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Table of contents

General insects (Notum - Pleura - Sternum) - The thorax of Dipters (Prothorax - Mesothorax - Metathorax) - Chaetotaxy - Bibliography - Web resources

The thorax is the second morphological region of the body and it bears the locomotion organs, represented by the legs and, in winged insects, by the wings. In all Insects it is composed by morphological and structural organization of the first three postcephalic segments, named prothorax, mesothorax, and metathorax, in order anteroposterior.

The level of degree of complexity and morphostructural differentiation of single thoracic segments is closely related to the development and the functionality of the organs of movement, especially the wings: in the wingless insects, the thoracic segments are relatively simple and poorly differentiated, while in the winged forms there is a strong development of meso- and metathorax, order to adapt the exoskeleton to the flight functionality and the attachment of the extrinsic muscles of wings. In the Diptera, in which the functionality and development is limited to forewings, the mesothorax is greater than the prothorax and metathorax and it occupies most of the region, with a morphological complexity due to the differentiation of more dorsal and lateral sclerites.

The pro-, meso-, and meta- suffixex are usually used to refer to the segment to which it belongs the sclerite or the appendix. For example, the fore, mid, and hind tibiae are respectively named also protibiae, mesotibiae, and metatibiae, so the pleura are distinct in propleura, mesopleura, and metapleura, etc. In the terminology related to the Diptera, often omit the suffix meso-, being an implicit reference to the second thoracic segment, while the suffixes pro- and meta- are always used to refer to other two segments.

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General insects

Secondary metamerism of the thorax
Fig. 1 - Secondary metameric divisions of the thorax.
I: prothorax; II: mesothorax; Bs: basisternite; N: notum; S: sternum; St: sternellum; as: acrosternite; at: acrotergite; fr: phragma; fu: furca; pm: primary intersegmental membrane; sm: secondary intersegmental membrane; s.a.: antecostal suture; s.s.: sternacostal suture.
Author: Giancarlo Dessì
(License: Creative Commons BY-NC-SA)

In Insects, the exoskeleton of each thoracic segments has a ring structure composed by the junction of four plates: the one dorsal and the one ventral are unpaired and medians, and two lateral are symmetrical. The dorsal plate is named tergum or tergite or notum, the ventral plate sternum or sternite, the lateral pleura. The origin of pleura, in Insects or, more generally, in the clade of Tracheata (including Hexapoda and Myriapoda), is controversial and has been the subject of various theories which has occurred since the end of 19th century. Historically, the most accepted hypothesis is that the pleura derived form the differentiation of a part of a basal segment of legs, called subcoxa Heymons, 1896, although this hypothesis was formulated in different ways in the details.

The general morphology of dorsal, lateral, and ventral plates is complicated, especially in Pterygota, by the secondary metamerism and the division of plates in more sclerites. The secondary metamerism derived from the fusion of tergite and sclerite of a segment with the posterior parts of tergite and sternite fo previous segment, that are called, respectively acrotergite and acrosternite.

In the tergum, an antecostal suture remains as a trace of the intersegmental junction. Here, the endoskeleton forms internal dorsal processes called phragmata, to which the extrinsic muscles of wings attach. The acrosternite remains separed from sternite of the next segment by a primary intersegmental membrane. Both the acrotergite and the acrosternite are separed from their original segment by a secondary membrane.

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Notum

The structure of the notum or tergum is characterized by the possible sclerotization of secondary membrane. In this case, the acrotergite is called postnotum and is bounded anteriorly by the sclerotized membrane and posteriorly by the antecostal suture. So, the posnotum is a sclerite derived from a segment and intimately associated with the tergite of the next segment.

