\Un"der*wing`\, n.
1. (Zo["o]l.) One of the posterior wings of an insect.
2. (Zo["o]l.) 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}.