I'd add one more task for future researches: to carry out chemical analysis of the liquid. By the way, what is the functional significance of human spitting?

Excellent summary on this phenomenon.
A couple of observations to add.

1. When I observed a bee fly blowing bubbles it was not at the tip of the proboscis but at the base and seemed to be related to cleaning actibity on the proboscis.
2. Whilst the commonest bubbles are clear or only slightly opaque I have seen the same muscid fly species blow bubbles from transparent through to dark coffee coloured but have not seen any colour change in bubbles blown by one fly in sequence.
3. I do wonder if bubble blowing enables the fly to introduce new salivary gland enzyme activity into the stomach contents.

Overall I suspect that whilst most bubble blowing is to to with digestion one way or another I suspect some bubble blowing may be for other purposes (eg cleaning).

One thing we have achieved though is to show that this is a common activity

Brian V.

I would have thought that human spitting is to get rid of something distatesful in the mouth. That may be why it is considered wrong to spit at someone, and why if you are eating soup, if you spit into the soup you will find it hard to continue eating that same soup even though it is your own spit and cannot harm you. It seems an inherited tendency hard wired in.

Robert - the flies are not spitting in this case- they always suck the droplet contents back in.
Brian V.

Had an interesting comment left on a flickr pic of this behaviour.

"Regurgitating aerates the semi-digested food and helps in oxidation processes. Speeds up digestion"

Hadn't thought of aeration as being a possibility.
Brian v.

i think it is possible for such a phenomen to take place ,,, it. not supprising cause flies ,mosquitos etc are known for vomiting what they have into the host or food and they take fresh blood or juices .. so this might have that they will be very full. and they just bubble.

As Cor Zonneweld points out:- one really needs experiment to falsify an hypothesis. All else is but interesting speculation. At least in tephritids there is some experimental work on bubbling and here are a few of the papers I've come accross:-
(1) Hendricks et al (Physiol. Entom. 17: 152-61 [1992]) followed weight changes through feeding and bubbling in tephritids and concluded that bubbling serves to eliminate excess water through evapouration. They discounted thermoregulatory mechanisms.
(2) Hendricks et al (J. Insect Behav. 6:43-64 (1993)) fed tephritids yeast hydrolysate & sugar solutions. Multivariate logistic regression suggested that bubbling behaviour is determined by liquid feed volume, degree of dilution, hunger and temperature.
(3) Aluja et al (An.Ent. Soc. Am. 86: 776-93 [1993]) showed in tephritids a goood correlation between bubbling and regurgitation behviour and feeding times [ie it is a feeding phenomenon].
So. at least in Tephritidae, it seems that bubbling is most likely to be a feeding response. As ever in biology, one phenomenon is often intimately linked with others so I don't think one could rule out thermoregulatory spinn-offs or exolytic secretion for example... and alternative explanations might be appropriate in isects other than tephritids. But the hendricks model of water regulation during and after feeding is based on pretty sound experimental data
Hope this helps
cheers
Adrian

When I first saw a fly doing this, I thought it was like a dog returning to its vomit. I've not seen anything since to contradict that idea. My most recent observation was of a Euxesta pechumani dining on some fresh anole feces. It spent a lot of time dining - long enough for me to go get my camera and then take lots of pictures. After some time dining, it blew a bubble (why are they called bubbled? they're really drops). The drop it exuded was crystal clear. It continued dining later. https://www.inaturalist.org/observations/112196442