Hoverfly

A hoverfly (family Syrphidae, order Diptera), most probably a Sphaerophoria scripta female.

I say it is a female because its eyes are well separated, its abdomen is thick and tapered, and it was apparently laying eggs on this flower head (in spite of the presence of several aphids).  [Note added 2009-12-06: It may be that the hoverfly deliberately lays it eggs on a flower head with aphids so that its larvae will have a source of food when they hatch out.]  For comparison, here is a photo of a male Sphaerophoria scripta that I took last year.

Photos taken Reading University grounds, Reading, UK, on 2009-06-24.