These are really cool little critters! We have these on this side of the pond, too and I've seen them a few times. I did some checking and here a few biological "factoids." These are actually pupae rather than larvae (though they are still able to swim around at this stage - in contrast to what we usually think of when we hear the word pupa). These are called Phantom Midges (Family Chaoboridae) rather than mosquitoes (mosquito pupae are similar, but typically more curled up than this). They use the little bunny "ears" to breath at the surface.
Steve
Absolutely right. I didn't want to get too technical and of course they are pupae, not larvae, of the phantom midge Chaoborus crystallinus. I enclose a rough shot of the actual Larvae. Not a good shot but I am working on these at the moment. Very common in ponds a this time of year. Strange things the way they hang in mid water- great food for my Discus fish too !
JohnR