Scorpion-Tailed Spider from Australia

Spider with prehensile tail.
April 7, 2010
Ok, this is definitely the wackiest spider I have ever seen – it has a prehensile tail!
The spider is about 20mm long, with it’s tail, and sits in it’s web facing downwards, with it’s tail pointing up (picture 3). the spinnarets are in the middle of the body, not at the end of the tail. If it didn’t have a tail, it’d have basically exactly the same body-shape as, and a similar web to an orb spider.
I think it’s an Arachnura (scorpion-tailed spider), like the one here: http://www.biodiversityexplorer.org/arachnids/spiders/araneidae/ . Any idea what the tail is for? Balance?
By the way, that’s some Justifiable Carnage, right there: I don’t like European cockroaches. This spider did though >:)
Cheers
naught101
Newcastle, Australia

Dear naught101,
You are correct.  This is a Scorpion-Tailed Spider, Arachnura higginsi, which is also pictured on the Insects and Spiders of Brisbane website.  This is the first time we have had a Scorpion-Tailed Spider image submitted to our website, and our first attempts at finding out any information have not produced an answer for your questions, though balance doesn’t really seem to be the purpose or more spiders would have this shape.

The spinnerets being located so far from the tip of the abdomen eliminates the possibility that the shape has any web spinning purpose.  Perhaps continued research willl reveal an evolutionary purpose for this odd anatomy.  The need to eat is never a consideration in our determination of Unnecessary Carnage.

Hi Daniel,
thanks for your response. One point:
>”…balance doesn’t really seem to be the purpose or more spiders would have this shape.”
Evolution doesn’t work this way, it’s entirely possible that only this one genus has evolved this attribute. In any case, it must be beneficial in some way, or they would quickly die out, since they seem to inhabit the same niche as orb spiders, and a bunch of other genera. In any case, they do seem to use the “tail” for balance, but that doesn’t explain the strange shape and colouration of the tip of the tail. Perhaps Batesian mimicry?
>”The need to eat is never a consideration in our determination of Unnecessary Carnage.”
I don’t understand what you mean. I was implying Carnage on my part – I caught the cockroach, and fed it to the spider. I’m dreaming of having a massive spider army with a taste for european cockraches 😀
cheers
ned

Hi Ned,
The balance issue is all speculation.  It is often stated that Orbweavers are quite clumsy if they fall from their webs.  They lead a relatively sedentary life.  We do not believe the tail is for balance.  It seems more likely that it would confuse a predator that might strike at a less vital part of the spider’s anatomy.
Our self determined definition of Unnecessary Carnage involves creatures being killed out of fear and disgust, though in a sense, your disgust for the European Cockroach led you to feed it to this spider.  The death was not an end though, and a greater purpose was served.  Thanks so much for your thought-provoking letters.