Males will bite through the exoskeleton and deliver sperm without performing the somersault seen in males mating with adult females. Juvenile female redbacks nearing their final moulting and adulthood have fully formed reproductive organs, but lack openings in the exoskeleton that allow access to the organs. Some redback males have been observed using an alternative tactic that also ensures more of their genetic material is passed on. Although this prohibits future mating for the males, this is not a serious disadvantage, because the spiders are sufficiently sparse that less than 20% of males ever find a potential mate during their lifetimes, and in any case, the male is functionally sterile if he has used the contents of both of his palps in the first mating. The second is females which have eaten a male are more likely to reject subsequent males. The first is the eating process allows for a longer period of copulation and thus fertilisation of more eggs. Sacrifice during mating is thought to confer two advantages to the males. Males which are not eaten die of their injuries soon after mating. In about two of three cases, the female fully consumes the male while mating continues. In the process of mating, the much smaller male somersaults to place his abdomen over the female's mouthparts. The redback spider is one of only two animals known where the male has been found to actively assist the female in sexual cannibalism. It then tries and often succeeds in inserting the other palp into the female's second orifice. This is thought to be the sole method by which males assess a female's reproductive status, and their courtship dismantles much of the pheremone-marked web.ĭuring mating, the male redback attempts to copulate by inserting one of its palps into the one of the female's two spermathecae, each of which has its own insemination orifice.
They are attracted by pheromones, which are secreted by unmated sexually-mature female redback spiders onto their webs and include a serine derivative -2-methylbutyryl-L-serine). How males find females is unclear, and it is possible they may balloon like juveniles.Ī Western Australian field study found that most males took 6 to 8 weeks to travel around 3 to 3.5 metres with occasional journeys of over 8 m, but that only around 11–13% successfully found a mate. The male spider does not eat during this period. Small compared to the female, the male redback is 3–4 mm long and is light brown, with white markings on the upper side of the abdomen and a pale hourglass marking on the underside.Īnother species in Australia with a similar physique, ''Steatoda capensis'', has been termed the "false redback spider", but it is uniformly black, and does not display the red stripe.īefore a juvenile male leaves its mother's web, it builds a small sperm web on which it deposits its sperm from its gonads and then collects it back into each of its two palps, because the gonads and palps are not internally connected.Īfter it moults into its last instar, it sets off wandering to seek a female. Each spider has a pair of venom glands each attached to each of its chelicerae with very small fangs.
The bright red colours may serve as a warning to potential predators. Juvenile females have additional white markings on the abdomen. Redback spiderlings are grey with dark spots, and become darker with each moult. The cephalothorax is much smaller than the abdomen, and is black. The round abdomen is a deep black, with a red longitudinal stripe on the upper surface and an hourglass-shaped red/orange streak on the underside.įemales with incomplete markings or all-black abdomens occasionally occur. The adult female redback has a body around 1 centimetre long, with slender legs, the first pair of which are longer than the rest.
Females have a body length of about 10 millimetres, while the male is much smaller, being only 3–4 mm long.
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