Its collect tiny bits

Kate HarveyMy Book of Insects, Seashore Animals and Fish, Londres, MacMillan and Co, 1944, p. 85.

The most interesting part of the life story of the caddis-fly has to do with the larva. When a larva hatches from an egg, dropped in a little jelly by the female fly, it begin to make a case in which to live. It collects tiny bits of sand and sticks the pieces together round its body by threads of silk, which come from silk-glands below the mouth. The inside of the case is lined with silk. The larva, or caddis-worm, hides most of its soft body Inside the case, but it can push out its hard brown head and six legs. Little knobs and hooks at the end of the body enable it to hold on to the Inside of the case, so that it can walk about dragging its home along as it goes. There are several kinds of caddis-flies, and the caddis-worms make different kinds of cases. Some use bits of sticks, others use tiny stones, and still others weave together pieces of green leaf. All of them line the inside with soft silk.