Which acid is found in nettles and ant stings?

Formic acid, named directly from formica, the Latin for ant - it was first isolated in the seventeenth century by distilling large quantities of crushed ants, which is exactly as unpleasant a piece of chemistry as it sounds.

Nettles are more complicated than the folk explanation. The stinging hairs are silica tubes that snap off to leave a needle and inject a cocktail - histamine, acetylcholine, serotonin and formic acid among others - so the pain is chemical warfare rather than a single acid burn. Which also means the traditional remedy of rubbing a dock leaf on it does very little beyond the cooling and the distraction.

Wood ants spray it as a defence and can do so accurately over a distance - approaching a large nest in a European forest and disturbing it produces a fine acidic mist you can smell. Some birds exploit this by anting: deliberately sitting on a nest and letting ants swarm through their feathers, or rubbing crushed ants over themselves, which appears to work as a parasite treatment.