Particles of quartz sand

A. S. Moffat, The Secrets of Angling, Edimbourg, A. & C. Black, 1865, pp. 199-202.

Caddis, Caddies, or Cad-baits, are the nymphae or larvae of various species of aquatic insects, whivh, on issuing from the ovum, immediately commence to envelop themselves in artificial cases constructed of bits of straws, rush, loose particles of sand and fine gravel, which they contrive in a very wonderful manner to cement together into cylindrical tubes by means of a glutinous secretion which exudes from their bodies for the purpose, in order to afford them protection during this helpless stage of their existence. Many of the cases of the smaller species are composed of particles of quartz sand, most artistically arranged so as to fit axurately to each other , and with the smooth facets towards the interior; and when viewed through a magnifying lens, the beautifully disposed different-coloured crystals present no mean imitation of an elaborate piece of mosaic work ; while the interior of the tube is smooth and polished as glass, and lined by a coating of the same glutinous cement which holds the particles of sand together. Multitudes of these larvae of different species and size, and encrusted with various substances, may be found adhering to stones and stumps, or crawling about the bottom of every little rill and pool in the vicinity of a river, like animated pieces of straw, the head and fore-legs alone protruding from the orifice of their unique habitations.
The Piper Caddis, so called from the shape and material of which its case is composed, is the larva of the stone-fly, and the largest of the tribe; being about an inch in length, and forms one of the best angling baits among the caddies. This larva is very common in northern and Welsh streams, as well as the fly which produces it; and is said never to be found but where the bed of the river is either composed of limestone or large pebbles. This I can believe, as it the natural habit of the stone-fly to hide underneath the stones and large gravel by the reiver-side during the height of the day ; hences its name of the stone-fly; and it only comes abroad on the wing, morning and evening. During their seasons, both the larva and the fly are very commopn, and in some districts abundant in the rough stony streams of Northumberland, Cumberland, Westmoreland, Scotland, and Wales ; but in muddy rivers they are never seen. This larva gets the name of piper caddis from its hollow wooden case, closely resembling the drone reds in a set of bagpipes. The fly generally appears in the perfect state towards the middle or end of April or beginning of May, according to the forwardness of the season and the earliness of the locality, and disappears again in July.
The larva of the green and grey drakes form their tubes of longitudinal pieces of slips of coarse straw, glued lengthways together, and are the next in size to that of the stone-fly, being three-quarters of an inch in length. This caddis usually abounds in all the rills and pools in the vicinity of rivers running over muddy or sandy bottoms, but seldom exists to any extent in the neighborhood of swift stony streams, and is likewise a large, conspicuous, and excellent bait for most fish. The fly usually appears from the 20 th or end of May to the middle of June, according to the locality-generally about the latter period in Northumberland.
Other species form their cases of short pieces of rush and stalks of grass, while others again use nothing but particles of dand agglutinated together, as I have before mentionned ; and from them issue the different tribes of duns, yellows, etc- all purely aquatic flies, and lures of the first exellence for trout.
These caddis may easily be procured in their respective seasons before they assume the winged state- piper caddis in April and the larvae of the drakes in May- by searching the small rills and wet ditches which adjoin the rivers, when they will be found crawling about the bottom in their strange habitations among the stone and mud.