Tarantula self-defence: the stridulation story

Alright, imagine a spider the size of a dinner plate. Now imagine it raising its legs and hissing like a cat. Terrified? You're in good company. Stridulation is the term for defensive noises made by spiders and other organisms, usually by rubbing two parts of the body together. It is used predominantly by Arthropods, however certain snakes and at least one hedgehog are capable of doing it as well. Though often animals develop stridulation in order to be able to sing for a mate (think of crickets) it has been shown experimentally to have a deterrent effect on predators of all stripes, and who can blame them? In tarantulas, the hissing combined with raised legs and a flash of colour around the fangs should be enough to deter anyone really.

• This just looks like too much trouble to eat

Scary serenading

If you're a tarantula and you're threatened, the first step is trying to convince a potential predator exactly this - that tangling with you is too much trouble. For most mygalomorphs (primitive spiders), the mechanism for doing so lies in a hardened 'file' on the sides of their fangs and a series of stiff hairs on the inside of the palps (the 'plectrum') which they can rub against the file. It's a bit like someone running a wooden spoon down a wash board over and over.

In other spiders, these stridulatory organs may be on other sets of legs, on the spinnerets or the abdomen. And they aren't always used solely for defense in spiders either. Some, such as wolf spiders (Lycosidae) actually use them in finding a partner. For these keen Romeos, all the world is a stage as they send a vibratory love song through the substrate to woo their potential mates.

• I do believe that's the sound of a renaissance man in the distance...

But what if you can't sing?

Many tarantulas of the new world tropics such as this beautiful Avicularia

lack the ability to stridulate. Instead, they have a secret defense a great deal more unpleasant. They are capable of kicking up hairs from a special patch on their abdomens which cause itching and irritation of the skin and mucous membranes. Having experienced this in person, I can confirm its not at all pleasant (it leaves the skin with a burning sensation) and there is some real danger associated with getting these hairs in your eyes.

Others, such as this fiery Psalmopoeus

are simply blazingly fast.

And now you're thinking about venom, aren't you?

For those of you wondering why all these tarantulas don't use venom as a primary defense, the answer is simply: they aren't that venomous. The vast majority of tarantulas' bites would leave you with nothing more than a couple of nasty holes in your hand (or wherever you'd been bitten). Certainly the mechanical damage from one of the 'dinner-plate size' variety would be impressive and take a while to heal but that would be about it. Some species, such as this intricately marked Poecilotheria from India

are reported to have slightly more serious bites, with evidence for neurological symptoms such as sweating and muscle spasms. But in the bigger picture, their bites are nowhere near a cause for concern. And chances are, you would never get to find out anyway. Even the largest or most aggressive of tarantulas are generally trying to go about their nights without human interference. They certainly aren't going to go out of their way to bite you. Like most other spiders, they'll only bite under duress. So the next time you run into one of these beauties, feel free to appreciate it... from a distance ;)

Well, that's it for today
Happy Steeming People,
The Wise Fox

Sources:
https://www.livescience.com/41795-tarantula-bites-harmful.html
https://link.springer.com/article/10.1007%2FBF00293305?LI=true
http://britishspiders.org.uk/bulletin/100301.pdf
https://www.livescience.com/51006-wolf-spiders-vibrate-for-courtship.html
http://www.americanarachnology.org/JoA_free/JoA_v33_n2/arac-033-02-0597.pdf
http://weirdanimalreport.com/article/stridulation-sing-without-mouth

All images are from Wikimedia Commons