Tarantulas can and do bite people. However, they only bite when they feel threatened, and only as a last resort. Most tarantulas in the Americas have barbed hairs that they use defensively before biting. Species from other parts of the world lack these hairs and are more aggressive.
Tarantulas make threat displays and attempt to flee before biting a human being. Even when they do bite, they often perform "dry" bites, or bites that lack venom. Venom is adapted for subduing prey and is not often used as a defensive measure. While the effects of tarantula venom vary widely by species, those native to the United States do not have venom that is dangerous. Their bite is comparable to a bee sting. Other species, such as the African tarantula Pelinobius muticus, can cause strong muscle cramps and hallucinations.