About the morphology and the function performed, the wings are characters of great interest in many orders of insects. The etymology of the name of various orders, on other hands, refers to a recurring feature of the wings. This consideration also applies the Diptera, due some features and the specialization acquired during the evolution.

An apomorphic condition of the Diptera is the transformation of the metathoracic wings into the halteres and the consequent reduction of true wings to a single pair. This feature, present without exception through the entire order, is rarely observed in other insects. The presence of a single pair of wings, however, have distinctive traits which make a fly easily recognizable. Moreover, the etymology of the scientific name of the order refers to the presence of only two wings.

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General morphology

Wing morphology
Fig. 1 - Generic morphology of the wing.
a: tip; an: anal lobe; ax: alula; c: cell; cm: costal margin; fv: false veins; pm: posterior margin; pt: pterostigma; rem: remigium; v: veins.
Author: Giancarlo Dessì
(License: Creative Commons BY-NC-SA)

As in most insects, the wing is derived from a tergopleural expansion which inserts between the tergite and the pleuron and articulates, by axillary sclerites, to special structures differentiated from the thoracic plates and composed of processes and sclerites. In the Diptera there is a strong differentiation of posterior wings, called halteres, which have got a sharply reduction and profound changes in form and function. So the halteres are transformed into sensory organs accessories of the flight. The forewings mantain the primary function with structural adjustments that bring the ability to flight at the highest levels of efficiency within the class of Insects.

The forewing is divided into three areas, from proximal to distal (McAlpine, 1981):

The blade is composed of a membranous double layer, bare or more or less pubescent, hyaline or variously pigmented. Inside there are sclerotized tubules called veins. So there are two structural elements: the venation, composed of the veins, and the membrane. This is composed of of a number of more or less large areas called cells, delimited by the veins and the margin. A single cell is closed if completely enclosed by veins, and open if part of the perimeter coincides with the wing margin without veins.

The basic morphological elements of the wing are as follows:

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Axillary and basal areas

Anatomy of the wing base
Fig. 2 - Structure of the base of wing.
1: tegula or costal plate; 2: basicosta or humeral plate; 3: subcostal sclerite; 4: first axillary sclerite; 5: second axillary sclerite; 6: third axillary sclerite; 7: stem vein; 8: proximal median plate; 9: distal median plate; 10: anterior notal wing process (mesonotum); 11: pleural wing process (mesopleuron); 12: posterior notal wing process (mesonotum); 13: costagial break; 14: humeral break; 15: lower calypter or squamula thoracica; 16: upper calypter or squamula; 17: alula or axillary lobe; 18: alular incision; 19 anal lobe; A1: first anal vein; A2: second anal; C: costa; CuA: anterior branch of cubitus; CuP: posterior branch of cubitus; M: posterior branch of media; MA: anterior branch of media; R: radius; Sc: subcosta; a1: anal cell; bc: basal costal cell; bm: basal medial cell; br: basal radial cell; c: costal cell; cup: posterior cubital cell; h: humeral crossvein.
Author: Giancarlo Dessì
(License: Creative Commons BY-NC-SA)

This is a small region including the complex system which connects the wing to the thorax. This area is composed by small sclerites and membranes joined proximally to the pleural sclerites and notal wing processes and distally to the longitudinal veins. They also provide support for the insertion of direct wing muscles. This region is divided into a proximal area, containing the axillary sclerites, and a basal stalk, containing the bases of longitudinal veins. The anatomical structure, rather complicated, is outlined in Figure 2.

The axillary area sensu stricto includes, in order anteroposterior and proximal-distal, these following sclerites:

In some groups, the distal part of the posterior notal wing process is separated from the rest and become a plate of the axillary area forming the fourth axillary sclerite.

The basal stalk is distally close to the axillary area and includes the proximal and distal median sclerites. These are two plates distal to axillary sclerites and placed behind the stem vein and in front to the alula. Proximal and distal sclerites are separated by a fold.

The stalk contains also the bases of longitudinal veins (costa, subcosta, radius, posterior media, cubitus, and anals) and the humeral crossvein. Finally, it entirely includes the stem vein and the anterior branch of media. The first one is the basal section of the radius and appears as a strong and short vein articulated to the first axillary sclerite and aligned with the radius sensu stricto. The second one is ha short crossvein which arises from the bases of the posterior media and anterior cubitus and reach the base of radius.

