Sarah wrote Some type of hoverfly larva? Specifically that known as the
'rat-tailed
maggot', the larva of the Drone Fly _Eristalis tenax_

I've had those, in a fishless pond, they are very kewl critters.
We brought them inside (of course!) and watched them noodle around and breathe
through their extended 'tails'.
Mine were grey in colour, very maggoty in appearance, excepting the breathing
tube.

k30a