Some type of hoverfly larva? Specifically that known as the 'rat-tailed
maggot', the larva of the Drone Fly _Eristalis tenax_ (the tube is very
extensible). I have these in my half-barrel 'pond' and would not be
surprised to find them in a water butt to which the adults could gain
access. They are harmless, and the adults are good pollinators.

And our hens just love the larvae ...

Thanks for the site.

Mary