The Red Underwing (Catocala nupta) is a moth of the family Noctuidae which contains 25000 species.
This is a large (80mm wingspan) nocturnal European species which, like most noctuids, is drably colored to aid concealment during the day (unless, like the one shown, it chooses a white wall).
This species and other Catacala moths have brightly colored underwings, in this case more orange than red. 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.
As the Red Underwing takes off, the sudden flash of color may confuse the attacker, and when it lands and immediately closes its wings it may seem to disappear as the color is “switched off”.
The adult feeds on nectar, the caterpillars eat willow and poplar leaves.