Fireflies, glow-worms, same thing. Le luciole = French for firefly (Italian luciola). Thanks so much for locating that, Andrew; it's something I've been wondering about a long time.
So the fireflies are in the score and not just the idea of some production designer. For years I'd heard about the way the Met did the fireflies during the Act I love duet -- until they hired a Japanese stage director for a new production and he pointed out there are no fireflies in Japan.
The first Met production of Butterfly I ever saw came right after that revelation, so I never got to see the fireflies. Instead, cherry blossoms dropped down onto the roof of Cio-cio-san's house. It was lovely, but not the same as itty-bitty lights going on and off and sometimes moving.