T. T. Macan, The Study of Stoneflies, Mayflies and Caddis Flies, volume 17 of the Amateur Entomologist, The Amateur Entomologist’ Society, 1982, p. 31

Most of the case-building make a careful selection of materials, often cutting it to the desired size if it is vegetable, and then stick the pieces to the outside of the tube, either along the long axis, across it or diagonally. The material used depends to some extent on what is available, and many worker have removed a larva from its case and then place dit among materials which the species does not normally use. Most of the larvae tested have proved to be adaptable. Some species change the nature of the material or its arrangement as they increase in size. After ecdysis a larva adds to the front of the case and then bites off the hind end.

H. M. Hanna describes how a larva of Limnephilus politus starts a case by laying down a band of suitable material and then covering it with secretion. It then lies on its back over the band, seizes the ends with its legs, brings them together and glues them, after which material is added to the front in the usual way.  Another method is to spin a loose girdle round the abdomen and cover it with extraneous material. The first part of the case is generally makeshift and its bitten off.