Broken glass

E. S. BrownLife in Fresh Water, Londres, Oxford University Press, 1955, p. 15.

A caddis-fly larva, having been goaded out of its case by inserting a pin gently into the rear end, is put in clean water in a small vessel with nothing but fragments of broken glass. The larva will usually make a new transparent case out of the glass, through which its movements can be watched. Now and the nit makes undulating movements with its body to drive a current of water over the thread-like gills.