This guy is obviously a wasp mimic, but how can it possibly fly with those stubby little wings?
it also looks like it's trying to mimic a Thread-waisted wasp, but for what reason?
To try to attract and then attack Thread-waisted wasps?

Actually these guys don't eat other insects like robber flies, but use their proboscis to suck up nectar.
Their strangely shaped abdomen is however another story, and it uses it to inject its eggs into its host insect
(very likely thread-waisted wasps) while in flight. Then, after the eggs hatch, the larvea eat the host wasp from the inside out.
Now you can see how those little wings make sense,
since larger wings would interfere with the wings of the flying host insect and both bugs would go crashing to the ground.
Clever! Very clever indeed!

Click on the following link to view the high resolution image on Flickr.