The force of the current

Roger Woolley, The Fly-Fisher’s Flies, Londres, The Fishing Gazette, 1938, pp.28-29.

In pratically every ditch, pond and brooklet their larvae are to found, carrying their houses on their backs, for they are the cad-bait, stick-bait, straw-bait that are the wonder of all lads when first noticed moving along the bed of stream or ditch. The eggs are deposited in the water by the female flies in various ways, and almost immediately the larvae have emerged from th egg they begin to build their house, which they carry about with the mobn their backs. As the larvae are very slow in movement and a very tempting morsel to the fish and become extinct had not. Nature endowed them with the instinctive power to develop an artificial covering about them for their lodging and protection. These case are cylinder-shaped, very tough and very smooth on the inside, and as the larva grows it increase the size of its house. By an adhesive secretion natural to it, the larva as it progresses, covers the outside of its house with rougher materials, sand, grit, bits of stick, straw, and portions of vegetable materials in a wonderful manner, some very neatly built, some very irregular in shape, but all marvellously balanced and adjusted, so that it is not too heavy for the larva to be able to move with it easily, yet heavy enough to withstand the force of the current, but not so buoyant that the case will float away with its occupant. When crawling about the bed of the stream, the head, thorax and legs protude beyond the case, and the body Inside the case is provided with hooks at the tail end, with which the larva holds on to its house and drag it along.