Redback Spiders The redback spider (Latrodectus hasselti), also known as the Australian black widow, is a highly venomous spider believed to have originated in Australia. It is now also found in Southeast Asia and New Zealand, with colonies elsewhere outside Australia. Part of the widow spider genus Latrodectus, the adult female is easily identified by her black body with a distinctive red stripe on her abdomen. She typically measures about 10 millimeters in body length, while the male is significantly smaller at only 3–4 mm.
Primarily active at night, the female redback creates a messy web in warm, sheltered areas, often near human dwellings. Her diet includes insects, spiders, and small vertebrates, which she captures using a complex venom injected through her fangs. Prey is immobilised with a glue-like substance from her spinnerets, trussed in silk, and then repeatedly bitten before being taken to her retreat. The male often lives at the edge of the female’s web, scavenging leftovers. Other spiders and parasitoid wasps prey on redbacks, which are known for sexual cannibalism during mating.
After mating, females store sperm in their reproductive organs, allowing them to fertilize eggs over a two-year period, with each clutch averaging 250 eggs contained in a silken sac. While the redback has established colonies in various countries due to accidental introductions, it is one of the few spiders capable of causing significant harm to humans. Its neurotoxic venom can lead to latrodectism, characterized by severe pain and other symptoms. Fortunately, an antivenom has been available since 1956.
Commonly known as “redback” due to the red stripe on its back, the spider is also called by various other names like red-striped spider and jockey spider. Described in 1870 by Swedish arachnologist Tamerlan Thorell, the redback’s taxonomy was debated until DNA analysis clarified its classification. The species, found in the family Theridiidae, is closely related to the black widow. Though there is evidence of interbreeding between redbacks and katipō, they remain distinct species.
The adult female redback has a black, spherical abdomen with a red stripe on top and an hourglass mark underneath. Males are much smaller and light brown with white and pale markings. The false redback spider, Steatoda capensis, resembles the redback but lacks the red stripe.
Nocturnally active, the female spins her web at night, often remaining in one location throughout her life. The web is a tangled structure designed to trap prey and includes a funnel-like area where the spider resides. Prey, often insects, is trapped by vertical threads and hauled into the air. Larger animals like lizards and snakes can also become ensnared. The redback spiderlings feed on appropriate-sized prey such as fruit flies and mealworm larvae. Human activity, which attracts insects, often brings redbacks into close proximity with people.
Redback spiders are fascinating creatures with distinct behaviors and life cycles. When a redback detects prey caught in its web, it approaches cautiously, about a leg’s length away, and immobilizes the victim by encasing it in sticky silk. Unlike other spiders, the redback doesn’t spin its prey while wrapping it. Instead, it injects venom that liquefies the insides of its catch. Once subdued, the spider carries its meal to a safe spot and consumes the liquefied contents, typically within 5 to 20 minutes of the initial capture. Redbacks rarely drink unless deprived of food.
These spiders often engage in prey-stealing, where larger females take food from other spiders’ webs. They may even battle and consume other redbacks, including those of the opposite sex. If a male is accepted by a female, he can feed on her web’s captures. Baby spiders, on the other hand, sometimes steal food from their mother and may even resort to eating their siblings.
In terms of their life cycle, redback spiderlings emerge from eggs after about eight days. Cooler temperatures can delay this process. After hatching, they spend roughly a week in the egg sac, feeding and molting. Baby spiders typically appear between September and January. Males mature quickly, within 45–90 days, while females take 75–120 days. Males live up to seven months, but females can survive two to three years, even without food for extended periods. They are resilient to a wide temperature range, though need a relatively warm summer to reproduce successfully.
Young spiders live in their mother’s web initially, with cannibalism sometimes observed. Eventually, they use a technique called ballooning to leave home, spinning silk threads that carry them away on the wind. Juveniles might cooperate when building webs.
Regarding reproduction, males prepare for mating by creating a sperm web before leaving their mother’s web. They wander in search of a female, guided by pheromones. During mating, males perform a somersault, offering themselves as a meal to the female—a tactic that might prolong copulation and increase egg fertilization. Some males opt for an alternative strategy, mating with juvenile females who possess mature organs but lack entry points, thus bypassing the typical somersault sacrifice.
Females store sperm in their spermathecae, using it to fertilize multiple egg sacs over time. They can produce several sacs, each with many eggs, shortly after mating.
The redback spider is native to Australia but has spread to several other regions, including New Zealand and parts of Asia and Europe. It thrives in dry, dark environments, often near human structures. In urban areas, redbacks are commonly found in sheltered places like sheds, logs, or beneath household items. Managing populations typically involves removing these habitats and physically destroying spiders and egg sacs, though pesticide use is generally discouraged due to its limited effectiveness and potential toxicity.
The redback spider, known for inhabiting areas modified by humans, has spread globally through international shipping. Its ability to endure colder climates means it could settle in many temperate regions beyond Australia. This spread raises concerns about potential risks to unaware individuals who might be bitten by this venomous spider and the potential impact on endangered local insect species.
Redback spiders have also established small colonies in parts of New Zealand, often found in steel or car shipments. Introduced in the early 1980s, they are now present in Central Otago in the South Island and New Plymouth in the North Island. Redback spiders have been a concern in the United Arab Emirates since 1990, and they have formed colonies in greenhouses in Belgium. Isolated sightings suggest they could also be in New Guinea, the Philippines, and India. In England, redbacks were discovered in Preston after a shipment from Australia, and there have been other isolated sightings in the country.
