A demonstration of a Mexican Red-kneed Tarantula (Brachypelma smithi) flicking its urticating hairs. (AKA butt fuzzies.)
Urticating hairs are one of a New World tarantulas main defense mechanisms.
When a tarantula feels threatened it will kick urticating hairs off of its abdomen, aiming them at potential predators. The hairs can cause temporary irritation of the skin. It’s itchy as heck.
Because this is their first defense reaction, they aren’t likely to bite!
A lot of New World tarantulas are very calm and not inclined to shoot their urticating hairs or bite, and this is why they make great pets. They also have very weak venom. There has never been a single death from a tarantula bite, but that doesn’t stop people from making up stories.