underwing

from WordNet (r) 3.0 (2006)

underwing
    n 1: moth having dull forewings and brightly colored hind wings

from The Collaborative International Dictionary of English v.0.48

Underwing \Un"der*wing`\, n.
   1. (Zool.) One of the posterior wings of an insect.
      [1913 Webster]

   2. (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}.
      [1913 Webster]