Flies have forewings that are membranous and hind wings, called halteres,a that are shaped like balls on the end of a stalk. In a previous series of posts, I discussed the role of the halteres in providing feedback information to the fly. Flies typically beat the forewings and the halteres at the same frequency. However, in most flies, the wings and the halteres beat in anti-phase rather than in phase. A group of scientists* investigated the mechanism of wing coupling. Using freshly killed flies, moving the wing on one side with forceps casued the opposite wing to move in phase and the halteres to move in antiphase. This is clear evidence of mechanical coupling. In the dead flies, the nervous system was inoperable ruling out a nervous system mechanism of coordination.
The movements of the forewings are coupled by the up and down movements of the thorax. The vibrations of the thorax are transmitted to wings causing them to beat in phase. The muscles that move the forewings and muscles that move the halteres are separate muscles that are not necessarily coupled. The scientists discovered a ridge of cuticle that coupled the right forewing with the right haltere and the left forewing with the left haltere. Surgically severing the ridge allowed the movements of the forewing and haltere to uncouple. If the linkage on only the right side was severed, only the right wing and right haltere uncouple; the left wing and left haltere remain in antiphase. The tight mechanical coupling of the wings and halteres is important to their flight dynamics.
*Tanvi Deora, Amit Kumar Singh, and Sanjay P. Sane. Biomechanical basis of wing and haltere coordination in flies. PNAS February 3, 2015. 112, no. 5 pp. 1481–1486.