Why does the caddis-worm, or larva of the water-moth, encase itself with shells, stems, leaves, straws, etc.?

Because its form is but little adapted for swimming: its long body, encumbered with six legs, is specifically heavier than water, the element in which it has to seek its food ; and it thus attaches itself to these bodies, to counterpoise its own excess of gravity, and enable it to swim on the surface, in the centre, or at the bottom of the water. This explains the variety and singularity in the materials of the coats of these insects. When they want to ascend, the quantity of hollow and buoyant substances attached to them is increased; in order to descend, the light and hollow substances are either diminished or counterpoised by an addition of heavy materials, such as shells or gravel ; the interior they line with a fine silk spun by themselves.