But how are they made?

Emma C. MacKean, Cinderella And Others Stories, Mc Loughlin, 1943, pp. 37-38.

The Caddis worm and Fly.
Some queer little house i am going to tell you about. The are built at the bottom of the river. But how are they made, and who makes them ? Only a worm, that was at first a tiny green egg, fastened to some stem of a weed. But no sooner is the egg hatched, small as it is, than out comes the worm and begins to build. He would be gobbled up by the fishes if he didn’t. Every one of the family builds droll houses, and no two alike. Some are of dead leaves glued together, and very safe ; some are of grass cut off and put to gether, like a bundle of straw ; a prettier one is made of shells stuck together.
As soon as the house is ready our caddis worm moves in, and hangs himself up by his tail. When he goes for his food, he takes his house with him. When he has eaten his fill, he them makes a little silk door, and hides from everything. This door is a queer thing, a curious net-work all interwoven securely on every side. What he does shut up in the dark nobody knows ; but after awhile he comes out of his little prison-house a beautiful fly with four gorgeous wings.
His home is now in the air, and he has forgotten he ever lived in the water. No more eating, for he has no mouth ; but his life is a short one, and he soon dies