Thorax of Tipulomorpha dorsal view
Fig. 2 - Morphological dorsal regions of thorax of Tipulomorpha.
1: prescutum; 2: scutum; 3: scutellum; 4: prescutal suture; 5: transverse suture; 6: scutoscutellar suture; 7: parapsidal suture or notaulus; 8: paratergite; 9: pronotum; 10: anepisternum; 11: anepimeron; 12: basalare; 13: subalare; 14: laterotergite; 15: mediotergite; 16: metanotum; 17: halter.
Author: Giancarlo Dessì
(License: Creative Commons BY-NC-SA)

In addition to these sutures, which define the primary and secondary metamerism, there are others that divide the notum in more sclerites. The most occurrent are the transverse suture and, posteriorly, the scutoscutellar suture. When all these sutures are present, the notum is divided in prescutum, scutum, and scutellum. Other sutures are present in restricted groups of insects, such as the notauli of Hymenoptera, represented by two simmetrical longitudinal sutures, near to lateral hedges of the scutum and posteriorly convergent.

In most winged insects, the presence of these transverse sutures divides the three thoracic tergites as follows:

This schematic division has a purely theoretical value because not all the sclerites are simultaneously present, especially in the metanotum, which usually has a simpler structure than the mesonotum. This simplification reaches the highest expression in the Dipters, where the metanotum is strongly reduced in front to mesonotum.

On each side, the notum generates two processes (notal wing processes, distinct in anterior and posterior) that offer attachment to pteral sclerites of wings.

Thorax of Tabanus atratus
Fig. 3 - Thorax of Tabanus atratus (Brachycera: Tabanidae) (lateral and dorsal view)
In evidence the wing processes (green) and the axillary sclerites (orange). 1P, 2P, 3P: first, second and third axillary sclerites; ANP: anterior notal wing process; AxC: axillary cord; epm: epimeron; eps: episternum; PN: postnotum; PNP: posterior notal wing process; psc: prescutum; scl: scutellum; sct: scutum; WP: pleural wing process.
Author: Robert Evans Snodgrass (1909)
Modified from the original drawing
(License: Public Domain)

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Pleura

The pleural region is characterized by the presence of the spiracles (meso- and metathorax), by the possible insertion of the wing, and by the articulation of the legs. The insertion of the wing is positioned at the top of the pleuron, while the articulation of the first segment of legs (coxa) is in the lower part. Even in this case it is often referred to the thoracic segment with the terms of proepisternum, mesoepisternum, metaepisternum, etc. An horizontal suture, not always present, can divide the episternum into two sclerites:

Similarly, another suture can divide the epimeron into two sclerites:

Thorax of Tipulomorpha lateral view
Fig. 4 - Morphological lateral regions of thorax of Tipulomorpha.
1: prescutum; 2: scutum; 3: scutellum; 4: prescutal suture; 5: transverse suture; 6: scutoscutellar suture; 7: parapsidal suture or notaulus; 8: paratergite; 9: antepronotum; 10: postpronotum; 11: cervical sclerites; 12: propleuron; 13: prosternum; 14: anepisternum; 15: katepisternum; 16: anepimeron; 17: katepimeron; 18: meron or hypopleuron; 19 anatergite (laterotergite); 20: katatergite (laterotergite); 21: mediotergite; 22: metanotum; 23: metanepisternum; 24: metepimeron; 25: metakatepisternum; 26: basalare, pleural wing process, and subalare (in anteroposterior order); 27: forecoxa; 28: midcoxa; 29: hindcoxa; 30: halter.
Author: Giancarlo Dessì
(License: Creative Commons BY-NC-SA)

These subdivisions are generally present in the mesopleuron and the metapleuron, while the propleuron is simple and undivided or divided in proepisternum and proepimeron.

The two spiracles open in the intersegmental area between the propleuron and the mesopleuron and between the mesopleuron and the metapleuron. By convention, they are considered originated by the posterior segment, so the anterior spiracle is called mesothoracic and the posterior is called metathoracic.

The morphology of the pleural region is completed by further sclerites, usually reduced, which are integral parts of the articulation of wings and legs.

