Symbolism, Imagery, Allegory
No, we don't mean "The Itsy Bitsy Spider"—we mean the rhyme that Keeper is murmuring to himself while he does a cats-cradle kind of string game with the embroidery thread he stole from Flora. Here's how it goes:
"How daintily the butterfly
Flits to the spider's lace
Entranced by glimm'ring silver strings
Entwined with glist'ning grace.
"How craftily the spider speaks
And whispers, 'All is well,'
Caresses it with poison'd feet
And sucks it to a shell." (18.105)
Azalea happens to overhear this when she returns to the pavilion alone to get her mom's brooch back from Keeper. And it should've been a warning, but, well, Azalea's not the most cautious of people, is she? (If you're not sure, check out her analysis in the "Characters" section.)
The meaning is pretty easy to unpack: Keeper is the spider, and the "glimm'ring silver strings" he uses to intrigue the butterfly refers to the beautiful silver forest and the enchanted pavilion. This, of course, makes Azalea and her sisters butterflies… but they're beautiful creatures so it fits. The spider speaks "craftily," weaving a subtle trap all along that the poor butterfly doesn't see in time—which corresponds, we learn, with how Keeper entraps the girls with his smooth charm and his invitation to come and keep dancing.
Seeing Keeper as a spider isn't actually that far-fetched. The first time Azalea goes to the pavilion alone, to try to get Mr. Bradford's watch back, she sees Keeper in an unconventional position: "There, on the ceiling like a big, black spider, was Keeper" (14.77). This is totally creepy because of how unexpected it is—and Keeper speaks soothingly to her, explaining: "Living in such a small pavilion for so many years makes one, ah, creative" (14.83). So we see Keeper speaking craftily, earning his prey's trust, even before the spider metaphor gets made explicit through the rhyme.
If Keeper is like a giant spider, all we can say is that we hope he ends up under someone's shoe heel.