The Collaborative International Dictionary of English v.0.48:

Patagium \Pa*ta"gi*um\, n.; pl. Patagia. [L., an edge or
   border.]
   1. (Anat.) In bats, an expansion of the integument uniting
      the fore limb with the body and extending between the
      elongated fingers to form the wing; in birds, the similar
      fold of integument uniting the fore limb with the body.
      [1913 Webster]

   2. (Zool.) One of a pair of small vesicular organs situated
      at the bases of the anterior wings of lepidopterous
      insects. See Illust. of Butterfly.
      [1913 Webster]