Bubble blowing fly

I didn't know what was in the fly's mouth but when I showed this to kwdiane she said that the fly was blowing bubbles. 

I looked this up and it seems that many flies and other insects blow bubbles although no-one seems to know exactly why they do this. 

Amongst the possible reasons are that it aids digestion, is a defensive mechanism, cleans their mouth or thermoregulation. Whatever the reason it is a new one on me.