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There is an "official" Checklist of all Italian fauna available on the Internet which I use as my bible. It isn't, of course, foolproof, but it's pretty reliable. According to this checklist, we do actually have C. festivum in my part of Italy, and also C. vernale and C. intermedium, so I guess that leaves the field wide open!
Here's the full list of Chrysotoxum species:
Chrysotoxum arcuatum (Linn?, 1758)
Chrysotoxum bicinctum (Linn?, 1758)
Chrysotoxum cautum (Harris, 1776)
Chrysotoxum cisalpinum Rondani, 1845
Chrysotoxum elegans Loew, 1841
Chrysotoxum fasciolatum (De Geer, 1776)
Chrysotoxum festivum (Linn?, 1758)
Chrysotoxum impressum Becker, 1921
Chrysotoxum intermedium Meigen, 1822
Chrysotoxum octomaculatum Curtis, 1832
Chrysotoxum vernale Loew, 1841
Thanks to you all, Sarah