Editing Halteres (section)

==Morphology==

[[File:Fly haltere and calyptra.jpg|350px|thumb|right|Electron micrograph of fly haltere and surrounding structures:<br>
'''1''' calyptra (squama)
'''2''' upper calypter (antisquama)
'''3''' haltere
'''4''' mesopleuron
'''5''' hypopleuron
'''6''' coxa
'''7''' wing
'''8''' abdominal segment
'''9''' mesonotum
'''c''' capitellum of haltere
'''p''' pedicel of haltere
'''s''' scabellum of haltere]]

The general structure of halteres are well recognized, but much variability exists between species. The more ancient family groups, such as [[Tipuloidea]] (crane flies), possess halteres with rather long stalks. This causes the haltere bulbs to be much further from the body and easily visible to the naked eye. More derived families, such as [[Calliphoridae]] (blow flies), have developed specialized structures called "calyptrae" or "squama", which are tiny flaps of wing, that cover the haltere. Pringle (1948) hypothesized that they prevented wind turbulence from affecting haltere movements, allowing more precise detection of body position, but this was never tested. The stalk of the haltere is also not always straight. Instead, the stalk's shape in more derived families tends to be reflective of the body shape of the individual. This minimizes the amount of air space between end-knobs and the sides of the abdomen and thorax. In these families, the halteres beat so close to the body that the distance between haltere and thorax is a fraction of the diameter of the haltere bulb.<ref name="Pringle 1948" /> An extreme example of this trait is in the family [[hoverfly|Syrphidae]] (hoverflies), where the bulb of the haltere is positioned nearly perpendicular to the stalk.<ref>{{cite web|title=Recognising hoverflies|url=http://www.biodiversityireland.ie/projects/irish-pollinator-initiative/hoverflies/recognising-hoverflies/|website=National Biodiversity Data Centre|publisher=Biodiversity Ireland|access-date=2 December 2015|archive-date=22 December 2015|archive-url=https://web.archive.org/web/20151222125934/http://www.biodiversityireland.ie/projects/irish-pollinator-initiative/hoverflies/recognising-hoverflies/|url-status=dead}}</ref>

Flies typically hold their halteres at a 90-degree offset. To visualize this, if you were to imagine a person holding their arms out sideways, this would be a 180-degree offset. If that person then moved their arms backward so that the angle created between their fingertips and spine was 90 degrees, this would be a 90-degree offset. The halteres of flies work the same way. They are positioned behind their bodies, forming a 90-degree angle between the haltere bulbs and the center of their thorax. It is necessary for the halteres to be positioned like this in order to detect the three axes of motion. Those axes are yaw, pitch and roll, as illustrated in the above figure (Directions of rotation). The [[mechanoreceptor]]s at the base of the halteres are only able to measure force in two directions (horizontal and vertical), so a single haltere is only able to measure rotations along two of the three axes. Because the halteres are set at different angles (90-degree offset), they also beat along two separate horizontal and vertical axes. This gives them the ability to acquire information from two non-parallel planes and allows sensation of rotation in all three directions. However, flies are most sensitive to pitch.<ref name="Pringle 1948"/><ref>{{cite web|last1=Neal|first1=Jonathan|title=Living With Halteres III|url=https://livingwithinsects.wordpress.com/2015/02/27/living-with-halteres-iii/|website=Living with insects blog|publisher=The Twenty Ten Theme. Blog at WordPress.com|access-date=17 November 2015|date=27 February 2015}}</ref>
[[File:Diversehalteres.tif|thumb|360x360px|Haltere length and shape varies from species to species.]]

===Neurophysiology===

When halteres are experimentally induced to flap, volleys of [[action potential]]s within the [[haltere#haltere nerve|haltere nerve]] occur in synchrony with the haltere-beat frequency.<ref name="Pringle 1948" /> When flies are then rotated, these volleys break down, likely in response to different groups of [[sensillum|sensilla]] being activated to inform the fly of its recently changed body position. Haltere [[afferent nerve fiber|afferents]] have also been shown to terminate in the [[mesothorax|mesothoracic]] [[neuropil]] where flight muscle neurons are located.<ref name="Chan 1998" /> Haltere afferent activity responding to rotations and wing steering behavior converge in this processing region.<ref name="Chan 1998" />

====The haltere nerve====

[[sensory neuron|Sensory inputs]] from five sensory fields located at the base of the haltere all converge onto one nerve, the haltere nerve. How these sensory fields are organized at the level of the central nervous system is currently unknown. It has been determined that those five sensory fields all project to the thorax in a "region-specific" way and afferents originating from the forewing were also shown to converge in the same regions. Not every specific target for the haltere afferents have been determined, but a few connections between [[motor neuron]]s known to be involved in wing steering control and particular haltere sensory fields have been identified, particularly one synapse between the haltere nerve and a wing steering motor neuron known as mnb1.<ref name="Chan 1998" /><ref>{{cite journal|last1=Fayyazuddin|first1=A|last2=Dickinson|first2=MH|title=Haltere afferents provide direct, electrotonic input to a steering motor neuron in the blowfly, Calliphora |journal=The Journal of Neuroscience|date=15 August 1996|volume=16|issue=16|pages=5225–32|pmid=8756451|pmc=6579303|doi=10.1523/JNEUROSCI.16-16-05225.1996}}</ref>

