Small China Mark Cataclysta lemnata
This is yet another species of which the caterpillar lives under the surface of the water: the Small China Mark. Males and females differ extremely. Males are small (wingspan up to 15 mm) and almost entirely white (top picture). Females are much bigger (wingspan up to 24 mm) and brownish (bottom picture). The larvae eat Duck-weed and Water-lily. The species is on the wing from May to October and can be seen all over the British Isles.