Fly

http://nationalinsectweek.co.uk/

It's national insect week so here is day 2.

It's a fly on a buttercup, no idea what sort.

Anybody?

I did try using an online key to identify it but, do you know, that the order diptera contains more animals than any other?

So, my clever friends, tell me and point me to a field guide for insects which may give me a chance?