Patio Life: Rat-tailed Maggot (Dronefly Larva)

Ok bit of a story with this...

My friend has a worm house thingy, looks like a compost heap but its for feeding refuge to worms... they are very happy worms I might add! Anyway, there is a spout and the worm pee (I think it may be called leachate) drains off, it makes a brilliant concentrated fertilizer. It has lots of dead creatures in it that had the misfortune of getting in it while it was an open bucket under the spout.

She poured it into one of those big air tight jars like a mason but bigger, and I brought it home. A few days later I used some in my watering, and sealed it back up.

A few days ago, I opened it up to use again and these were worming around in it!! Now its been sealed pretty much air tight, setting in a shadyish place and what on earth can live in worm pee!!
I asssure you this guy is very much alive, and there are probably about 10-15 more in the jar.

So what is he, what does he want, is he really content to live in the worm pee or will he be ok in my tiny make shift pond? That appears to be some sort of tail on it, but it can move that as well...I've got NO idea.

The worm pee will run out at some stage there is only a few inches of it in the glass jar.

Update: Thanks to Derbyshire Harrier for the help with the ID on these guys! And of course all the other useful suggestions... :O