underwing

noun3

1. •
   A hind wing on an insect.

   • The upper wings of the moth which Jacob held were undoubtedly marked with kidney-shaped spots of a fulvous hue. But there was no crescent upon the underwing. — 1922
2. •
   The underside of a bird's wing.

   • The ‘white-tailed’ kites in the genus Elanus (‘kite’) are small, gull-like, grey-and-white hawks with black forewing patches and varying amounts of black on the underwings. — 2019
3. •
   A member of the genus Catocala, a nocturnal moth which usually has brightly coloured underwings.