The wings of Neoptera are articulated to the thorax in the axillary region, between the tergum and the pleuron. The pleura completes the complex of articulation giving two sclerites, the basalare and the subalare, and a pleural wing process; the basalare is differentiated by the episternum, the subalare by the epimeron. The pleural wing process, finally, is represented by the junction of the episternum with the epimeron and is positioned above the pleural suture. The pleural sclerites and process, with the notal processes and the pteral sclerites, the complex system of articulations which joins the base of the wing with the transition between notum and pleuron. On these sclerites are inserted the direct wing muscles, that allow the folding of the wing on the abdomen and its rotation around the longitudinal axis.

The articulation of the leg to the pleuron has different structure, with the presence of sclerites of uncertain and controversial origin. The coxa is the first segment of the leg and it connect to the pleural region by a membrane or a true articulation with intermediated sclerites. Among these it appears the trochantin, anterior to the base of the coxa, and the meron, positioned posteriorly. The meron is originated from the division of coxa by a suture (basicostal suture) and in some groups, including the Dipters, it can be flattened and appears as a sclerite of the pleuron, placed below the epimeron.

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Sternum

The sternum is the ventral plate that closes the exoskeleton of each thoracic segment. In the winged insects is poorly developed than the notum and the pleura, while in wingless insects is large and usually flattened. The presence of sutures and joining membranes divides the sternum in various sclerites. The maximum number of sternal sclerites is four. In antero-posterior order, they are called presternum, basisternum (or sternum sensu stricto), sternellum (or furcasternum), poststernum (or spinasternum).

The basilary suddivision is originated by the sternocostal suture, which separates the basisternum from the sternellum and run between the coxae. This suture is inside associated with the ventral process of the endoskeleton called furca. Anteriorly, the presence of a presternal suture separates the basisternum from the presternum, that appears as a narrow sclerite. The poststernellum is instead generated by the secondary metamerism and so it identifies with the acrosternite. Unlike the acrotergite, that can merge with the notum of the next segment, the poststernellum remains separate from both adjacent segments, and therefore appears as a intermediate sclerite. It is called also spinasternum because bears inside a process spine-like of the endoskeleton, placed behind the furca.

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The thorax of Dipters

Thorax of Cyclorrhapha dorsal view
Fig. 5 - Morphological dorsal regions of thorax of Cyclorrhaphous.
1: presutural area; 2: postsutural area; 3: scutellum; 4: postpronotum or humeral call; 5: notopleuron; 6: prealar callus; 7: supralar area; 8: postalar callus; 9: transverse suture; 10: scuto-scutellar suture; 11: anepisternum or dorsal episternum; 12: anepimeron or dorsal epimeron; 13: laterotergite or pleurotergite (postnotum); 14: mediotergite (postnotum).
Author: Giancarlo Dessì
(License: Creative Commons BY-NC-SA)

The Diptera is one of the most advanced lineages within the Neoptera and have developed a special aptitude for flight. But they also have concentrated the function of flight sensu stricto in the forewings, while the metathoracic wings have specialized in a sensory function closely related to the flight. These two aspects affect the peculiarities that differ this order from other insects: first, there is a simplification in the morphology and structure of prothorax and metathorax, but there is also a greater complexity in the mesothorax. So, the thorax of Diptera is apparently simpler than other order, especially when referring to the loss of metameric subdivision in pro-, meso-, and metathorax. On the other hand, this apparent morphological simplification is due to the functional specialization of the mosothorax, where the structure becomes more complex in the tergal and pleural regions, which are the most involved in the flight.

The terminology recurring in the literature is confusing because affexted by ambiguities due to difficult interpretation of homologies, sometimes incorrect or controversial. Terms used to call various sclerites was sometimes inappropriate (McAlpine, 1981). It follows that a sclerite may have different names adopted by different Authors. Herein we will use the terminology proposed and adopted in the Manual of Nearctic Diptera.

