Tarantula hair. Coloured scanning electron micrograph SEM of hair that has been shed by a Mexican red_kneed tarantula Brachypelma smithi. Tarantulas are poisonous spiders. This tarantula has a defence mechanism that involves flicking hairs from its legs the knee hairs are red. The hairs are intensely irritating to human skin. This view shows how the hairs are loosely attached to the skin. The hairs are also used to sense motion and noise, vibrating to alert the spider. B. smithi has a body length of around 10 centimetres. It is nocturnal, and it is found in the deserts of Mexico and the southern USA.