Tarantulas, spiders of the family Theraphosidae, are the largest, longest-lived, and most docile spiders. They range in size from 2.5-7.5 cm, and females can live up to 35 years, while males only live 5-7 years. They are found in most tropical and subtropical regions of the world where they often live in burrows. Tarantulas are very territorial, have poor eyesight, and use vibrations to sense their prey.

Tarantula venom is rarely toxic to humans, but it can be fatal to dogs and other animals. Tarantulas rarely bite, but if threatened and unable to retreat, they will defend themselves. The bite is often painful, but the venom typically only causes a local histamine reaction. Joint stiffness lasting weeks has been reported.

Tarantula hairs, not bites, are the cause of most human injury. Tarantulas, when threatened, are able to flick off thousands of hairs from their abdomen toward their aggressor. These barbed hairs can mechanically irritate the skin, cornea, and, occasionally, the respiratory tract.

When tarantula hairs penetrate the skin, they can cause a severe pruritic reaction with localized edema and erythema (pruritic papules). The itching can last for weeks. Anaphylaxis has been rarely reported. The degree of reaction is dependent on the type of tarantula hairs. Type I hairs, found in all tarantulas within the United States, do not penetrate as deep as Type III hairs found on Caribbean and Central and South American species. Type II hairs are not flicked off but rather used to build silk webs, while Type IV hairs from one species in South America, Grammostola, has been shown to irritate the respiratory tract in small animals and potentially cause asphyxiation.

If the hairs penetrate the cornea, they initially will cause pain and pruritus. If not removed quickly, the hairs can migrate farther into the eye and can cause disease ranging from conjunctivitis or anterior chamber inflammation to ophthalmia nodosa and panuveitis.

Although not common, inhalation of tarantula hairs may cause allergic rhinitis.