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Prothorax

The prothorax of Dipters is poorly developed and closely associated to the mesothorax. Only some sclerites, placed in the anterior part of thorax remains, as traces of this segment, but they are hardly visible from the dorsal view. It joins to the head by a membrane, named neck or cervix, in which can differentiate small cervical sclerites.

The dorsal region (pronotum) is longitudinally divided into two sclerites, called respectively antepronotum and postpronotum, which have various size and shape. The antepronotum is usually poorly developed, with the exception of the Nematocera; in this case it can differentiate two lateral and symmetrical expansions, called antepronotal lobes. The postpronotum is instead well developed in the upper Diptera, but is intimately fused with the anterior region of mesonotum. Of the postpronotum are well visible two lateral expansions, called postpronotal lobes or humeral calli.

The lateral region (propleuron) is reduced to a small area antero-lateral. The presence of a pleural suture, often incomplete, divides the propleuron into two sclerites, the proepisternum (anterior) and the proepimeron (posterior).

The ventral region (prosternum) consists of a single sclerite but can be also divided into a presternum (anterior) and a basisternum (posterior). Usually it is separated from the pleura by a membranous area, but in some lower Brachycera can differentiate a sclerotization, named precoxal bridge, which join the prosternum to the proepisternum.

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Mesothorax

The mesothorax is the morphological region most important, due both to its exceptional development and its complex articulation of sclerites, which can be hardly indentifiable in the homologies. The importance in the taxonomy developed a detailed terminology in the literature, sometimes confused due different interpretations of the homologies.

Mesonotum

The mesonotum occupies most of dorsal view of the thorax. In order anteroposterior we can identify four parts, called as follow (McAlpine, 1981): prescutum, scutum, scutellum, and postnotum. The scutum is the most large, condition considered apomorphic for the entire order of Diptera.

The anterior region, strongly reduced, is the prescutum and is visible only in some primitive groups (Tipulidae, Ptychopteridae) because of presence of prescutal suture, that is placed in the anterolateral region between the prescutum and the scutum. In manu nematocerous and orthorrhaphous Diptera, the posterolateral area between of prescutum is crossed by a longitudinal suture which separates a narrow lateral sclerite, called paratergite.

In the scutum, the most significant character is the differentiation in two areas, due to the transverse suture, called presutural and postsutural. The size and the shape of the transverse suture differ depending on the systematic group:

In Nematocerous may be other sutures. Among these are cited the lateral parapsidal sutures (notauli), which run longitudinally and symmetrically near the lateral hedges of the scutum, and the median or acrostichal scutal suture, which runs longitudinally along the median axis. These structures are present only in specific taxonomic groups. In the upper Brachycerous, at the lateral end of the transverse suture there is a dorsolateral sclerite, paired and symmetrical, called notopleuron and placed between the humeral callus and the insertion of the wing. This sclerite could be homologous of the paratergite fo lower Diptera (McAlpine, 1981).

The lateral area of the scutum, related to the insertion of the wing, usually shows swellings and ridges which have a specific terminology. In this area there are an anterior and posterior swelling, named respectively prealar callus and postalar callus. The lateral hedge of the scutum between the calli is named supralar area. Posteriorly and ventrally to postalar callus may be a relief, named postalar ridge.

Posteriorly, the scutum is separated from the scutellum by a suture named scutoscutellar. The scutellum is the morphological feature more visibile in the dorsum after the scutum and it is placed immediately behind the latter. Usually it appears as asubrouded or subtriangular sclerite , sometimes with particular shapes. From the lateral view it appears often as prominence that overcomes the other posterior structures, represented by the postnotum and the metanotum, which are hidden from dorsal view.

The postnotum is located behind the scutellum and is visible from lateral view. Some Author called it "metanotum" or "postscutellum", but these names are considered incorrect by McAlpine (1981). It consists of three sclerites, the one unpaired and median (mediotergite or subscutellum), the other lateral, paired and symmetrical (laterotergites or pleurotergites or mesopleurotergites). The laterotergites are often divided in two small plates: the dorsal one called anatergite, the ventral one called katatergite. La anterior portion of mediotergite is usually concave, but the Tachinidae and some other flies show a convex profile, called subscutellum.

