Tony Irwin wrote:
Hi Hans
High speed flash photography has shown that a fly (such as a housefly) will fly along in normal orientation under the ceiling, but with its front legs raised above its head. When the legs hook onto a suitable projection, the front part of the fly stops, but the rear part continues in a half-back-somersault, so the fly comes to rest upside-down and facing in the opposite direction.
There's a multi-flash picture of this action in Stephen Dalton's book "Borne on the Wind", which makes my explanation much clearer!;)