Latrodectus hasselti, a venomous spider found throughout Australia, related to the black widow spider of America and the katipo spider of New Zealand.
These spiders have are black or brown in colour and have a red or orange stripe on their back with an "hourglass" shaped red/orange spot on the bottom of their abdomen. Adult male spiders reach about 3mm in size and females are usually up to three times larger.
Female spiders eat males after mating. After mating, and the subsequent dinner, she spins up to 8 round balls of web for her eggs, each containing as many as 300 eggs which hatch in about 2 weeks time. Not all reach maturity as the red backs are known for cannibalism.
Mum builds the web in dry sheltered areas such as under rocks, in logs, junk-piles, sheds and dunnies. It consists of a funnel shaped retreat area with sticky threads running vertically to ground attachments.
Unlike the Funnelweb spider, the Red-back is not aggressive but will guard her eggs fiercely or will bite if trapped in clothing or rubbish being collected.
The common culprit in red back spider bites are female spiders as male spiders are too small to bite humans effectively. The bite can be fatal but no deaths have occurred since 1956 when antivenom was successfully introduced.
Red back spider venom toxin is a protein that depletes either acetylcholine, catecholamines (or both) at nerve endings. The mechanism of pain is not fully understood.
The symptoms vary according to the amount of venom injected, the vascularity of the bite site and the age, size and health of the victim.
Bites are commonly on the extremities. Bitten humans usually suffer an immediate painful bite, though occasionally it may only be felt as a mild sting. Local sweating often develops. The pain and sweating usually worsen and spread proximally. Lymphadenopathy and tenderness of the draining lymph nodes may occur. As the venom spreads (through the lymphatic system), nausea, vomiting and headache may occur as can progressive muscular weakness, migratory joint pains and abdominal pain.
Without treatment, in non-fatal cases of red back envenomation, resolution of muscle spasm and weakness may take months.
Since the advent of modern sewage systems, there have been no recorded cases of people being bitten on their butts when sitting on a toilet seat.