Drop a little ink near the open end of the case

John Clegg, The Secret World of the Pond, The Project Club, Project Book 013, Londres, Wolfe Publishing, 1970, pp. 38-39.

Watch a caddis larva build its case
Insert the blunt end of a pin into the rear or a caddis case and move it about gently. The larva will then come out. Place it in a dish of water in which there is material similar to that used in its own case. If this includes pieces of stick or plant stem, put long uncut pieces in the dish. The caddis will start building a new case and you will be intigued to see how it seems to measure the pieces so that they are the right lenght. Provide small glass beads or pieces of mica, and the larva will make a transparent case and you will be able to watch the caddis move its body up and down regularly. Drop a little ink near the open end of the case ans see the movements of the larva causing currents of water to flow over its body. This brings fresh supplies of oxygen to the hair-like. Now let the larva crawl back into its own case and return it to the pond.