Red underwing

This is a large (80 mm wingspan) nocturnal European species which, like most noctuids, is drably coloured to aid concealment during the day. It flies in August and September, and comes freely to both light and sugar.
This species and other Catocala moths have brightly coloured underwings, in this case orange, red, or pink. These are not visible at rest, being hidden by the dull forewings, but they help the moth avoid predators such as birds if it is disturbed during the day.