Redback spiders
Redback spiders are members of the widely distributed Latrodectus genus known as widow spiders. Other members include the black widow (L. mactans) and brown widow (L. indistinctus) of North America, the thirteen-spotted spider (L. tredecimguttatus) of Europe and the katipo (L. katipo) of New Zealand. In Australia, redbacks vary considerably in numbers from region to region and suburb to suburb. Although widely distributed, they are more common in temperate regions than the colder, southern areas. Redbacks have adapted well to an urban environment, making their homes in sheds and garages, under houses, in industrial areas and in outside toilets. Redback spider bite is thought to be the commonest serious spider bite in Australia, particularly over the summer months. An antivenom was introduced in 1955 and no deaths attributed to treated redback spider bites have been reported since. Prior to this, at least 14 fatalities had occurred.
Female redback spiders are considered the most dangerous. They are easy to detect, because they usually have a red, orange or brownish stripe on their black globular abdomen (upper). A red/orange hourglass shaped spot is also often present on the underside of the abdomen. Juvenile spiders are smaller, more variably coloured and may lack any spots or stripes. Once the female has mated she can use and store the sperm for up to two years. Over this period she produces several batches of eggs, with each egg sac containing 200 to 500 eggs. Only female redbacks spin webs. Male redbacks are much smaller, only 3-4 mm long, and are light brown. Their upper abdominal marking is often white, and like the females, they have the hourglass marking on their underside. Male redbacks tend to have a much shorter life than females because most are eaten by the female during the mating process. The fangs of male redbacks are unable to penetrate the skin of humans to induce envenomation.
Each year, more than 250 cases of redback spider bite require antivenom. Perhaps ten times more people are bitten, but the cases are mild or unrecognized and do not receive antivenom. In humans, males appear to be more frequently bitten than females. This is probably in relation to occupational exposure. Redback spiders are generally shy and retiring and only bite defensively when disturbed. Humans unwittingly desturb them in the garden, shed and in clothing (especially footwear) or sit on them. 75% of redback bites occur on the limbs. The venom acts directly on the nerves, causing the release and subsequent depletion of neurotransmitters.
In Australia, the closest relatives of the redback spider are the brown house spiders (also called cupboard spider or false widow spider) and grey house spiders. Both species are less harmful to humans than redbacks. They are slightly smaller than redbacks and have a similar body shape, but they lack the distinctive upper abdomonal red spot.
Daddy-long-leg spiders and white-tailed spiders are known to catch and kill redbacks.
