My publication of "Fahrenheit 451" is the "Literature Connections" volume issued by McDougal Littell. On page 78-79, in the second section of the novel, you can find this quotation. Montag has just gotten off the phone from talking with Faber. Mildred announces that "the ladies are coming over this evening." The two of them get into a conversation as to whether or not Beatty knows that Montag has books. Montage tells Mildred that he doesn't think Beatty knows "which book I stole." It is here that Montag remembers speaking to Beatty and hearing his Captain describe what it is like to see books burn. This is the quotation you have referenced.
"like the petals of a flower...each becomes a black butterfly...swarms of black moths that had died in a single storm."