The redback spider, Latrodectus hasseltii, is a venomous spider endemic to Australia. It is a member of the worldwide genus Latrodectus, the widow spiders. The female is easily recognisable by her black body with a prominent red stripe on the upper side of her abdomen. Females have a body length of about 1 centimetre (0.39 in), while the male is smaller, being only 3–4 mm (0.12–0.16 in) long.
The redback is one of few arachnids that usually display sexual cannibalism while mating.
The adult female redback has a round body about 1 centimetre (0.4 in) long, with long, slender legs. The body is a deep black colour (occasionally brownish), with a red (sometimes orange) longitudinal stripe on the upper abdomen. On the underside of the abdomen is an hourglass-shaped red/orange streak. Redback spiderlings are grey with dark spots, and become darker with each moult. The female red stripe also develops through this process, starting as a red spot, then multiple spots which later merge. Juvenile females have additional white markings on the abdomen.
The male redback is 3–4 mm (0.12–0.16 in) long and is light brown in colour, with white markings on the upper side of the abdomen and a pale hourglass marking on the underside.
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 female may live between two and three years. Even without food, spiders may survive for an average of 100 days, and sometimes over 300 days. Redbacks can survive temperatures from below freezing point to 40 °C.
Young redback spiders leave the maternal web by being carried on the wind. The spider extends its abdomen high in the air and produces a droplet of silk. The liquid silk is drawn out into a long gossamer thread that, when long enough, carries the spider away. Eventually, the silken thread will adhere to an object where the young spider will establish its own web.
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 its palp (copulatory organ), because the gonads and palp are not internally connected. Unmated sexually mature female redback spiders secrete pheromones including a serine derivative onto their web. This is thought to be the sole method by which males assess a female's reproductive status, and their courtship dismantles much of the pheromone-marked web.
The redback spider is one of only two animals to date 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 that 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 the possibility of future mating for the males, this is not a serious disadvantage, because the spiders are sufficiently sparse that only 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 paired palps in the first mating.
Once the female has mated, the sperm is stored in one or both of her two spermathecae (sperm storage organs). The sperm can be used to lay several batches of eggs, over a period of up to two years (estimated from observations of closely related species) but typically restarts pheremone production advertising her sexual availability about three months after mating. A female spider may lay four to ten egg sacs, each of which averages around 250 eggs, though can be as few as 40 or as many as 500. She can produce a new egg sac as early as one to three weeks after laying a previous one. The spiderlings hatch after about 8 days and emerge from the egg sac as early as 11 days after being laid, but cooler temperatures can significantly slow the development, so that emergence does not occur for months.
The female spider lives in an untidy web in warm sheltered locations, and preys on insects, spiders and lizards that it entraps, but it can live for over 100 days without food. It is preyed upon by other species of spider and parasitoid wasps.
Redbacks usually prey on insects, but can capture larger animals that become entangled in the web, including king crickets, trapdoor spiders, and small lizards. Food scraps and lighting attract insect prey to areas of human activity, which brings their hunters. 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 female accepts the male, it is permitted to feed on the victims snared in the female's web.
The redback stays concealed during the day, and the female spins a web during the night, usually in the same location for most of its adult life. The web is a disorganised, irregular tangle of fine but strong silk. 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 serve two purposes, it snares prey and small insects and can be lifted in the air, and secondly acts as a trip wire to alert the spider to the presence of prey or threats. The horizontal strands, known as guy lines, break when prey thresh around. These webs are usually placed between two flat surfaces, one beneath the other. Rare observations suggest that they occasionally utilize dead leaves to construct a more enclosed nest.
The redback spider's origins are uncertain, and it may have been spread by human activities. The species was known by 1850 in South Australia, only 14 years after European settlement there, but was not reported in early spider collections in other colonies. Because it was first found at seaports, it has even been suggested that the redback may not be native to Australia. It is now found in all but the most inhospitable environments in Australia and its cities. Outside urban areas, the redback is more often found in dryer habitats ranging from sclerophyll forest to desert, even as harsh as the Simpson Desert. 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 under surface of toilet seats are common sites.
The redback is one of the most dangerous species of spider. It has a widespread distribution in Australia, and colonies have also been established in a number of other countries. It is especially found co-located with humans and is responsible for the large majority of Australian spider bites with serious medical consequences. Anti-venom is commercially available, and since its introduction in 1956, no deaths directly due to redback bites have been formally recorded.
Bites from redback spiders produce a syndrome known as latrodectism, with symptoms similar to bites from other Latrodectus spiders. The syndrome is generally characterised by extreme pain. A red spot is usually visible at the puncture site. The bite may be painful from the start, but sometimes only feels like a pinprick or mild burning sensation. Within an hour, victims generally develop more severe local pain with local swelling and sometimes piloerection (goosebumps). Pain, swelling and redness can spread proximally from the site. Systemic envenoming is heralded by swollen or tender regional lymph nodes; associated features include malaise, nausea, vomiting, abdominal or chest pain, generalised sweating, headache, fever, hypertension and tremor. Rare complications include seizure, coma, pulmonary edema, and respiratory failure or localised skin infection. Localised sweating and stiffness may occur at the bite site, and lymphatic involvement may result in lower limb pain.
Populations can be controlled by clearing habitats, squashing the spiders and their egg sacs, and the use of insecticide in outhouses. Some experts recommend against the use of spider insecticides, because of their toxicity, and the fact that redbacks are rapid recolonists.
Some photos and information are provided by Bayer.
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