Field of Science

Tarantulas sans Tarantella

It's interesting how different people perceive levels of risk. Someone once asked how I could be completely unafraid of spiders, but be extremely nervous around cars (I am - a friend of mine once banned me from riding in the passenger seat when she was driving, because the sight of my knuckles turning white as I gripped onto the handlebar would make her nervous). I asked him in return how I could possibly be otherwise - hardly anyone is ever seriously hurt by a spider, but cars kill large numbers of people on a regular basis. The point of that little anecdote, in case you were wondering, is to introduce a family of spiders that have provided stock horror film fodder for years, but are widely known to be fairly harmless - the Theraphosidae.

Theraphosidae are a family of large spiders found mostly in ex-Gondwanan landmasses - South America, Africa, India and Australia, as well as in south-east Asia. These are the spiders best known as bird-eating spiders or tarantulas, though the name "tarantula" originally applied to a member of a quite different family of spiders, the European wolf spider Lycosa tarantula. The photo at the top of the post (from here) shows an Australian species of Selenocosmia. The photo just above this paragraph (from Tarantulas from Uruguay*) of Theraphosa leblondi gives a good idea of the size some theraphosids reach. Theraphosidae include the largest living spiders - indeed, since the Carboniferous Megarachne was reidentified as an eurypterid, modern Theraphosidae include the largest spiders known to have existed ever.

*I rather enjoyed the Tarantulas from Uruguay page, but if you're at work you might want to be forewarned that the page does play music at you.

Theraphosidae belong to the group of spiders known as mygalomorphs. Spiders can be divided into three major groups - liphistiomorphs, mygalomorphs and araneomorphs. Liphistiomorphs are a small group found in eastern Asia that represent the sister group of all other spiders, and can be distinguished from other spiders by their retaining an obviously segmented abdomen. The other two groups of spiders can most easily be distinguished by their chelicerae (fangs). Mygalomorphs retain the more primitive condition of having the fangs directed straight up and down, and so are only able to stab down with them. Araneomorphs, by far the larger and more diverse of the three groups, have the fangs directed towards each other and are able to pinch prey or attackers between the chelicerae (the Wikipedia page for Araneomorphae has a good pair of photos showing the difference). Mygalomorphs are mostly relatively large spiders (there are a few exceptions). They also tend to be far less sexually dimorphic than many araneomorphs, with relatively little difference between males and females.

While the bites of Theraphosidae are apparently not particularly notable as far as humans are concerned, of more concern for people handling tarantulas is the presence on the abdomen of many South American species of urticating hairs - specialised hairs with minute barbs that can break off and irritate the skin of any threatening predators. Members of the subfamily Theraphosinae can even propel the hairs directly at a threat by rubbing the legs against the abdomen. Members of two genera of theraphosids have also been recorded to incorporate shed urticating hairs into the silk of egg-sacs, which was demonstrated to increase the defense offered by the egg-sac against insect egg predators (Marshall & Uetz, 1990).

The South American Avicularia metallica (image from here).

Many species of Theraphosidae are popular as pets, and females may live for up to thirty years in captivity (males, in contrast, do not survive long after mating). Unfortunately, while pet individuals of the more popular species such as the red-kneed tarantula (Brachypelma smithi) are generally captive-bred, a substantial market (in many places, such as Australia, a largely illegal market) exists in wild-caught specimens, especially of rare and unusual species. Many theraphosid species have very limited ranges, and are severely threatened by collection for the pet trade, and I have been informed that at least some Australian species have actually become extinct due to over-collection. This is especially tragic as a large proportion of the Australian theraphosid population remains undescribed, necessitating a race against time to recognise their diversity before the opportunity to protect it is lost forever.


Marshall, S. D., & G. W. Uetz. 1990. Incorporation of urticating hairs into silk: a novel defense mechanism in two Neotropical tarantulas (Araneae, Theraphosidae). Journal of Arachnology 18: 143-149.


  1. Tarantulas terrify me, yet I am fascinated by them. My cousin had a BIG pet tarantula, and I was petrified when he picked it up. I could see those massive fangs a mile away.

    I didn't know there existed a MORE primitive brand of spider--I'll have to look those critters up. I always learn something new at Catalogue of Organisms!

  2. i know someone who worked on spiders but simply can't abide by opilionids.

  3. I was once told a story by a woman who works on spiders in the US of going into a cave and finding a particularly massive conglomeration of Opiliones (opilionids can form clusters of up to several thousand individuals) which icked her out so much she fled the cave.

  4. I'm probably arguing semantics, but although everyone I know would say I was an arachnophobe, but I don't consider it to be a fear as much as an abject repulsion. I don't think the spider is going to eat me, but I am disgusted by the thought of it crawling all over me, and I suspect I would feel the same way about any arthropod of a similar size.

    However, my husband is terrified of wasps. It's not the ick factor, it's not that they make his flesh crawl, it's unadulterated terror. And the main difference is that I can be in the same room as a gigantic house spider (unless it's the bedroom), but Paul cannot be within audible distance of a wasp, and certainly not a hornet.

    My odd "but why aren't you afraid of them?" thing is snakes. No fear of snakes whatsoever, other than when I'm on a horse, because I know a horse is not going to make a rational backing-away-from-the-rattler movement but rather will throw me down that very steep hill.

  5. Your post brings back some very pleasant memories from my childhood (now there's a first sentence you really don't expect to see in a comment about a post on tarantulas). I lived for three years at Death Valley and then five more at Grand Canyon. We (the kids) looked forward to late spring. Why? Because that's then the tarantula boys went out looking to get some.

    We would capture a tarantula (small (2-3 inches across), brown and furry) and convince Mom to take us to the Visitor Center (at Death Valley it was a couple mile drive, but at the canyon we could walk). We then entered the visitor centre and enjoyed the shocked expressions and screams of terror (and the occasional shout of glee) directed toward the tarantula on my shoulder.

    The rangers at the front desk knew us (or knew OF us) and would (as the Park Service is so fond of saying) sieze the interpretive moment and give a short, impromptu talk about the spiders and why they were all wandering around (sex), how dangerous they were (not very (I rate the bite just above a sweat bee but far below a bald faced hornet)), and why they should not be picked up except by people with lots of experience (at which point the ranger would introduce us as NPSBs (National Park Service Brats)).

    I have lived back east since the late 70s and miss the annual tarantula orgies.

    Again, thanks for the good memories.

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