Gerald Durrell, How to shoot an Amateur Naturalist, Glasgow, Fontana/Collins, 1984, p. 164-165.

Take the common caddis-fly larvae, for example. In any placid ponds, you may find those who have spun themselves a silken tube to live in and then camouflaged the outside with sand or tiny bits of vegetable debris. (When I was young, I used somewhat unfairly to remove a larva from its home and then, while it was spinning another, provide it with different-coloured materials, such as brick dust and powdered slate, and thus get multicoloured caddis larvae.)

In the still waters of a pond a camouflage of plant debris will suffice, but in a fast-flowing stream or river the larva needs something more substantial to help anchor it and prevent it friom being swept away by the current, soi t uses tiny pebbles which to the larva are as big as boulders. When you are examining the pebbly bottom of a stream you are sometimes taken aback when what appears to be a small pile of pebbles walks away.