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]

WordNet (r) 3.0 (2006):

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