Frederic Michael Halford , Floating flies and how to dress them: a treatise on the most modern method, New York, Scribner and Welford, 1886, p. 78-79.

ON ARTIFICIAL FLIES

Soon after the eggs are hatched, the grub, commonly know as a “caddis“ commences collecting a most extraordinary conglomeration of all sorts and descriptions of pieces of stone, gravel, stick, sedge, &c., out of which it proceeds to construct a case of such dimensions and proportions, that it is just able to house itself comfortably in it. The tiny atoms of which this case is built are cemented together by a glutinous compound which exudes from the grub itself; the thin end of the case, corresponding to the tail-end of the grub, is closed, and from the other end the head and a considerable portion of the body of the grub may be seen protruding as he is crawling along on the gravel of a shallow, dragging his little house after him. The selection and distribution of the materials of which the case is constructed are so carefully selected that the presence of a single bubble of air will cause the entire structure, with its inmate, to rise and flat, tail upwards, on the surface of the water, while at the smallest alarm the air-bubble is expelled and the grub in its case sinks to the bottom. When about to hatch, the grub closes the head-end of the case, and remains in a torpid state for a short time, after which it crawls out of its house to the bank, and emerges a fully-formed fly.