False colour scanning electron micrograph (SEM) of the tip (foreground) & base (background) of two stinging hairs on the surface of a common nettle leaf, Urtica dioica. The stings are long hairs (short ones are harmless) with silicified tips (intact here) which break off when the nettle is brushed against or stroked. The sting penetrates the skin, injecting a mixture of acetylcholine & histamine; the former a neurotransmitter substance normally produced at nerve terminals; the latter a chemical mediator responsible for many of the unpleasant symptoms of allergic reactions. The result is painful. Magnification: X320 (10x8), X84 (35mm).