I found these chaps in an old oak water butt I had just put in. They look like normal larvae, but have a very very long tail. The water butt had only recently filled, so I couldn't decide if they were just living in the wood and then got a nasty wet surprise, or whether they are aquatic. Any help identifying them would be much appreciated. The second picture shows a whole mass of them in the hole at the top of the butt.
These super maggots have long tails that look a bit rat-like, so they became commonly known as Rat-tailed Maggots. They are the larvae of hoverflies and likely to be either one of the Eristalis species or Myathropa floreais a common species in my garden. These larvae feed on detritus and rotting vegetable matter in the water and use this tail as a snorkel to enable them to breath in stagnant water.
To encourage these fab flies I stand buckets and other containers full of wood debris and weeds in quiet corners of the garden and simply let them catch the rain, the larvae then feed on this waterlogged resource. Equally you need to provide for the adults and they simply need flowers (which is great because so do I) as they feed on nectar and pollen. My yellow flowered fennel is a blur of hoverfly species at the moment as well as many other insects.
Thanks for the info - last year we had hornet-mimic hoverfly larvae (identified through this site) so now that's a couple of species in residence. Our garden is a mass of adult hoverflies too so we'll be placing buckets of old wood and weeds around the garden now to encourage them even more. Does it matter if the contents become completely submerged in the water or do the adults need something to land on before laying their eggs??
Quite a few adults seem to get stuck in the lean-to at the back of the house, but we've found if you hold up a small scrunched-up duster, they think they're flowers and land on them, so we can then put them outside again!