UNDERWING
underwing
(noun) moth having dull forewings and brightly colored hind wings
Source: WordNet® 3.1
Etymology
Noun
underwing (plural underwings)
A hind wing on an insect.
A member of the genus Catocala, a nocturnal moth which usually has brightly coloured underwings.
(ornithology) The underside of a bird's wing.
Source: Wiktionary
Un"der*wing`, n.
1. (Zoöl.)
Definition: One of the posterior wings of an insect.
2. (Zoöl.)
Definition: 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.
Source: Webster’s Unabridged Dictionary 1913 Edition