Maybe this one has passed the scene before in all those years, but the O.E. origins are funny and obscure.I used online etymology. Maybe there are better sources.The French Italian origins seem to be more logic.

Butterfly O.E. buttorfleoge, perhaps based on the old notion that the insects (or witches disguised as butterflies) consume butter or milk that is left uncovered. Or, less creatively, simply because the pale yellow color of many species' wings suggests the color of butter. Another theory connects it to the color of the insect's excrement, based on Du. cognate boterschijte. A fascinating overview of words for "butterfly" in various languages can be found here:

The small point in this is that in all the long months of camping out in our life in a place with many butterflies around they never showed any interest in butter or milk of cream.

PapillonÉtymol. et Hist. A. 1. a) Fin du xiies. papeillon «insecte lépidoptère» (Audigier, éd. O. Jodogne, 42); b) α) 1685 «esprit léger, volage» (La Fontaine, Poésies diverses)

Farfalle Looking into French and Italian etymologies the word origins from latin papilio. with the redoubling (batting and lifting...)of the root 'peb' which stands for flying.
I has remained almost unchanged in French: papillon and in Italian: padiglione, which means 'pavilion'. The Italian word for butterfly : ' farfalle' is however largely remote from the latin, constructed by the same criteria ; the repetition of the syllable to remind of the double movement of the lightweight wings.