Eristalis tenax

This is the Drone Fly (Eristalis tenax). It's called that because it is disguised as a honey bee drone.  You might think this hover fly was a bee but you can tell it is a fly because it only has one left wing and one right wing.  Bees and wasps and most insects other than flies have two wings on each side - four altogether. 

Drone flies hover around all summer at about head height but are completely harmless to humans.  This one died on my window sill the other day, sadly unable to get out and visit flowers in the sunshine it loves.

Notice the tiny U-shaped kink in the vein near the end of each wing.  All the Eristalis hover flies have this and a few of their close relatives.

Female Drone flies lay their eggs in stagnant water.  The larva is called the rat-tailed maggot because it has a long breathing tube like a tail to help it survive in water where there is a shortage of oxygen.  You can often catch one if you go pond-dipping.   Here are three in some water in an old tyre (not my photo)

When they pupate they crawl out of the water and find a dry place to hide.  I've never seen one of these but they are only one centimetre long and as they keep their tail they look a bit like tiny mice.