Haltere

In dipterous insects, halteres (/hælˈtɪəriːz/; singular halter or haltere) are minute dumbbell-shaped organs that have been modified from hindwings to provide a means of encoding body rotations during flight. Halteres are rapidly oscillated simultaneously with the wings, allowing them to experience forces resulting from body rotations. If the body of the insect rotates about one of its three axes (yaw, pitch or roll), the rotation exerts a force on the vibrating halteres – this is known as the Coriolis effect (see below). The insect detects this force with sensory organs called campaniform sensilla and chordotonal organs located at the base of the halteres and uses this information to interpret and correct its position in space. Halteres act as a balance and guidance system by providing rapid feedback to the wing-steering muscles, as well as those responsible for stabilizing the head. This is what allows flies to perform their fast acrobatic maneuvers.

The majority of insects have two pairs of wings. What makes flies unique is that they possess only one set of lift-generating wings and yet are still regarded as some of the most skillful fliers. The order name for flies, "Diptera" literally means two wings, but there is another order of insect which has evolved flight with only two wings: strepsipterans, which are more commonly referred to as twisted wing parasites; they are the only other organism that possess two wings and two halteres. The only difference is that strepsipterans have adapted their forewings into halteres, whereas dipterans have adapted their hindwings into halteres. This unique structure which detects rotations/perturbations during flight has never been described in nature elsewhere.

Halteres are able to sense small deviations in body position using the gyroscopic properties of moving mass. What this means is that halteres beat up and down in time with the flapping of the wings along a linear pathway, but when the fly's body begins to rotate, the path of the beating halteres also changes. Now, instead of the halteres following a linear path, they begin to follow a curved path. The larger the perturbation they experience, the farther the halteres move from their original linear path. During these periods, the haltere is no longer moving in only two directions (up and down), but four (up, down, left, and right). The force exerted on the halteres in response to this left right movement is known as Coriolis force and can be produced when any moving object is rotated in the three directions of rotation, yaw, pitch or roll (see figure). When this occurs, tiny bell-shaped structures at the base of the haltere experience strain as the haltere stalk bends in their direction. The nervous system can then transform the bending of these hairs into electrical signals, which the fly interprets as body rotation information. The fly uses this information to make corrections to its position and thereby restabilizes itself during flight. Further details explaining the dynamics and physiology of halteres are described below.

...
Wikipedia