Halteres

Halteres ( /hælˈtɪəriːz/; singular halter or haltere) are small knobbed structures modified from the hind wings in some two-winged insects. They are flapped rapidly and function as gyroscopes, informing the insect about rotation of the body during flight. The word 'halter' comes from Greek ἁλτήρ, a double knobbed device used in Ancient Greece by athletes during training in jumping.