The main membranous areas of the base are composed of the basal costal cell (bc), the alula, and the calypteres.

The alula, called also axillary lobe, is a posterior expansion of the membrane, usually well developed in most brachycerous families and reduced or lacking in most of Nematocerous. It is delimited anteriorly by the median sclerites and laterally by two indentations more or less deep: the proximal one separates the alula from the upper calypter, the distal one, called alular incision, from the anal lobe.

The upper calypter, or squamula alaris, is a membranous axillary expansion involved in the movements of the wing and is located between the alula, the third axillary sclerite, and the posterior notal wing process; it is considered homologous of the jugal region of other Neoptera- The lower calypter, or squamula thoracica, is the membranous lobe that connects the posterior margin of the wing to the mesonotum; unlike the upper calypter it is fixed. The calypteres are separated by an indentation of the margin more or less deep. When the wing is at rest, the posterior edge is folded so that the upper calypter covers at least in part the lower. In most Brachycerous, the upper calypter is larger than the lower, but in some groups this ratio is inverted. This condition occurs in most Calyptratae and in some families of lower Brachycerous, such as Tabanidae.

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The venation is a morphological element of basical importance in the taxonomic diagnosis, so the dipterologists pay special attention to the interpretation of the homologies and the resultig terminology. Other fields of application, of the study of the venation, concern the phylogeny, the evolution, and the paleontology. The main problem that recurs in the literature is the controversial interpretation of some homologies, due to a substantial simplification of the venation in the Diptera, a trend that occurs in the more advanced orders of insects with membranous wings (Diptera and Hymenoptera): the loss of both longitudinal and crossveins makes it difficult to identify the origin of certain veins and generates incongruity of the terminology adopted in the taxonomic descriptions. Since the late 19th century to today various models of venation scheme are developed; they consist of implementations and adaptations of the original system developed by Comstock & Needham (1898-1899). After the publication of the Manual of Nearctic Diptera (1981) a wide consistency of terminology has emerged, althoug there are still different interpretations by some Authors (Amorim & Rindal, 2007).

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The primary veins in the ground-plan of Neoptera

Groundplan of wing venation of Neoptera
Fig. 3 - Groundplan of wing venation of Neoptera.
A1-3: first, second, third anal; C: costa; CuA: anterior cubitus; CuA1-2: branches of the anterior cubitus; CuP: posterior cubitus; h: humeral; M: media (posterior branch); M1-4: branches of the posterior media; MA: anterior media or arculus or phragma; R: radius; R1: anterior branch of the radius; Rs: radial sector; R2-5: branches of the radial sector; Sc: subcosta; Sc1-2: branches of the subcosta.
Author: Giancarlo Dessì
(License: Creative Commons BY-NC-SA)

It is believed that the veins are residues of the hemocoel containing tracheae (McAlpine, 1981). As a general condition, the wing of insects includes six primary veins, that are called, from the anterior to posterior, as follows:

At least in theory, each vein divides into two branches, one anterior, convex respect to the costal margin, and one posterior, concave. Each branch arise from a common hemocoelic sinus and may further divide in other secondary branches.

This condition, in fact, occurs only in fossil forms of primitive insects, while in the existing there is a reduction more or less significant of the venation, specially in more advanced groups of the Neoptera. The structural adaptions that may be encountered, respect to the groundplan, are as follows:

In particular, in Neoptera, we find the following conditions:

  1. The costa is a simple undivided vein.
  2. The subcosta is usually unbranched, but in primitive forms (Neuropteroidea, Mecoptera) it divides often into two terminal branches referred to as Sc1 and Sc2[1].
  3. The radius divides into two branches, called anterior branch of radius (R1), unbranched, and radial sector (Rs). The latter have two further dichotomous divisions and gives rise to four branches (R2, R3, R4, and R5).
  4. The media loses the primary basal stem and seems arising from the base of the cubitus (Figure 2). The anterior media (MA) is strongly reduced and becomes a basal crossvein, called also arculus or phragma, that reaches the base of the radius. The posterior media (M) has two dichotomous division into four branches (M1, M2, M3, and M4).
  5. The cubitus divides into two branches. The anterior (CuA) divides further into CuA1 and CuA2 branches, while the posterior (CuP) remains unbranched.
  6. The anals are simple and free (A1, A2, ...).
  7. The anterior veins (costa, subcosta, and radius) are usually parallel to the costal margin and lose the primitive concavity of the posterior branches.
  8. The posterior veins (media, cubitus, and anals) bend more or less sharply to the posterior margin and lose the primitive convexity of the anterior branches.

