The redback spider (Latrodectus hasseltii), also known as the Australian black widow is a species of highly venomous spider believed to originate in the South Australian or adjacent Western Australian deserts, but now found throughout Australia, Southeast Asia and New Zealand, with colonies elsewhere outside Australia.
The adult female is easily recognised by her spherical black body with a prominent red stripe on the upper side of her abdomen and an hourglass-shaped red/orange streak on the underside. Females usually have a body length of about 10 millimetres (0.4 in), while the male is much smaller, being only 3 - 4 mm (0.12 - 0.16 in) long.
The redback is mainly nocturnal; the female remains concealed during the day, and spins her web during the night, usually remaining in the same location for most of her adult life. Classified as a gum-footed tangle web, the web is an irregular-looking tangle of fine but strong silk.
The redback and its relatives in the genus Latrodectus are considered dangerous, alongside funnel-web spiders (Atrax and Hadronyche), mouse spiders (Missulena), banana spiders (Phoneutria) and recluse spiders (Loxosceles). Venom is produced by holocrine glands in the spider's chelicerae (mouth parts). Venom accumulates in the lumen of the glands and passes through paired ducts into the spider's two hollow fangs.
It contains a complex mixture of cellular constituents, enzymes and a number of high-molecular-weight toxins, including insect toxins and a vertebrate neurotoxin called alpha-latrotoxin, which causes intense pain in humans.
Female redbacks have an average of around 0.08 - 0.10 mg of venom, and experiments indicate that the median lethal dose (LD50) for mice at room temperature is 10–20% of this quantity (0.27–0.91 mg/kg based on the mass of the mice used), but that it is considerably deadlier for mice kept at lower or higher temperatures. Pure alpha-latrotoxin has an LD50 in mice of 20–40 µg/kg.
Envenomation from a redback spider bite produces a syndrome known as latrodectism. A small but significant percentage of people bitten develop significant pain or systemic symptoms.
Symptoms typically include nausea, vomiting, abdominal or chest pain, agitation, headache, generalised sweating and hypertension.
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