They exhibit a fine symmetry

Curtis C.F. Riley, « Some Phases of Spring Nature Study », School Sciences and Mathematics, Menasha Wisconsin, The Central Association of Science and Mathematics Teachers, vol. 18, juin, 1918, p. 544.

….In the shallower water of the brook, where it forms small ripples as the current flows over the rocks, stones, and gravelly bottom, frequently are found the larvae of caddice fly. In such a location, food can be carried to the larvae by means of the current.  These creatures are observed to be some distance below the surface of the water, atttached to rocks, stones, aquatics plants and the gravelly bed of the stream. Some kinds of caddice fly larvae are more or less in obtaining food. If a number of the larvae are examined, it is noticed that the case, or sheaths, which vary very much in shape, entirely surround their bodies. Careful observation discloses that some of the cases, containing larvae, are not attached to any solid object, and although the sheaths, frequently, are constructed of heavy materials, they are carried about easily, because of the buoyancy of the water. In general, the cases are cylindrical in outline. However, they may be flattened, irregular, rough with sharp projections, smooth, or symmetrical in configuration. The body of each larva is surrounded by a tube, made from a silky material which is produced by the animal itself. This silk-like cylinder a lining within the sheath, which forms the outer wall around the body of the larva.

The cases of the caddice fly larvae are built out of various differents materials, such as portions of leaves, particles of gravel ans stone, and small pieces of wood. Sheaths have been found that were formed from fragments of aquatic plants, the genus Equisetum for example. Some are so constructed that they exhibit a fine symmetry, while others are totally lacking in « this quality ». The various parts of which the cases are composed are fastened together by means of the silk-like material which is spun by the larvae. This substance is somewhat viscid, which property is of decided value in holding together the individual parts of the sheath. The larvae of the caddice fly are often spoken of as caddice worms. They are worm-like, or caterpillar-like, in general appearance.