Thomas Milner, The Gallery of Natur: a Pictorial and Desriptive Tout Through Creation, Londres, Wm. S . Orr & Co., 1848, p. 745-746.
The most remarkable deposit is an indusial limestone, so called from the Latin indusium, a case, because essentially composed of the case of a species of insects in its larva state, incrusted with travertin, and cemented into a rock. The reader has no doubt often observed, when by the side of a clear and shallow pool of water, little oblong masses moving along the bottom, resembling pieces of straw, wood, or even stones. There are the straw, or caddis-worms, employed by the fisherman as a bait, really the larvae of a tribe of four-winged insects, of which nothing is seen in the water but the head and legs, by means of which they move, and drag along the case in which the rest of the body is enclosed, and into which, on any alarm, they wholly retire. « The construction of these habitations, » says Kirby and Spence, « is is very various. Some select four or five pieces of the leaves of grass, which they glue together into a shapely polygonal case ; others employ portions of the stems of rushes, placed side by side, so as to form an elegant fluted cylinder ; some arrange round them pieces of leaves like a spirally-rolled ribband ; other inclose themselves in a mass of the leaves of any aquatic plants united without regularity ; and others again form their abode of minute pieces of wood either fresh or decayed. Other species construct house which may be called alive, forming them of the shells of various aquatic snails of different kinds and size, even while inhabited, all of which are immovable fixed to them, and dragged about at pleasure- a covering himself with squirrel’s skins, should sew together into a coat the animal the animals themselves. Not having the powers of swimming, but only of walking at the bottom of the water by aid 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 water, it is of great 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 too great buoyancy ; and it is as essential that it should be so equally ballasted in every part as to be readily moveable in any position. Under these circumstances our Caddis-worms evince their proficiency in hydrostatics, selecting the most suitable substances ; and, if the cell be too heavy, gluing to it a bit of leaf or straw ; or, if too light, a shell or piece of gravel. »
In a precisely similar way the cases which compose the indusial limestones we are noticing are composed. Around the larva dwelling the shells of a small spiral univalve belonging to the genus paludina, a tribe of fresh-water snails, are aggregated, and both the insects and molluscs must have existed in countless swarms in the ancient lakes of central France, since ten or twelve cases may be packed within the space of a cubic inch, and some single strata of the indusial limestones are six feet in thickness, and may be traced over an area of several miles.