===Muscles===

Flies use indirect flight muscles to accomplish wing movement, and the beating haltere movements are driven by the same group of muscles (see dynamics section). In addition to the indirect flight muscles which are responsible for the flapping motion, there are also steering muscle which control the rotation/angle of the wings. Because halteres evolved from hindwings, the same complement of steering muscles exists for the other directions of movement as well. Chan ''et al.'' (1998) identified 10 direct control muscles in the haltere similar to those found in the forewing. In 1998, Chan and Dickinson proposed that the planned haltere movements (without external forces acting on them) were what initiated planned turns. To explain this, imagine a fly that wishes to turn to the right. Unfortunately, as soon as it does, the halteres sense a body rotation and [[reflex]]ively correct the turn, preventing the fly from changing direction. Chan and Dickinson (1998) suggested that what the fly does to prevent this from occurring is to first move its halteres as if it were being pushed in the opposite direction that it wants to go. The fly has not moved, but the halteres have sensed a perturbation. This would allow the haltere-initiated reflex to occur, correcting the imagined perturbation. The fly would then be able to execute its turn in the desired direction.<ref name="Chan 1998">{{cite journal |last1=Chan |first1=Wai Pang |last2=Prete |first2=Frederick |last3=Dickinson |first3=Michael H. |title=Visual Input to the Efferent Control System of a Fly's 'Gyroscope' |journal=Science |date=10 April 1998 |volume=280 |issue=5361 |pages=289–292 |doi=10.1126/science.280.5361.289 |pmid=9535659 |bibcode=1998Sci...280..289P |s2cid=41890194 }}</ref> This is not how flies actually turn. Mureli and Fox (2015) showed that flies are still capable of performing planned turns even when their halteres have been removed entirely.<ref name="Mureli 2015"/>
[[File:Major fields of campaniform sensilla on the haltere.tif|thumb|379x379px|Diagram of the six major fields of campaniforms on the haltere. Four fields are located dorsally -- the dorsal Hick's papillae (dHP), dorsal basal plate (dBP), dorsal scapal plate (dSP), and the dorsal flanking sensilla (FS). Two fields are located ventrally, the ventral Hick's papillae (vHP) and the ventral scapal plate (vSP).]]

===Campaniform sensilla===

The way in which rotation sensation is accomplished is that there are five distinct sensory fields located at the base of the haltere. These fields, which actually contain the majority of [[campaniform sensilla]] found on the [[exoskeleton]] of blowflies (more than 400 campaniform sensilla per haltere),<ref name="Chan 1998" /><ref name="Gnatzy 1987">{{cite journal|last1=Gnatzy|first1=Werner|last2=Grunert|first2=Ulrike|last3=Bender|first3=Manfred|title=Campaniform sensilla of Calliphora vicina (Insecta, Diptera)|journal=Zoomorphology|date=March 1987|volume=106|issue=5|pages=312–319|doi=10.1007/BF00312005|s2cid=20420858}}</ref> are activated in response to strain created by movements at the haltere base in different directions (due to Coriolis forces acting on the end knobs).<ref name="Pringle 1948" /><ref name="Fraenkel 1938"/><ref name="Nalbach 1993"/> Campaniform sensilla are cap-shaped protrusions located on the surface of the exoskeleton (cuticle) of insects. Attached inside the cap is the tip of a [[dendrite|dendritic]] projection (or [[sensory nerve]] fiber). The outer segment of the dendritic projection is attached to the inside surface of the cap. When the haltere is pushed to one side, the cuticle of the insect bends and the surface of the cap is distorted. The inner dendrite is able to detect this distortion and convert it to an electrical signal which is sent to the central nervous system to be interpreted.<ref>{{cite journal|last1=Keil|first1=TA|title=Functional morphology of insect mechanoreceptors |journal=Microscopy Research and Technique|date=15 December 1997|volume=39|issue=6|pages=506–31|pmid=9438251|doi=10.1002/(sici)1097-0029(19971215)39:6<506::aid-jemt5>3.0.co;2-b|s2cid=5552615 }}</ref>

===Chordotonal organs===

[[Chordotonal organs]] detect and transmit distortions in their position/shape in the same way that campaniform sensilla do. They differ slightly at their site of detection. Chordotonal organs, unlike campaniform sensilla, exist beneath the cuticle and typically respond to stretch as opposed to distortion or bending. Their sensory nerve endings attach between two internal points and when those points are stretched, the difference in length is what is detected and transformed into electrical signaling. There are far fewer chordotonal organs at the base of the haltere than campaniform sensilla (on the order of hundreds), so it is assumed that they are far less important for detecting and transmitting rotational information from haltere movements.<ref name="Pringle 1948" />