Particularly clearly seen here are the halteres, a pair of drumstick-shaped organs which flies have in place of hind-wings. The halteres beat up and down in opposite phase to the wingbeats, and seem to function like gyroscopes. They serve as sense organs to monitor angular accelerations and trigger corrective responses, and so help enable flies to remain stable in flight.
Railway station shelter (northbound line), Rosyth Halt, Fife, Scotland. About 14:30 BST, Thursday July 29th, 2009.