Rat-tailed maggots. They hatch into hoverflies or drone flies. On closer inspection of the photo I think I can see the dead adult which probably laid the eggs and then couldn't escape the water butt again.
Much thanks, Hester. Glad to know I didn't inadvertently mix up some weird primordial soup (i.e. that it can be identified). My only wonder now is whether these are out of place in a compost tea, in that are they bad for, a hazard to, the garden? Rat-tail maggot doesn't sound so good but neither does, i'm sure, many beneficial insect larvae names.
I think they are just one of those unfortunate benign creatures that somehow evoke a primitive "yuck" reaction in humans. I don't think they're going to do you any harm unless you are thinking of drinking the stuff. The adults will either be beneficial pollinators or actively predatory. They're all part of the big picture.
Dear Abbey;-), I was wondering if the one stone from the air pump would be enough. Last year I used the two stones that come with that pump. This year I tried 2 rain barrels with one stone each. My intention was to try molasses in one and none in the other. The rest was to be about the same, in amendments, manures and green matter. I read on a website an emphatic NO to molasses for compost tea is why i'm trying 2 teas. Seems I oughta bought two air pumps. Hmm, maybe if i sink the stone lower closer to the bottom of the rain barrel it would have a fuller oxygenating effect. Will adjust and sweeten the other pot too, as i've yet to add the molasses. Busy and kinda forgot about it. Tons too due!
I would guess you don't have enough aeration for a proper aerated compost tea if you're having larvae like this grow in it. How long are you brewing it for? what are your ingredients? what size pump are you using? what is the air being pushed through?
EDIT: in my experience compost tea should not stink, it should smell lightly sweet and clean, not rancid.
I wouldn't worry, too much, Nick, although I second the call for way more oxygen. I have a dumb question, though: do you have a lid on your brew barrel? Is is covered, or is it open to the air? If it's not sufficiently covered, covering it could prevent all but the smallest insects from getting in there to lay eggs in the first place. I have done it before with reclaimed window screens. I am sure you could just use a snap ring and bug netting, too (I was using large diameter pipes, because that's what I had, and really big hose clamps).
A human being should be able to change a diaper, plan an invasion, butcher a hog, conn a ship, design a building, write a sonnet, balance accounts, build a wall, set a bone, comfort the dying, take orders, give orders, cooperate, act alone, solve equations, analyze a new problem, pitch manure, program a computer, cook a tasty meal, fight efficiently, die gallantly. Specialization is for insects.
-Robert A. Heinlein
The Greenhouse of the Future ebook by Francis Gendron