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).
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.
REFERENCES
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.