A male brimstone butterfly, Gonepteryx rhamni (Lepidoptera: Pieridae). In this species the males have bright yellow uppersides to their wings which makes them appear yellow when flying. However, when they alight they always fold their wings so only the green undersides show. This gives them the appearance of a green leaf. Another example of camouflage in butterflies.
Photo taken in Chazey Wood, near Caversham, UK, on 2013-04-07.