William Shipley, A true treatise on the art of fly-fishng, trollin etc., Londres, Simpkin Marshall, 1838.

The food of the grayling, as ascertained by examination, besides the various flies-imitations of which are successfull used by anglers-consists also of the larvae of Phryganea, Ephemera, and Libellula ; the remains of the cases of the former, and the Tough skins of all of them, being frequently found in their stomachs. We have found also several small shells, examples of the Physa and Neritina fluvialis. Dead shells and small pebbles are also found; but whether these last are taken up by the fish to serve any useful purpose, as in the stomachs of gallinaceous birds, or have only formed part of the case of the Phryganea, may be questioned.