Saliva bubbles?

I have posted earlier a photo of a bee-fly whith a droplet on the proboscis (june 13th) hypothesizing that it was doing maintenance as the droplet appeared and disappeared many times over the few minutes that I was photographing it. The Ectophasia fly of todays blip has got only a very small proboscis and still I noticed exactly the same phenomena  (see also in extra a profile photo). Research has shown that the long proboscis of butterflies in their complex simplicity are delf-cleaning and they have been used as models for surgery device,  but I find nothing on the self cleaning of fly-proboscis. Mayby in both cases the flies I captured were just entertaining themselves making saliva bubbles like small children sometimes do!