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.