The capitate stalk on both sides of the thorax having developed from the dorsal metathoracic wing-buds, it is freely movable and capable of vibration, representing the hind wing of Diptera; used for flight balance. [ http://flybase.org/reports/gg.html https://www.worldcat.org/search?q=bn%3A3110148986 http://en.wikipedia.org/wiki/Haltere ]