Dr. David Lees from the Department of Zoology of the University of Cambridge, United Kingdom, helped me identifying this species.
I met David in Madagascar in January 2014 during his research work in Andasibe-Mantadia NP and had the pleasure to spent a few days together.
David has extensive field research experience in Madagascar and is considered the leading expert on Madagascar butterflies.

Dr. David Lees informed me that this is Daphnis kitchingi, a large hawk-moth recently discovered (2011) in Madagascar.
It resembles the much more common Daphnis nerii (Oleander Hawk Moth) which also occurs on Madagascar.
It is amazing that something so beautiful can have been discovered so recently.

Andasibe-Mantadia National Park, Madagascar, 12 January 2014