John Clegg, Studying Insects, Ill. E.C. Mansell, Herts, Bruce & Gawthorn, circa 1950, p. 42.

The eggs of caddis are  laid on water plants or stones and when the larvae hatch they spin a silken covering for their soft bodies and attach to that pieces of stick or other material . If you insert the blunt end od a pin into the rear of the caddis case the grub within will usually leaves somewhat reluctantly. If you then provide it with transparent material, such as tiny pieces of mica or celluloid, it will often make a new case out of these material and you will be able to watch not only how it makes the case, but also how, once Inside, it keeps its body moving up and down to draw a current of  water through the case.