The nature of the fragments

 E. C. H. Day, « The  Neuropters », New York, Scientific American, an Illustrated Journal of Art, Science & Mechanics, vol. XXV, 5 août 1871, p. 82.

So the larva of the stone fly, the caddis worm, living in the water, beneath stones and other shelter, makes itself a little tube, composed of fragments cemented together. The nature of the fragments  will vary according to the species, each species confining itself, it is said, to one kind of material ; some use grains of coarse sand, others particles of broken shells, or minute shells themselves, others, again, bits of wood or short lengths of rushes, or of the stems of grappes. Within such cases, the grubs live, feebly moving about, and seizing such small insect prey as comes unawares within their reach. When fuly grown, they attach their case to some object, close it tightly, and pass into a pupa, or chrysalis stage.