Redback spider
The redback spider, also known as the Australian black widow, is a species of highly venomous spider believed to originate in Australia, but which is now found in Southeast Asia, Japan and New Zealand. It has also been found in packing crates in the United States 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, while the male is much smaller, being only 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.
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 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, 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, 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.Description
The adult female redback has a body around long, with slender legs, the first pair of which are longer than the rest. The round abdomen is a deep black, with a red 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 scarlet red colours may serve as a warning to potential predators. Each spider has a pair of venom glands, one attached to each of its chelicerae with very small fangs. Small compared to the female, the male redback is 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, and does not display the red stripe.
Behaviour
Web
The redback is mainly nocturnal; the female remains concealed during the day, and spins her web during the night, usually remaining in the same location for most of her adult life. Classified as a gum-footed tangle web, the web is an irregular-looking tangle of fine but strong silk. Although the threads seem random, they are strategically placed for support and entrapment of prey. The rear portion of the web forms a funnel-like retreat area where the spider and egg sacs are found. This area has vertical, sticky catching threads that run to ground attachments. The vertical strands act as trip wires to initially alert the spider to the presence of prey or threats. They also snare and haul prey into the air when weaker horizontal strands that hold them down, known as guy lines, break when prey thrash around. These webs are usually placed between two flat surfaces, one beneath the other. The female spends more time in the funnel and less time moving around during cooler weather.The individual web filaments are quite strong, able to entangle and hold small reptiles.
Prey
Redbacks usually prey on insects, but can capture larger animals that become entangled in the web, including trapdoor spiders, small lizards, and even on rare occasion snakes. One web was recorded as containing a dead mouse. The woodlouse is a particularly common food item. Developing spiderlings need size-appropriate prey, and laboratory studies show that they are willing to consume common fruit flies, mealworm larvae, muscoid flies and early nymphs of cockroaches. Food scraps and lighting attract insect prey to areas of human activity, which brings the redbacks. 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. It then bites its victim repeatedly on the head, body and leg joints and wraps it in sticky and dry silk. Unlike other spiders, it does not rotate its prey while wrapping in silk, but like other spiders, it then injects a venom that liquefies its victim's innards. Once it has trussed the prey, the redback takes it to its retreat and begins sucking out the liquefied insides, generally 5 to 20 minutes after first attacking it. Redback spiders do not usually drink, except when starved.Commonly, prey-stealing occurs where larger females take food items stored in other spiders' webs. When they encounter other spiders of the same species, often including those of the opposite sex, they engage in battle, and the defeated spider is eaten. If a male redback is accepted by a female, it is permitted to feed on the victims snared in the female's web. Baby spiders also steal food from their mother, which she tries to prevent. They also consume sticky silk as well as small midges and flies. Spiderlings are cannibalistic, more active ones sometimes eating their less active siblings.