Georg Fleming Richardson, Geology for beginners, Londres, Longman, Brown, Green & Longmans, 1843, p.439-440.
Others beds are composed in a manner still more strikling, and calculated yet more strongly to display the power of Supreme, as regards the most apparently trivial of his creatures. There is an insect, called the phryganea, or may-fly, which, in its larva state, is called the caddis-worm, and is then empyed by the angler in his cruel sport for the purpose of bait, and which possesses the following remarkable instinct as described by Kerbt and Spence, in their valuable treatise on entomology. « Not having the power of swimming, but only of walking at the bottom of the water, by means of the six legs attached to the fore part of the body, which is usually protruded out of the case, and the insect itself being heavier than wter, it is of greatt importance that its house should be of a specific gravity, so nearly that of the element in which it resides, as while walking neither to incommode it by its weight, nor by its too great buoyancy ; and it is as essential that its should be so ballasted in every part as to be readily movable in any position. Under these circumstances our caddis-worms evince their proficiency in hydrostatics, selecting the most suitable subtances, and if the cell be too heavy, gluing to it bits of leaf or straw; or, if too light, shells or pieces of gravel. » The accompanying illustration depicts a case of this kind in the possession of the author, in which the attached objects are a minute species of paludina a fresh-water shell.
In a similar manner, a large and extinct species of caddis worm, which abounded in the ancient lakes of Auvergne, was accustomed to cover its case with the shells of a small species of the univalve genus paludina. Vast beds of the limestone of this region are essentially composed of these objects in a fossil state, and are termed indusial limestone, from the Latin indusium, a case. When, as Mr. Scrope observes, we consider that ten or twelve of these cases may be packed within the space of a cubic inch, and that some single beds of this limestones may be traced over an area several miles in extent, we may form some idea of the countless numbers of insects and molluscs, whose remains have contributed to form this remarkably constitued rock ; while we can but be struck with the incalculable period required for its deposition, and the cycles of ages during which the tranquil waters of the lakes must have continud to deposit these singular sediments.