In my search for fireflies, I found the Butterfly piano score (It/Eng) first. The stage direction (in English only) refers to glow-worms. We also have the (It/Ger) full score with Italian stage-directions, in this case referring to "le lucciole", a word I don't know, but obviously a glowing thing of some sort, which shine around the lovers among the flowers and the foliage of the trees.