Redback spider

Appearance

Females have a body length of about 10 millimetres, while the male is much smaller, being only 3–4 mm long.

The adult female redback has a body around 1 centimetre 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 red/orange 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 red 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 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.

Naming

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.

Distribution

The redback spider is probably native to Australia; however, it has been suggested that it may have been spread to Australia by human activities, because it was first found at seaports. Queensland researchers Robert Raven and Julie-Ann Gallon also argued its preference for habitats altered by humans to be further evidence of this.

Spider expert Barbara York Main queried that were it indeed introduced, it would be odd that Australia was missing from the worldwide distribution of "Latrodectus" species. The redback's close relationship with the New Zealand katipo also supports the native status of both in their respective countries.

Reproduction

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, 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 with occasional journeys of over 8 m, 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 -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 males. 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, 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 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 3 mm 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.

Predators

The black house spider, daddy-long-legs spider and the giant daddy-long-legs spider are known to prey on the redback spider, and redbacks are often absent if these species are present in significant numbers. "Agenioideus nigricornis", a spider wasp, is a parasitoid of the adult redback. Other wasps of the families Eurytomidae and Ichneumonidae parasitise redback eggs, and mantid lacewings prey on redback eggs.

Defense

The redback and its relatives in the genus "Lactrodectus" are among the most dangerous spider genera, alongside funnel-web spiders, mouse spiders, banana spiders and recluse spiders.

Venom is produced by holocrine glands in the spider's chelicerae. 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 number of high-molecular-weight toxins, including insect toxins and a vertebrate neurotoxin called alpha-latrotoxin, which is active 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 the clinical manifestations of envenomation, although the precise mechanisms are not well-understood. Female redbacks have an average of around 0.08–0.10 mg of venom, and experiments indicate that the median lethal dose for mice at room temperature is 10–20% of this quantity, but that it is considerably more deadly 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 "Latrodectus" genus 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 of South Africa.

Cultural

Indigenous Australians in New South Wales mixed the venom with that of snakes and pine tree gum to form a broth used to coat spear tips.