We've been more or less consistent with stanzas up until this point – now we get a tiny, one-line thing hanging out in the middle of the poem. What's up with that?
One of the things a one-line stanza does to a poem is slow it way down. You have to pause before you get to the one line, read just one line, and then pause again right away. After all that talk of hotness and fierceness, this little line might just be what the poem needs to keep it from going entirely crazy!
So. We've slowed down…for what? A fly, apparently.
The fly, in direct contrast to everything that's been happening up until this point, is "cool," further slowing and balancing the motion of the poem.
So now there are two bugs – the bee buzzing around hotly inside the flower, and the fly, almost indifferently, pausing for a minute on the edge of one petal.
That petal, by the way, is probably the "flame fringe," what with all the talk of heat we've just been doing.