The Collaborative International Dictionary of English v.0.48:

Instar \In"star`\, n.
   a postembryonic stage of life of an arthropod, especially an
   insect, between two successive molts; also, the arthropod
   when in that stage of life.
   [PJC]

The Collaborative International Dictionary of English v.0.48:

Instar \In*star"\, v. t.
   To stud as with stars. [R.] "A golden throne instarred with
   gems." --J. Barlow.
   [1913 Webster]

WordNet (r) 3.0 (2006):

instar
    n 1: an insect or other arthropod between molts

Bouvier's Law Dictionary, Revised 6th Ed (1856):

INSTAR. Likeness; resemblance; equivalent as, instar dentium, like teeth;
instar omnium, equivalent to all.