Duprat places the caddis fly larvae in a controlled environment, supplying them with precious materials. Individual insects build with different levels of skill, some preferring beads and others pearls. 

In an interview with Christian Besson, Duprat mentions that an American entomologist, Charles T. Brues, observed caddis fly larva in a river in northern Nevada adding bright blue natural opals to their cases when they could, indicating that the insects use more than purely utilitarian considerations.

Fascinating stuff.

More: another interview with Duprat +  a video of the caddis at work.