If no other material is at hand

E.G. Boulenger, Richard St. Barbe Baker, L.C. Bushby etc., Nature in Britain: An illustrated Survey, Londres, Batsford T., 1936, p. 125.

Best know of all aquatic fly larva are the caddis worm, so favoured as bait by bottom fishermen. These fly larvae build cylindrical tubes of stick, shells, gravel, etc in which to secrete their unprotected bodies ; the shell-and grit-using species being satisfied in an aquarium to use glass beads for their home-making if no other material is at hand. Thus highly decorative results may be obtained. No jungle is more rife with carnage than the seemingly.