Dead organic material

Walter Housley Wellhouse, How insects Live, An Elementary Entomology, New York, MacMillan, 1926, p. 190.

The larvae are called caddis-worms. They have well-developed jaws and devour small animals, plants and bits of dead organic material which float within their reach. Each species makes a special kind of case in which to live and most of the caseds can be dragged about from place to place by the larva within. In moving its case the larva extends its head and thorax outside through the open end and uses the thoracic legs to pull itself along. Some cases are made of grains of sand, some of coarse gravel, some of sticks piled log-cabin fashion, some of leaves which have fallen into the water, and others of silk alone. All are cemented together and lined with silk.