Animated butterfly with drop shadow

Dave has experimented over the years with different sorts of animated butterflies and we have many on our website.
A few years back he tried to give a shadow effect in the hope of making the animation more realistic.
I found this animation, one that has never been published and think that Dave nearly got it right. Maybe he should experiment a little further.
The big problem, I think, is that the shadow will only look right on the correct colour background. In Flash you can actually make the shadow transparent.
Dave has worked quite a bit with Flash but decided a few years back that for the majority of people it is much easier to insert an animated gif into a website.