The correct measurement

Evelyn Cheesman, Insect Behaviour, Londres, P. Allan, 1932, p.71-72

Take the various species of caddis-worms, for instance, which live in water, and makes cases to cover their bodies. The building materials of these cases vary according to the species. In some species the larvae cut pieces of leaf and arrange them tilewise in rows, attaching each fragment separately and securing it with their glutinous saliva Others cover the case with grit, find sand, and pieces of shell, or sometimes whole shells if these are tiny enough. Others cut a hollow stem of the correct measurement and clothe themselves with it by getting inside ; others again arrangesmall stems all pointing back-wards so that they look something like miniature elongated porcupines . Then there are the water-cress caddis-worms which are the bane of water-cress growers, which cover the case with small irregular bits cut out of the leaves. All these caddis-worms are very nearly related, but there are any number of different instincts to direct them in the choice of material for the cases and the special method of constructing them.