Bug of the Month: Red-back Spider

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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

Comments (8)

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Ken Walker 1 February, 2012 16:00
Thanks Tim - Well research and well written. A very informative piece. Cheers Ken
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Metung Holiday Villas 1 February, 2012 19:08
Wonderful post!I learned a lot about the Red-back spider. Hope to find more interesting posts from you. Thanks!
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Matthew 2 February, 2012 11:43
I am curious as to whether the bite of male red-back spiders is as venomous as the female's bite given that they have less obvious markings and are presumably therefore harder to spot?
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Tim 4 February, 2012 11:36

Hi Matthew, Thanks for your question. There have only been two recorded cases of bites inflicted by male Red-back Spiders (these were recorded in a 2003 study in the Medical Journal of Australia by Isbister and Gray). One bite was inflicted by a sub-adult and the other by an adult. Neither of the bites had systemic effects, but both caused pain to some degree, with the sub-adult's bite being more effective and inducing severe pain of a somewhat prolonged duration. 

Unfortunately more documented male-inflicted bites are required in order to compare (with reliable statistical analyses) the toxicity of males’ bites with those of females. Bites from males are significantly rarer than bites from females. This is assumed to be due to the male's reduced biting capabilities, which result from its smaller size (and fangs) rather than a less-potent venom. The absence (or pronounced reduction) of aposematic (warning) colouration in males (as compared to females, which possess bright red/red-orange markings) is assumed to be an honest indicator of their less malicious nature. More research would potentially facilitate a more definitive answer to your question.

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Lynne 15 October, 2013 09:15
Hi I was bitten twice during my sleep by a male redback early Wednesday morning. The bites were extremely painful and felt like an ant sting. The pain lasted for four days, and now nearing 7 days since, the area in my arm near the bite is itchy and still sore. My muscle is still sore. I was prescribed pain killers. Hoping the pain goes soon, and am scared to go to sleep at night in the event of another spider bite!
Mcmillans 9 February, 2012 12:17
Hi Tim. I love your post! It's interesting and very informative.
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Mohsen 18 May, 2012 05:27
Hi i want to know how long it takes for a redback spider to turns to adult?
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Tim 20 June, 2012 09:41

Hi Mohsen, maturity is attained after about three months in males and four months in females, but these time frames are dependent on environmental factors such as food availability and average temperature.

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