I’ve only seen these twice. Spotting them in my water-butt I told my partner Annie without giving the common name, as I think it’s a bit pejorative. (Rat-tailed Maggots.)
She got this video, risking dropping her phone in the water. You can see their intestine through their skin. We can’t work out if they’re feeding on floating vegetable matter or drowned insects.
She was delighted with them as I thought she would be, and found out they are the larvae of a kind of hoverfly – news to me.
Her suggested new name for them is Chevron-larvae.