may serve as a warning to potential predators. Each spider has a pair of venom glands each attached to each of its chelicerae with very small fangs. Small compared to the female, the male redback is 3–4 mm (0.12–0.16 in) long and is light brown, with white markings on the upper side of the abdomen and a pale hourglass marking on the underside.

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.

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