Editing Halteres (section)

==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: [[strepsiptera]]ns, or stylops;<ref name="MW"/> they are the only other organisms that possess two wings and two halteres.<ref name="Pix 1993">{{cite journal|last1=Pix|first1=W|last2=Nalbach|first2=G|last3=Zeil|first3=J|title=Strepsipteran Forewings Are Haltere-Like Organs of Equilibrium|journal=Naturwissenschaften|date=1993|volume=80|issue=8|pages=371–374|doi=10.1007/bf01138795|bibcode=1993NW.....80..371P|s2cid=43790345}}</ref> 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.<ref name="Pringle 1948" />

[[File:Flight dynamics with text.png|thumb|220px|Directions of rotation]]

Halteres are able to sense small deviations in body position using the [[gyroscope|gyroscopic]] properties of moving mass.<ref name="Pringle 1948" /> 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.<ref name="Fraenkel 1938">{{cite journal|last1=FRAENKEL|first1=G.|last2=PRINGLE|first2=J. W. S.|title=Biological Sciences: Halteres of Flies as Gyroscopic Organs of Equilibrium|journal=Nature|date=21 May 1938|volume=141|issue=3577|pages=919–920|doi=10.1038/141919a0|bibcode=1938Natur.141..919F|s2cid=4100772}}</ref> 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 effect|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.<ref name="Fraenkel 1938"/><ref name="Nalbach 1993">{{cite journal|last1=Nalbach|first1=G.|title=The halteres of the blowfly Calliphora|journal=Journal of Comparative Physiology A|date=1993|issue=173|pages=293–300|doi=10.1007/BF00191842|volume=175|s2cid=15326397}}</ref> 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.<ref name="Pringle 1948" /> 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.<ref name="Hengstenberg 1988" /><ref>{{cite journal|last1=Hengstenberg|first1=R.|last2=Sandeman|first2=D. C.|last3=Hengstenberg|first3=B.|title=Compensatory Head Roll in the Blowfly Calliphora during Flight|journal=Proceedings of the Royal Society of London B |date=22 May 1986|volume=227|issue=1249|pages=455–482|doi=10.1098/rspb.1986.0034|bibcode=1986RSPSB.227..455H|hdl=11858/00-001M-0000-0013-EFCF-0|s2cid=108935705|hdl-access=free}}</ref> 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.<ref name="Hall 2015" /> Specific examples of what has been found are described below.