These dainty butterflies are about half the size of the Dainty or Dingy Swallowtail. The upper wing is almost transparent, with only the back markings and a light dusting of white.
They have constantly returned to the buddleia to feed during the last two days, and may have been visiting it far longer.
This was the first of their species I have seen outside books and web sites. Another exciting discovery, at least for me.
Postscript: later in the season I saw a few Glasswing Butterflies frequenting the banana passionfruit. Apparently they can use this as a larvae food plant. Hopefully that means more Glasswings in the future!