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.