Chitin

from WordNet (r) 3.0 (2006)

chitin
    n 1: a tough semitransparent horny substance; the principal
         component of the exoskeletons of arthropods and the cell
         walls of certain fungi

from The Collaborative International Dictionary of English v.0.48

Chitin \Chi"tin\, n. [See {Chiton}.] (Chem.)
   A white amorphous horny substance forming the harder part of
   the outer integument of insects, crustacea, and various other
   invertebrates; entomolin.
   [1913 Webster]