The Collaborative International Dictionary of English v.0.48:

Taenidium \Tae*nid"i*um\, n.; pl. Taenidia. [NL., dim. fr. L.
   taenia a ribbon.] (Zool.)
   The chitinous fiber forming the spiral thread of the tracheae
   of insects. See Illust. of Trachea.
   [1913 Webster]