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Think I've worked out what's going on when flies blow bubbles (actually droplets rather than bubbles.)

The labellum of the fly's proboscis sucks up liqud food thru numerous fine tubes. From time to time these tubes must get blocked. To clear blockages the fly reverses the flow and blows a droplet which is then sucked back in. Seem to remember from animal physiology lectures many years ago that fish use a similar "reverse thrust" to clear their gills.

Apologies if this is old news!
Malcolm Storey