Video: You may have seen halteres on flies, but you probably didn't know what you were seeing. They're the odd little twig-like extensions behind the fly's wings. These up-close videos give you a much better look, so you'll always notice from now on.
Flies live in a dizzying world. They can crawl across a table, up a vertical surface like a wall, and invert themselves to land on the ceiling. When they fly, they change orientation with a frequency and a rapidity that would leave a human stunned on the ground. The halteres are how they keep themselves oriented.
Those tiny parts moving up and down in the video are the halteres. These organs are exclusive to flies, and evolved from the hind wings. When flies walk or fly, they swing the halteres around (different flies move them in different ways) and use the halteres as a kind of external inner-ear.