Japan has an established redback population, particularly in Osaka, likely arriving with shipments of wood chips. In 2008, a significant number were found near Fukuoka City’s container terminal. The means of their distribution within Japan remain unclear, but it’s believed they spread by walking or being transported on vehicles. Awareness signs have been put up in parks around Fukuoka City after a bite incident in 2012.
Predators and Parasitoids
The redback spider has natural predators like the black house spider, cellar spider, and giant daddy-long-legs spider, which can deter its presence. The spider wasp Agenioideus nigricornis is a known parasitoid of adult redbacks. Additionally, wasps from the Eurytomidae and Ichneumonidae families, along with mantid lacewings, prey on redback eggs.
Bites to Humans
Incidence
Historically, the redback spider was responsible for more cases of envenomation needing antivenom than any other animal in Australia. By 2017, however, it was linked to around 250 annual envenomations requiring antivenom. Estimates suggest that between 2,000 and 10,000 people are bitten by redbacks each year across Australia. The larger female redback is responsible for most bites, as the male’s smaller size and fangs usually mean less severe outcomes. Female bites occur mostly during warmer months from December to April and usually happen when a person accidentally disturbs the spider’s web. Common situations involve reaching into dark spaces or disturbing hidden spiders in clothing or outdoor items.
A 2004 study indicated that 46% of bites occur on distal limbs, with others on proximal limbs, the trunk, and some on the head or neck. Precautions to avoid bites include wearing gloves and shoes while gardening, not leaving clothes on the floor, and checking shoes and gloves before wearing them. Educating children about not touching spiders can also help prevent bites.
Redback Spider Venom and Antivenom Overview
Redback Spider Venom
The redback spider, a member of the Latrodectus genus, is one of the few spiders considered dangerous to humans, alongside funnel-web, mouse, wandering, and recluse spiders. The venom is created in the spider’s mouth parts and delivered through its fangs. This venom includes complex mixtures like alpha-latrotoxin, which causes severe pain in humans by destabilizing cell membranes and causing excessive neurotransmitter release. This leads to symptoms such as muscle and cardiovascular issues due to acetylcholine and norepinephrine release. Female redbacks carry around 0.08-0.10 mg of venom, with a lethal dose for mice being just a fraction of this quantity. In 2012, the specific vertebrate toxin in redbacks was sequenced, showing similarities with other Latrodectus species.
Antivenom for Redback Spider Bites
The redback antivenom, developed by Commonwealth Serum Laboratories, is created using antibodies from horses inoculated with small venom doses. These antibodies are extracted, treated, and purified into a form that can neutralize redback venom. The antivenom has been safely used even during pregnancy and is clinically active against bites from various widow spiders. However, recent studies suggest its effectiveness is minimal, often comparable to placebo effects, and it’s not recommended for mild cases or envenomation by related species.
Symptoms of Redback Spider Bites
A redback bite can lead to latrodectism, characterized by pain, swelling, and a burning sensation at the bite site, often spreading along the limb. Symptoms like sweating, nausea, and hypertension may develop within hours or up to a day later. Severe reactions can persist for weeks, with rare complications including seizures and respiratory failure. Children, infants, and those with pre-existing conditions are at a higher risk of severe outcomes. Though fatalities are rare, they have occurred in infants and can take much longer in adults. Unlike some other spider bites, redback bites do not cause necrosis but can be misdiagnosed as other medical conditions.
Treatment
The approach to treating redback spider bites depends on how severe the envenomation is. Most cases don’t necessitate medical intervention. If someone experiences localized pain, swelling, or redness, applying ice and taking simple oral pain relievers like paracetamol usually suffice. It’s important not to use pressure immobilization at the wound site, but rather keep the person calm and still.
If the pain continues despite basic pain relief or if systemic symptoms appear, a hospital evaluation is recommended. In some cases, stronger pain relief, like opioid analgesics, might be needed. Historically, antivenom was given for severe cases or when systemic symptoms emerged, such as spreading pain, chest or abdominal pain, or excessive sweating. However, only a small percentage of bites result in envenomation or require antivenom.
Antivenom is typically administered intramuscularly (IM), with intravenous (IV) administration reserved for life-threatening situations. While IM was once thought to be less effective than IV, recent findings suggest the difference is minimal. Despite anecdotal evidence supporting antivenom’s efficacy, controlled studies have shown its effects are comparable to placebo, raising questions about its routine use. Nevertheless, it remains a treatment option, with precautions like having adrenaline ready for any severe allergic reactions.
Prior to antivenom, treatments like benzodiazepines and intravenous calcium gluconate were used, though calcium’s benefits haven’t been proven in trials. Overall, antivenom is considered safe, with a small risk of acute reactions. Most symptoms typically resolve within a week, and fatalities are exceedingly rare since the introduction of antivenom.
Bites to Animals
In animals, redback spider bites are hard to diagnose unless observed directly. Dogs seem relatively resistant, needing treatment only after multiple bites. Cats, however, are more susceptible and often require antivenom. Other animals like guinea pigs, horses, and camels are highly susceptible. Symptoms in animals mirror those in humans but may also include vomiting, diarrhea, and muscle tremors.
Historical Treatment of Bites
Many historical first-aid methods for redback bites, such as making incisions or applying alkaline solutions, are ineffective or dangerous. Treatments like sucking venom out or using strychnine were once common but are discouraged today. Modern first aid advises against incising, sucking, or using tourniquets.
Cultural Impact
Redback spiders have left a mark on Australian culture. Indigenous Australians historically used them in concoctions for spear tips. Slim Newton’s song “The Redback on the Toilet Seat” brought the spider into popular culture, winning a Golden Guitar award. The spider has since inspired sculptures, album titles, and brands, becoming an icon in various Australian symbols and products.
