Caddis fly larvae are known to incorporate bits of whatever they can find into their cocoons, be it fish bone or bits of leaves. Hubert Duprat gave them gold, turquoise, gems and pearls.
imo the fact that these look like mini-egyptian sarcophagi adds a beautifully poetic element to both the allegorical significance of metamorphosis in culture and the obsession in ancient egypt with the afterlife and all the associated myths and elaborate practices