Mesopleuron

Thorax of Cyclorrhapha in lateral view
Fig. 6 - Morphological lateral regions of thorax of Cyclorrhaphous.
1: presutural area; 2: postsutural area; 3: scutellum; 4: postpronotum or humeral callus; 5: notopleuron; 6: proepisternum; 7: proepimeron; 8: anepisternum or dorsal episternum; 9: katepisternum or ventral episternum; 10: anepimeron or dorsal epimeron; 11: katepimeron or ventral epimeron; 12: meron or hypopleuron; 13: anatergite (laterotergite); 14: katatergite (laterotergite); 15: mediotergite; 16: metanotum; 17: anterior spiracle; 18: posterior spiracle; 19 basalare; 20: pleural wing process; 21: subalare; 22: transverse suture; 23: scutoscutellar suture; 24: anapleural suture; 25: pleural suture; 26: transepimeral suture; 27: halter; 28: procoxa; 29: mesocoxa; 30: metacoxa.
Author: Giancarlo Dessì
(License: Creative Commons BY-NC-SA)

As in other winged insects, a pleural suture go through the mesopleuron of Dipteres from the base of the coxa to the insertion of wing, and divides it into two pleural plates, called episternum (anterior) and epimeron (posterior). These plates are further divided by two sutures, more or less horizontal, that cross the pleural suture.

The episternum is divided by the anapleural suture into a dorsal and a ventral sclerite. The first one is called anepisternum or dorsal episternum and is placed between the anterior spiracle and the insertion of wing. Some Authors called the anepisternum as mesopleuron, but the use of this term is ambiguous, because it should refer to entire lateral region of the mesothorax (McAlpine, 1981). On the anepisternum there is a vertical membranous area, called episternal or anepisternal fissure or, sometimes, anepisternal membrane. At the posterodorsal end of this fissure the anepisternum differentiates a small plate, called basalare, which is the anterior pleural sclerite of the axillar region. The katepisternum appears as a convex plate with a subtriangular shape that occupies the lower part of mesothorax in front to the coxa. Also this sclerite has various and ambiguous names, referring to different interpretation of homologies.

The epimeron is also divided by a transepimeral suture into two sclerites, called anepimeron or dorsal epimeron and katepimeron or ventral epimeron. At the dorsal hedge, the anepimeron differentiates other the other two sclerites of the axillar region: the anterior one is a bulbous expansion called pleural wing process and is located at the dorsal end of the pleural suture; the posterior one is the third sclerite, called subalare. The katepimeron appears as a narrow and small plate placed under the anepimeron. Sometimes it is fused with the sclerite below, the meron making a single plate called meropleurite.

The meron, called also hypopleuron or meropleuron, is a part differentiated by the coxa which has been strongly flattened to become an integral morphological part of the mesopleuron. Due to the reduced size of the katepimeron, the meron is placed at the same height of katepisternum. Usually it is poorly developed in the primitive families.

Mesosternum

The mesosternum is a region poorly developed and invaginated, which has limited interest in taxonomy. It is well developed in some families of Nematocera only.

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Metathorax

The metathorax of Dipters is a reduced segment placed at the posterior part of thorax. It is closely joined to mesothorax and the first abdominal segment. Usually is divided into two areas which are further subdivided in other sclerites. The dorsal plate is identified as the metanotum and extends between the bases of halteres. Under the metanotum there is the metapleuron, that may be divided by a pleural suture into metaepisternum and metaepimeron. Further subdivisions may be occur but only in some groups. The metasternum, finally, is a strongly reduced and invaginated area, not visible in most of Diptera.

