Here’s supposedly the meaning:
There was something oddly restful about the fireflies. He couldn’t put his finger on it but he drew comfort from it anyway. The way they’d seemed not separate entities but a single being, a moving river of light that flowed above the dark water like its negative image and attained a transient and fragile dominion over the provinces of night. (p. 161)
And here’s where I feel I’ve missed something from the book; if the title was taken from this line it would seem to be of utmost import.
What the above, in the context of the story as it has unfolded, and here, in Fleming’s thoughts after meeting and falling in love with Raven Lee Halfacre, might suggest is his seeing the world in a new way. Maybe he can put some sense of order in his own family by seeing the whole of mankind as a temporary superior force: "transient and fragile…" Maybe he recognizes human nature as the single drive behind actions. Maybe he realizes that all this is just a state of mind.
"negative image" is a phrase that may denote a helplessness against what will be, that of life being brief and passing and leaving no mark behind it on the stability of the earth.
How does this concept tie in with my thoughts of movement?
The Lost Children: A Charity Anthology