Cut into a rectangle

Helen MellanbyAnimal Life in Fresh Water, Londres, Methuen & Co, 1938, p. 187.

They build cases (Fig.138b) of plant material, such as pieces of roots, leaves, or reeds. Each piece of plants is cut into a rectangle, and then a case is made out of these by arranging them in a spiral to form a cylinder, open at both ends, the spiral twisting to the left. In young larvae the case may be slightly conical.  The larvae may easily be induced to leave their cases by poking them from the hind end ; if the case is then removed and the larva supplied with some leaves it will quickly make a new case.