underwing

noun

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.”

2. A member of the genus Catocala, a nocturnal moth which usually has brightly coloured underwings.
3. 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.”