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Looks like germinating seeds?
BMIG website: www.bmig.org.uk (centipedes, millipedes & woodlice)
BMIG Newsletters ~ www.bmig.org.uk/view/resource/bmig-newsletter
BMIG Bulletins ~ www.bmig.org.uk/view/resource/bmig-bulletin
Nope that was a tongue of sorts darting in and out.
In that case very interesting observation, but I dunno! (I do terrestrial species).
All the best
You could keep a few in a jar of pond water and sediment and see what hatches out.
I watched the video and I agree with John that it is a larva that exit the 'tube'. These do look rather like a diptera.
Grow a few on as John suggests. Much more exciting than watching seeds germinate!
I think they are gathering sediment into their tube to feed on.
Lat/Lng: 51.5813, 0.4702
OS grid ref: TQ712898