Unexpectedly, while recently reading Goulson's A STING IN THE TALE: MY ADVENTURES WITH BUMBLEBEES , I came across some slightly sinister associations with their habits.
Here's the passage, coming in a footnote at the bottom of page 203 of Goulson's book:
"These spectacular insects normally hang around the tops
of oak trees, and so are seldom seen. One old-fashioned
technique that was used by butterfly collectors was to place
a well-rotted dead rat on a wood-land ride. Beautiful though
the butterflies are, they have a macabre taste for the juices that
leak from such a corpse and are often lured down."
I admit to being curious as to whether Tolkien, a keen observer of nature, was aware of the purple emperor's taste for corpse-juices, and if so whether this contributed to his decision to include them into his description of Mirkwood. I assume not, since the scene of Bilbo and the butterflies has no overt sinister overtones in the book, but the ghoulish habits of their real-world counterparts is interesting, to say the least.
current reading: THE LATHE OF HEAVEN by Le Guin 
currently watcing : THE LATHE OF HEAVEN