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Webster 1913 Edition

Noun.

(Zool.)

One of the posterior wings of an insect.

(Zool.)

Any one of numerous species of noctuid moths belonging to

Catocala

and allied genera, in which the hind wings are banded with red and black or other conspicuous colors. Many of the species are called

red underwing

underwing ‎(plural underwings)

A hind wing on an insect.
- 1922, Virginia Woolf, Jacob's Room Chapter 2
  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.
A member of the genus Catocala, a nocturnal moth which usually has brightly coloured underwings.
(ornithology) The underside of a bird's wing.