LordV wrote:
Ok just to shoot myself in the foot, spotted a Scatophaga stercoria bubble blowing this AM at an angle of about 120' from where the sun would have been if it was shining.
Maybe it's the IR radiation that's important :).

Dima DD wrote:
I can not say which hypothesis is correct. However, I would like to put some words about this branch. In fact, body of the fly is darker and absorbs much more radiation than shining droplet. Moreover, evaporation of water from the surface must decrease droplet temperature, not warm it. More intensive evaporation - less the droplet temperature. Therefore, according to this branch of hypotheses ("thermoregulative"...), fly can use bubbling for cooling of its overheated body (i.e. it is some kind of "perspiration" ).