What is the commonest British butterfly? Despite years of mapping and monitoring there is no agreed answer to this simple question, but it may be the Small Tortoiseshell, Aglais urticae. The most recent butterfly distribution maps (Emmet & Heath 1989) show that, in terms of the number of 10km squares occupied and geographical spread across the country, the Small Tortoiseshell is equal first with the Green-veined White, Pieris napi.