Willy Ley, « Prelude to engineering », Astounding Science-Fiction, vol. XXVII, n° 6, New York, août 1941, p. 115.
The caddis « worm » is the larvae of a small flying insect which belongs to the order Trichoptera and that, if paleontologist guess right, represents the ancestors of the butterflies. The adult insect or imago does nothing especially noteworthy, save for propagating the species. But the larvae-well, that’s a better story. They live in fresh water, as has been indicated, and strongly feel the need for additional protection. They spin a round cases first, shaped like a splender barrel with a bottom. Head and some of the legs protrude from the open end. After that is accomplished the larva really starts working. The silky « barrel » is covered on the outside with extra-organic material, with shells of young freshwater snails or with tiny pebbles, or with pieces of thin dead stalks of underwater plants, all neatly cut to the same lenght and put together to a hexagonal tube. It seems that the larva uses what happens to be around, but once a strengthening material is chosen the larva sticks to its choice. There are no « mixed » caddis-worm cases ; it’s enther sand grains, or snail shells, or miniature lumber.