underwing

plural underwings

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 October 26, Virginia Woolf, chapter 2, in Jacob’s Room, Richmond, London: […] Leonard & Virginia Woolf at the Hogarth Press, →OCLC; republished London: The Hogarth Press, 1960, →OCLC

2. A member of the genus Catocala, a nocturnal moth which usually has brightly coloured underwings.
3. biologyThe 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, Stephen Debus, “Small Kites, Genus Elanus”, in Birds of Prey of Australia: A Field Guide, 3rd edition, Clayton South, Vic.: CSIRO Publishing, →ISBN, part II (Handbook), page 113