Halteres

Halteres (; singular halter or haltere) (from Ancient Greek: ἁλτῆρες, hand-held weights to give an impetus in leaping) are a pair of small club-shaped organs on the body of some flying insects that provide information about body rotations during flight. They are present on insects belonging to two orders, Diptera and Strepsiptera. In dipterans, the halteres evolved from a pair of ancestral hindwings, while males of the much smaller strepsipterans (stylops) have halteres evolved from a pair of ancestral forewings. Halteres oscillate rapidly along with the wings and operate like vibrating structure gyroscopes: any rotation of the plane of oscillation causes a force on the vibrating halteres by the Coriolis effect. 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 provide rapid feedback to the wing-steering muscles, as well as to the muscles responsible for stabilizing the head.

Background The majority of insects have two pairs of wings. Flies possess only one set of lift-generating wings and one set of halteres. 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, or stylops; they are the only other organisms that possess two wings and two halteres. The 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, though many flying insects have been shown to detect Coriolis forces from their non-specialised wings.

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. Halteres are typically only associated with flight stabilization, but their ability to detect body rotations can elicit compensatory reactions not only from the wing steering muscles, but also from neck muscles which are responsible for head position and gaze. Halteres may also be useful for other behaviors. Certain species of flies have been observed to oscillate their halteres while walking in addition to oscillating them during flight. In these individuals, halteres could thus be detecting sensory information during walking behavior as well. When the halteres are removed, these insects perform more poorly at certain walking challenges. However, how haltere information is processed and used during walking remains, with few exceptions, unclear. Specific examples of what has been found are described below.

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