Redback Spider Facts

Scientific Name: Latrodectus hasseltii

Redback spider with egg sacs
This female redback spider has two egg sacs.

AlexWang_AU / Getty Images

The redback spider (Latrodectus hasseltii) is a highly venomous spider that is originally from Australia, although it has colonized other regions. Redback spiders are closely related to black widows and females of both species have red hourglass markings on their abdomens. The redback spider also has a red stripe on its back. Redback spider bites may be painful, but are usually not a medical emergency and very rarely fatal.

Fast Facts: Redback Spider

  • Scientific Name: Latrodectus hasseltii
  • Common Names: Redback spider, Australian black widow, red-striped spider
  • Basic Animal Group: Invertebrate
  • Size: 0.4 inches (female); 0.12-0.16 inches (male)
  • Lifespan: 2-3 years (female); 6-7 months (male)
  • Diet: Carnivore
  • Habitat: Australia, New Zealand, Southeast Asia
  • Population: Abundant
  • Conservation Status: Not Evaluated

Description

The female redback spider is easy to recognize. She has a spherical, shiny black (sometimes brown) body with a red hourglass on her underside and a red stripe on her back. Females measure 1 centimeter or 0.4 inches in size. Sometimes all-black females occur. The male is much smaller than the female (3-4 millimeters or 0.12-0.16 inches). He is brown with white marks on his back and a pale hourglass on his underside. Spiderlings start out pale gray with darker spots. After a few molts, juvenile females darken and have the red stripe and hourglass, as well as white abdominal markings.

Male redback spider
The male redback spider is much smaller than the female and colored differently. Wocky / Creative Commons Attribution-Share Alike 3.0

Habitat and Distribution

Redback spiders are originally from Australia and are widespread throughout the country. International shipping has accidentally introduced the species to several other countries, including New Zealand, United Arab Emirates, Japan, New Guinea, the Philippines, India, and England.

The spiders thrive in dry habitats, such as deserts, and areas with human habitation. They build their webs in dark, dry, sheltered areas including rocks, shrubs, mailboxes, under toilet seats, inside tires, around sheds, and in outhouses.

Diet and Behavior

Like other spiders, redbacks are carnivores. They prey upon other spiders (including members of their own species), small snakes and lizards, mice, and wood lice. Juveniles eat fruit flies, cockroach nymphs, and mealworm larvae. Males and juvenile females may feed on an adult female's prey, but are just as likely to become her next meal.

Redbacks build an irregular web with sticky vertical strands and funnel-shaped retreat. The spider spends most of its time in the funnel and emerges to spin or repair its web at night. When a creature becomes ensnared in the web, the spider advances from its retreat, squirts liquid silk onto the target to immobilize it, then repeatedly bites its victim. Redbacks wrap their prey in silk, but do not rotate it during wrapping. Once wrapped, the spider carries its prey back to its retreat and sucks out the liquefied innards. The entire process takes between 5 and 20 minutes.

Reproduction and Offspring

Males are attracted to pheromones on the female's web. Once a male finds a receptive female, he exhibits sexual self-sacrifice, where he inserts his palps into the female's spermathecae (sperm storage organs) and somersaults so his abdomen is over her mouth. The female consumes the male during mating. Not all males mate using this method. Some bite through the exoskeleton of immature females to deliver sperm, so when the female performs her final molt she already contains fertilized eggs. Females can store sperm up to two years and use it to fertilize multiple batches of eggs, but they will accept new mates three months after mating. A female forms four to ten egg sacs, each about 1 centimeter (0.39 inches) round and containing 40 to 500 eggs. A new egg sac can be made every one to three weeks.

Spiderlings hatch after 8 days. They feed from the yolk and molt once before emerging at 11 days. Spiderlings live in the maternal web up to a week, feeding on their mother's prey and on each other. Then, they climb to a high point, produce a silk droplet, and are carried by the wind until their silk sticks to an object. The spiders build their webs and typically stay near the initial landing spot their entire lives. Males mature after instars (developmental molts) and 45-90 days, while females mature after seven or eight instars between 75 and 120 days. Males live six to seven months, while females live two to three years.

Baby redback spiders
Redback spiderlings are gray and resemble small house spiders. Bidgee / Creative Commons Attribution-Share Alike 3.0

Conservation Status

The redback spider has not been evaluated for a conservation status. The species is widespread across Australia. Redback spiders are preyed upon by many species, including the house spider, daddy-long-legs, and cellar spider. If these other spiders are present, redbacks tend to be absent. The use of pesticides to control redbacks is not recommended, as they kill other species and only temporarily control the spider population.

Redback Spiders and Humans

Redback spiders bite between 2,000 and 10,000 people in Australia annually. However, only one human death has been reported since an antivenom became available in 1956. Antivenom isn't actually more helpful than a standard analgesic for most human bites, but is effective for pet and livestock bites. While males bite, they don't cause significant symptoms. Juvenile and adult females can deliver either dry bites or venom. When venom is used, a syndrome called latrodectism occurs. Symptoms appear between an hour to 24 hours and include pain, swelling, and redness from the bite site. Sweating and goosebumps often occur. The bites rarely result in infection, seizure, respiratory failure, or pulmonary edema and never cause tissue necrosis. Redback spider bites are not considered a medical emergency for healthy adults. However, children, pregnant women, and elderly people may seek medical attention. Dogs resist redback venom, but cats, guinea pigs, camels, and horses are susceptible and benefit from the antivenom.

Sources

  • Brunet, Bert. Spiderwatch: A Guide to Australian Spiders. Reed, 1997. ISBN 0-7301-0486-9.
  • Forster, L. M. "The Stereotyped Behavior of Sexual Cannibalism in Latrodectus-Hasselti Thorell (Araneae, Theridiidae), the Australian Redback Spider." Australian Journal of Zoology. 40: 1, 1992. doi:10.1071/ZO9920001
  • Sutherland, Struan K. and James Tibballs. Australian Animal Toxins (2nd ed.). South Melbourne, Victoria: Oxford University Press, 2001. ISBN 0-19-550643-X.
  • Whyte, Robert and Greg Anderson. A Field Guide to Spiders of Australia. Clayton South, VIC, 2017. ISBN 9780643107076.