The Collaborative International Dictionary of English v.0.48:

Caseworm \Case"worm`\, n. (Zool.)
   A worm or grub that makes for itself a case. See Caddice.
   [1913 Webster]

WordNet (r) 3.0 (2006):

caseworm
    n 1: insect larva that constructs a protective case around its
         body