The Collaborative International Dictionary of English v.0.48:

Subimago \Sub`i*ma"go\, n. [NL. See Sub-, and Imago.]
   (Zool.)
   A stage in the development of certain insects, such as the
   May flies, intermediate between the pupa and imago. In this
   stage, the insect is able to fly, but subsequently sheds a
   skin before becoming mature. Called also pseudimago.
   [1913 Webster]