I saw this and took the photograph in Sri Lanka in July, it was wonderfully attractive because it looked just like a common UK wasp, but was bright blue. I only saw one of it; I've tried a basic search online but can't identify it. It looks like a wasp or a hover fly, I'd be greatful if you could shed some light on what it really is. Thanks. My first photo of it was blury, but might help:
It is a bee (no fly has antennae like that). Probably belonging to Thyreus (six species in Sri Lanka according to An Annotated List of Bees [Hymenoptera: Apoidea: Apiformis] of Sri Lanka by Anura Wijesekara - pdf here), possibly Thyreus insignis (see image 98 on page 406 of Ecology and Natural History of Tropical Bees ed by David W. Roubik). A pdf Keys for the identification of common bees of Sri Lanka can be found here.
Many thanks for that incredibly speedy, detailed reply. I am much happier looking at the photo now I that know a bit about the Thyreus species. I travel to Sri Lankan fairly often and I will look out for more of these incredible bees.