There is a virtual fly circus going on every time one of
these pesky insects comes in for an inverted landing.
Flies, like airplanes, quickly lose their lift when flying
upside down, immediately turning them from soarers to
sinkers.

The trick to landing upside down is all in the legs. Flies
have three pairs, and they start the maneuver by reaching
above their head with the first two legs, making contact
with the ceiling. The somersault continues with a body
tuck, which pulls the last two pairs of legs forward and up
to the ceiling. This feat of insect acrobatics allows the
fly to suspend itself upside down above your dinner plate
without having to complete a full roll.