W. Kirby & W. Spence, An introduction to Entomology or Elements of the NaturaL History of Insects, vol. 1, Londres, Longman, Hurst, Rees, Orme & Brown, 1818, p. 470-472.
If you are desirous of examining the insects to which I am alluding, you have only to place yourself by the side of a clear and shallow pool of water, and you cannot fail to observe at the bottom little oblong moving masses, resembling pieces of straw, wood, or even stone. These are larva in question, well known to fisherman by the title of caddis-worm, and which, if you take them out of the water, you will observe to inhabit cases of a very singular conformation. Of the larva itself, which somewhat resembles the caterpillar of many Lepidoptera, nothing is to be seen but the head and six legs, by means of which the rest of the body is enclosed, and into which, on any alarm, it wholle retires. The construction of these habitations 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 pices of wood either fresh or decayed. One, like the Sabellae, forms a horn-shaped case composed of grain of sand, so equal in size, and so nicely and regularly gummed together, their sides thoughout being of the thickness of one grain only, that the first time I viewed it I could scarcely persuade myself it could be the work of an insect. The case of P. bimaculata, which is less artificially constructed of a mixture of mud and sand, is pyriform, and has its end curiously stopped by a plate formed of grains of sands, with a central aperture. 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. However various may be the form of the case externally, within it is usually cylindrical and line with silk ; and though seldom apparently wider than just to admit the body of the insect, some species have the power of turning round in it, and of putting out their head at either end. Some larvae constantly make their cases of the same materials ; others employ indifferently any that are at hand ; and the new ones which they construct as they increase in size ( for they have not the faculty, like the larva of the moth, of enlarging them) have often an appearance quite dissimilar to that of the old. Even those that are most careless about the nature of the materials of their house, are solicitously attentive to one circumstance respecting them, namely their specific gravity. 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. It is from this necessity of regulating the specific gravity, that to the cases formed with the greatest regularity we often see attached a seemingly superfluous piece of wood, leaf, or the like.