Nettle stinging hairs. Coloured scanning electron micrograph (SEM) of stinging hairs on a female flower of a stinging nettle (Urtica dioica). Stinging nettles are covered in long poisonous hairs which cause pain and inflammation when brushed against. They inject a mixture of acetylcholine, a neurotransmitter which activates nerves, and histamine, which is produced in allergic reactions. The shorter hairs are harmless. Nettles are a common plant of wasteland and grassland, and have these hairs as a protection against herbivores. Magnification: x95 at 6x7cm size. x150 at 4x5"~PORTRAIT"