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Bug of the Month: Red-back Spider

Author
by Tim Blackburn
Publish date
1 February 2012
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The Red-back Spider, Latrodectus hasselti, is a type of widow spider. It is closely related to the Black Widow (L. mactans), native to North America, and the Katipo (L. atricus), native to New Zealand.

mature female Red-back This mature female Red-back has a dark-brown body and an orange-red dorsal stripe.
Image: Tim Blackburn
Source: Museum Victoria

First described scientifically in 1870, it was thought that the Red-back may have been a recent arrival to Australia since it was first reported some time after European settlement, from the port town of Rockhampton in central Queensland. Widow spiders can survive for months without food, and this enables them to travel long distances in cargo. The Red-back, however, is considered to be an Australian native by most experts, because of some notably distinctive characteristics that it does not share with overseas widow spiders.

Adult female Red-backs have a body length that is three to four times that of adult males, with females typically being 10-15mm long. Only females possess bright red or red-orange markings. They are usually black (sometimes dark brown) when mature, whereas males are usually light brown with white markings.

Juvenile female Red-back Spider in web Juvenile female Red-backs have different markings and colouration to the adults. This one is resting in the snare of her web.
Image: Patrick Honan
Source: Museum Victoria

Red-backs are found in all but the most inhospitable of Australian environments. They are usually found in their webs which they usually weave close to the ground in dry, sheltered areas, such as under rocks and logs, in junk piles, in sheds and outdoor toilets, and in empty tins and bottles. Electric lights and food scraps in people's houses and other buildings attract moths, flies, cockroaches and mosquitoes, which Red-backs feed on, and this may explain why these spiders prefer to live in and around places of human habitation over natural environments.

Red-back in Bugs Alive exhibition The Bugs Alive! Red-back display demonstrates a kind of habitat that Red-backs prefer. This one is littered with empty cans and containers and is kept relatively dry.
Image: Tim Blackburn
Source: Museum Victoria
 

The web of the female Red-back is an irregular mess of fine but strong silk. It usually contains a funnel-like upper retreat where the spider rests during the day, under which rests a mass of entangled, sticky strands that form a snare held to the ground or a wall by a number of trip-wires. These trip-wires contain globules of glue and are very elastic. When an insect or small vertebrate walks into one, the trip-wire snaps and catapults the victim into the snare above. Then the spider approaches its victim, wraps it in silk and bites it to envenomate and kill it. Male Red-backs do not spin webs and simply feed on prey items they salvage from the edge of the female's web.

Red-back spider feeding on large cockroach The Red-back’s web enables it to catch prey much larger than itself. This immature female is feeding on a cockroach that is more than twice its own body length and also much broader.
Image: Patrick Honan
Source: Museum Victoria
 

The venom of the Red-back is neurotoxic to humans, triggering an uncontrolled release of neurotransmitters – the chemicals that transmit signals between nerve endings. This can cause paralysis in the bite victim when the venom’s action severely depletes the neurotransmitter reserves required for normal muscle function. Most human victims of Red-back bites suffer little more than localised pain and swelling. In severe cases, however, bites can lead to chest and abdominal pain, nausea, vomiting, fever, muscle spasms, convulsions, coma and death (more likely in the young, elderly and frail). Before the development of an antivenom in 1956, at least 12 deaths had been recorded. The antivenom is assumed to have saved many lives as there have been no deaths since it became available - despite an increase in the number of bites reported. This increase is thought to be a consequence of expansion of habitats suitable for Red-backs in the urbanisation of Australia’s cities, and associated increases in human urban populations.

Links:

Red-back Spider infosheet

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