H. Cholmondeley-Pennell, Fishing Gossip, Edimbourg, A. & C. Black 1866, pp. 71-72.

The case, it must be observed, has almost invariably a seeming irregularity about it, to the curious observer. If made of small stones, it will generally be perceived that a small piece of lighter material- wood, leaf, or bits of rush- is attached to it. Again, when constructed of lighter substances-such as pieces of wood, leaves, or aquatic plants- a stone or two will be found adhering to the structure. Through at first sight this seems a rather incongruous sort of architecture, where so much ingenuity is displayed it can nevertheless be satisfactorily explained. The larva being of the same specific gravity as the water in which it lives, it follows, as a matter of course, that the case must be as nearly as possible of a similar weight. For if the case be heavier, the larva could not drag so weighty a house with it when ; whilst, on the other hand, if the case should be lighter, it would raise the larva from the ground, to be carried away by the current. Thus our little hydrostatic engineer, if it finds its case too light, ballasts it whith a stone or two, but if too heavy, instead of discomposing the case by throwing off ballast, the insect merely attaches a bit of wood or other light material, to give it the buoyancy required…/…
…/…In this new element, each larva, prompted by the unerring instinct of nature, commences to collecta round it a case composed of parts of plants, leaves, pieces of stick, small stones, sand, and even small fluviatilie shells with their living inmates. These materials are collected and secured by loose threads of a glutinous kind of silk spun from the mouth ; as practised by several caterpillars. The larva first collects a sufficiency of materials before it attempts to enclose itself, for it is obvious that the longer it builds, the less constructive action it can maintain. A remarkable instinct of adaptation of material is seen in those cases constructed of small stones, of all shapes, full of angles and irregularities, out of which the larva forms a tube as striaght, smooth, and uniform in the Inside as a gun-barrel. Nor is this all ; as the case is a movable house, which the insect drags about a twill, the under surface must be as smoth and free from projecting inequalities as the inside.