A piece of dead stick

Cholmondeley H. Pennell, The Modern Practical Angler, Londres, George Routledge, 1884, p. 231

The caddice (or as it name is sometimes abbreviated- « cad bait ») is the larva of the phryganea, of which there are many species, furnishing several of the insect most popular for imitation by the artificial fly.
In the larval state it is found in the banks of most streams amongst gravel, decayed red-rots, and other aquatic debris . It is envelopped in a shell or case, generally cylindrical, resembling commonly a piece of dead stick or rush, and from this is of course necessary to disencumber it before use.