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Chaetotaxy

The chaetotaxy of the thorax is one of the most important morphological elements for the taxonomic determination within the Brachycera and, in particular, the Schizophora. The recurrent terminology is again essential, however it has a substantial uniformity. Among the characters examined, in general, there is the presence, number, size, position. Great importance is given to macro- and microtrichia present in the dorsal and dorsolateral regions and in the mesepisternum, while the chaetotaxy of other regions is considered in specific cases. In general, setae and setulae give name from the sclerite in which they are inserted. Only on the scutum, for the large size of this region, we discern between different areas where it is more or less distinctly divided.

The definition of the scutal chaetotaxy is based on the position compared to the transverse suture and the media sagittal plane. Based on the first criterion, setae and hairs are distinct into presutural and postsutural, when placed respectively in front or behind the transverse suture. Under the second criterion, we distinguish between the following series:

  1. Acrostichal: these are the trichia adjacent to the median longitudinal axis of the scutum. They may be distributed more or less irregular, or align in two or more longitudinal rows. Usually, the descriptions do not consider the symmetry: the acrostichal zone is a median and unpaired band, therefore we consider the absolute number of acrostichal series. The number of rows, the number of setae in each row, the size and the thickness are elements given in the descriptions. In many groups, the acrostichal setae are lost and replaced by setulae or hairs. Sometimes there are two bristles, more developed than the other, inserted in front the scutoscutellar suture, so they are called prescutellar bristles.
  2. Dorsocentral: these are aligned along two rows adjacent to the acrostichal band. The descriptions consider the bilateral symmetry and refer to a single side.
  3. Intralar: these are aligned more or less regular near the dorsocentral serie. The position is not well defined, but it corresponds approximately to the band that extends to the sides of the dorsal part of the scutum.
  4. Posthumeral: they are aligned in the presutural area, parallel to the suture which separates the scutum from the humeral calli.
  5. Supralar: limited in number, these setae are located from prealar callus to the supralar area.
  6. Postalar: limited in number, they are located on the postalar callus and, therefore, near the lateral margin of the scutum behind the insertion of the wing.

Setae of other dorsal sclerites are easily identifiable due to well defined position. These setae are called respectively humeral or postpronotal (on the humeral calli), notopleural (on the notopleuron), scutellar (on the scutellum). Scutellar setae have different positions because they arise from the margin or, sometimes, also from the dorsal side of the scutellum (called disc). Scutellar setae on the margin are distinct into prebasal, basal, lateral, preapical, subapical, and apical. Those on the disc are usually scattered and poorly developed and are called discal.

About the chaetotaxy of the pleura, the characters taken into consideration are generally limited to the presence, the number, and the position of setae and group of hairs on the anepisternum and the katepisternum. These sclerites have often one o more well developed setae which have interest in taxonomic diagnoses. The presence of hairs or setae in other sclerites is considered in the descriptions of some groups. In these contexts the description may refers to hairs and setae present in the propleuron (proepisternal and proepimeral setae), the anepimeron (anepimeral), and the meron (meral or hypopleural).

Chaetotaxy of thorax of muscoid fly dorsal view
Chaetotaxy of thorax of muscoid fly lateral view
Fig. 7 - Chaetotaxy of the thorax on a muscoid fly (Brachycera: Schizophora). (A: dorsal view - B: lateral view )
1: acrostichal; 2: dorsocentral; 3: intralar; 4: supralar; 5: postalar; 6: humeral or postpronotal; 7: posthumeral; 8: notopleural; 9: prescutellar; 10: discal scutellar; 11: scutellar (apical and subapical); 12: scutellar (preapical and lateral); 13: scutellar (basal and prebasal); 14: presutural; 15: postsutural; 16: proepisternal; 17: proepimeral; 18: anepisternal; 19: katepisternal; 20: anepimeral; 21: hypopleural or meral.
Author: Giancarlo Dessì
(License: Creative Commons BY-NC-SA)

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Bibliography

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Web resources

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