In the air flies legs are down, but on the ceiling, they are up. How do they do the flip?

Answer 1

The leading theory for years was that the flies did a half barrel roll sideways a la the Blue Angels just before landing. This idea was shot down in 1958 when Natural History magazine published photos showing that in fact flies do a sort of backward somersault.

On approaching the ceiling, and while still flying right side up, flies extend their forelegs over their heads till they can grab a landing spot with the suction cups in their feet. Their momentum then enables them to swing their hind legs up, like a gymnast on a trapeze. Result: inverted fly, home and dry. So the answer to your query is, they get real close. Hope that'll do.

Answer 2

excerpt from "New insights into how flies fly":

" To make the turn, a fly must generate enough torque, or twisting force, to offset two forces working against it—the inertia of its own body and the viscous friction of air."

For the rest of the article, see link below.