Once mated, females lay one or more clusters of approximately 40 eggs in bark crevices or under lichens on the bark. They may fly to another location before depositing additional clusters of eggs. The nun moth embryo completes its development 2-6 weeks after the egg is laid and then enters diapause, a period of arrested development. First and second instars are capable of being dispersed by the wind for considerable distances. Pupation takes place in July.
Hind wings are generally gray-brown with minute dark or light patches, or both, at the edge.
The nun moth female has a wingspan of 45 to 55 mm, and the male, a wingspan of 35 to 45 mm. Color forms to the left are most common in Europe. Only the lightest forms of both sexes are present in Japan. Dark nun moth males can be distinguished from gypsy moth males by the presence of black flecks on the forewings rather than the distinct wavy lines characteristically found on gypsy moth males. Eggs of both the nun moth and gypsy moth are spherical, about 1 mm in diameter and slightly concave in the center.