2011-06-28 I rode my bicycle through our nearest natural park - the Lobau, in Vienna's 22nd district. I guess I spent as much time taking pictures as cycling. There were several of these butterflies (Argynnis paphia - Kaisermantel - Silver-washed Fritillary) obsessed with this dead leaf. I assume there was still some sap in the leaf and that it was a bit intoxicating.
Update: a bit of web browsing came up with the fact that the by-product of some molds is alcohol, and that butterflies can get drunk on alcohol. That's probably what is happening here.
Many thanks for your comments on the model of Vienna. I didn't get around to putting up a picture yesterday