Oh ye of little faith!! This from Wikipedia:
Besides the normal "hairs" covering the body, some tarantulas also have a dense covering of irritating hairs called urticating hairs, on the opisthosoma, that they sometimes use as protection against enemies.[6] These hairs are present on new-world species but not on specimens from the Old World.
Urticating hairs are usually kicked off the abdomen by the tarantula, but it is noteworthy that some may simply rub the abdomen against the target, such is so with the Avicularia genera. These fine hairs are barbed and serve to irritate. They can be lethal to small animals such as rodents. Some people are extremely sensitive to these hairs, and develop serious itching and rashes at the site. Exposure of the eyes and respiratory system to urticating hairs should be strictly avoided. Species with urticating hairs can kick these hairs off: they are flicked into the air at a target using their back pairs of legs. Tarantulas also use these hairs for other purposes such as to mark territory or to line their shelters (the latter such practice may discourage flies from feeding on the spiderlings). Urticating hairs do not grow back, but are replaced with each moult. The intensity, amount, and flotation of the hairs depends on the species of tarantula. Many[who?] owners of Goliath Birdeaters (T. blondi) claim that theraphosids have the worst urticating hairs.[citation needed].
To predators and other kinds of enemies, these hairs can range from being lethal to simply being a deterrent. With humans, they can cause irritation to eyes, nose, and skin, and more dangerously, the lungs and airways, if inhaled. The symptoms range from species to species, from person to person, from a burning itch to a minor rash. In some cases, tarantula hairs have caused permanent damage to human eyes.[7]
Before biting, tarantulas may signal their intention to attack by rearing up into a "threat posture", which may involve raising their prosoma and lifting their front legs into the air, spreading and extending their fangs, and (in certain species) making a loud hissing by stridulating. Their next step, short of biting, may be to slap down on the intruder with their raised front legs. If that response fails to deter the attacker, the tarantulas of the Americas may next turn away and flick urticating hairs toward the pursuing predator.
New-world tarantulas (those found in North and South America) are equipped with urticating hairs on their abdomen, and will almost always throw these barbed hairs as a first line of defense. These hairs will irritate sensitive areas of the body and especially seem to target curious animals who may sniff these hairs into the mucous membranes of the nose. Some species have more effective urticating hairs than others. The Goliath Birdeater is one species known for its particularly irritating urticating hairs. Urticating hairs can penetrate the cornea so eye protection should be worn when handling such tarantulas.[
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