The tubes are decorated

 Terry JenningsPond Life, The Young Scientist Investigates, Chicago, Children Press, 1985, p. 21.

The caddis fly lays its eggs in or near water. The larvae wich hatch from the eggs have a soft body. They protect themselves by making a tube around their bodies. Most caddis larvae make their tubes from sand grains or little pieces of plant material. Often the tubes are decorated with piece of shell or stone. This make it very nard for ennemies of the caddis larvae to spot them. Most kinds of caddis larvae feed on water plants.