Editing Halteres (section)

==Dynamics==

[[File:Insect wing muscles.png|350px|thumb|left|Fly thorax showing side view of dorsal longitudinal (DLM; upper left) and dorso-ventral (DVM; upper right) power flight muscles. Bottom image shows transverse cross section of fly.<ref name="Hedenström 2014 How Insect Flight Steering Muscles Work"/>]]

Dipteran insects along with the majority of other insect orders use what are known as [[insect flight|indirect flight]] muscles to accomplish flight. Indirect [[insect flight]] muscles are composed of two sets of perpendicular muscles (see left figure) that are attached to the thorax (instead of directly to the wing base as is the case for direct flight muscles). When the first set of muscles contracts, they deform the body of the insect and compress its thorax vertically, which lifts the wings. When the first set of muscles relaxes and the second set contracts, the thorax is squeezed in the opposite direction, which extends the body vertically and moves the wings downward.<ref name="Hedenström 2014 How Insect Flight Steering Muscles Work">{{cite journal |last1=Hedenström |first1=Anders |title=How Insect Flight Steering Muscles Work |journal=PLOS Biology |date=25 March 2014 |volume=12 |issue=3 |pages=e1001822 |doi=10.1371/journal.pbio.1001822 |pmc=3965378 |pmid=24667632 |doi-access=free }}</ref> The below figure demonstrates this movement with only the first set of muscles.

[[File:Motion of Insectwing.gif|thumb|200px|right|Motion of an insect wing: <br>'''a''' wings <br>'''b''' primary and secondary flight joints <br>'''c''' dorsoventral flight muscles <br>'''d''' longitudinal muscles]]

The movement of the wings and the halteres are mechanically coupled.<ref name="Sane 2015">{{cite journal|last1=Deora|first1=Tanvi|last2=Singh|first2=Amit Kumar|last3=Sane|first3=Sanjay P.|title=Biomechanical basis of wing and haltere coordination in flies|journal=Proceedings of the National Academy of Sciences|date=3 February 2015|volume=112|issue=5|pages=1481–1486|doi=10.1073/pnas.1412279112|pmid=25605915|pmc=4321282|bibcode=2015PNAS..112.1481D|doi-access=free}}</ref> Sane ''et al.'' (2015) demonstrated that in freshly killed flies, without any neural input, the movement of wings was still coupled with the movement of halteres. When a [[forceps|forcep]] was used to manually move a wing up and down, not only did the opposite wing move in synchrony, the halteres also beat in [[Phase (waves)#Phase difference|antiphase]] with both wings. The source of this coupling however, was not between the muscles which control the halteres and those that control the wings. Instead, two small ridges of [[cuticle]] known as the subepimeral ridges were found to be responsible. These ridges connect the right wing to the right haltere and the left wing to the left haltere.<ref name="Sane 2015"/>

Each side of the body must be synchronized and the two sides are also coupled. That is, the left and right wings and thus the left and right halteres always beat at the same frequency. However, the amplitude of the wingbeat does not always have to be the same on the left and right side. This is what allows the flies to turn and is accomplished using a [[Transmission (mechanics)|gearbox]], much like what you would find in an automobile. This gearbox can change the maximum amplitude of the wing movement and determine its speed of motion. The wings of flies even have a [[clutch]] structure at their base. The clutch moves between grooves in the gearbox, to engage and disengage the wing muscles and also modulate the wingbeat amplitude. When the amplitude of the left wing is less than the right, the fly will make a left turn. Even though haltere movement is controlled by separate muscles than the wings, because the wings are mechanically coupled with the halteres, changes in wingbeat frequency extend to the haltere-beat frequency as well, but haltere beat amplitude does not change.<ref name="Sane 2015"/>

Although halteres are coupled with the wings and with each other during flight, some flies oscillate their halteres while walking (without oscillating their wings). Because the haltere muscles are tiny in comparison to the flight muscles, flight muscle activity completely overshadows that of the haltere muscles during flight. It is unknown how haltere muscle activity during flight differs from walking. The left and right halteres show much more variable phase relationships while walking compared with flying, which may indicate decoupling of the left and right haltere muscles.<ref name="Hall 2015" />

===Differences between species===

Although halteres are always synchronized with the movements of the wings, the phase at which they oscillate differs between species. [[Brachycera]]n flies (short-antennaed) oscillate their halteres almost exactly opposite their wings (180 degrees). More ancient suborders such as the [[Nematocera]]ns (long-antennaed flies), which for example include crane flies and mosquitoes, exhibit a variety of wing-haltere phasing. These observed differences in wing-haltere coordination suggest that differences in sensory neuron output also exist between species. This means that the decoding mechanisms used by the [[central nervous system]] to interpret such movements and produce adequate motor output probably also vary depending on [[phylogenetics|phylogeny]].<ref name="Hall 2015" />