The Collaborative International Dictionary of English v.0.48:

Osmaterium \Os`ma*te"ri*um\, n.; pl. Osmateria. [NL., fr. Gr.
   ? smell.] (Zool.)
   One of a pair of scent organs which the larvae of certain
   butterflies emit from the first body segment, either above or
   below.
   [1913 Webster]