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The ground-plan of Dipters

In Diptera, at least in the ancestral condition, the venation takes up much of the ground-plan of Neoptera, but differs by some features, partly also subject to controversial interpretations.

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Longitudinal and crossveins

Wing venation of ancestral Diptera
Fig. 4 - Groundplan of wing venation of Diptera after McAlpine (1981).
A1-2: first and second anal; C: costa; CuA: anterior cubitus; CuA1-2: branches of anterior cubitus; CuP: posterior cubitus; h: humeral; M: media (posterior branch); M1-3: branches of posterior media; MA: anterior media; m-cu: medial-cubital; m-m: medial; R: radius; R1: anterior branch of radius; Rs: radial sector; R2-5: branches of radial sector; r-m: radial-medial; Sc: subcosta; sc-r: subcostal-radial.
Author: Giancarlo Dessì
(License: Creative Commons BY-NC-SA)

With regard to the longitudinal veins, in addition to features mentioned for the ground-plan of Neoptera, in the Diptera the primitive condition is characterized ad follows.

Costa. It extends along the entire margin, but appears more strong in the costal margin.

Subcosta. Certain Authors believed that the most primitive forms (Tanyderidae, Tipulomorpha) have the subcosta bifurcated into two terminal branches. Sc2 was instead identified by other authors as a crossvein which connects the subcosta to the radiu (sc-r). The dispute was not resolved even in the most recent literature. Referring to the Manual of Nearctic Diptera, McAlpine (1981), in the chapter about the general morphology, indicate this as crossvein, but says that the problem has not importance for taxonomic purpose. In the same nook, in the chapter on Tipulidae sensu lato (including Limoniidae and Cylindrotomidae), Alexander & Byers (1981) consider instead the subcosta as divided into two branches. Finally, no mention is shown in the chapters on Tanyderidae and Trichoceridae. Referring to the Manual of Palaearctic Diptera, in the chapter on the general morphology, Mertz & Haenni (2000) implicity consider the subcsosta unbranched, because report in the glossary the definition of sc-r. Dahl & Krzemińska (1997), for the Trichoceridae, and Krzemiński & Judd (1997), for the Tanyderidae, also refer to the subcosta as a simple vein.

Posterior veins according to Comstock and according to Tillyard
Fig. 5 - Homologies and nomenclature about the posterior veins as interpreted by Comstock and Tillyard.
CuA: anterior cubitus; CuA1, CuA1: branches of the anterior cubitus; CuP: posterior cubitus; M1, M2, M3, M4: branches of the posterior media; m-cu: medial-cubitalal.
Author: Giancarlo Dessì
(License: Creative Commons BY-NC-SA)

Radius. The base of radius shows a constriction like a suture, approximately near the humeral vein. This constriction separates the stem vein from the rest of the radius. Even in the Diptera the double dichotomic division of the radius recurs as primitive condition.

Media. On the structure of the median vein, there are two funtamental characteristics: the reduction of the base, as in the ground-plan of Neoptera, and the distal division of the posterior media. Regarding to the latter, in all Diptera, including the most primitive groups, the branching of posterior media and the anterior cubitus originates a maximum of five veins, instead of six veins in the ancestral condition of Neoptera (four branches from the media and two from the cubitus). However, in the literature there are two opposite interpretations of the homology of the fourth vein, with important consequences on the nomenclature:

These different theories are reflected in the nomenclature adopted. Referring to the primitive structure of the venation (see figure 5), there are these following matches:

These conflicting theories have different relevance since the twenties. The hypothesis by Tillyard found a wide agreement between the Authors until the eighties, while after the pubblication of the Manual of Nearctic Diptera the interpretation by Comstock has prevailed, due its adoption of McAlpine (1981). However, in recent decades some Authors have followed the Tillyard's model, at least for Nematocera.

Cubitus. Concerning the anterior cubitus, see above. The posterior cubitus is a reduced, weak and incomplete vein; it is also untrachead and close to anterior cubitus.

Anals. The number is reduced to two veins only. The first anal is relatively stout, has a convex shape and usually reaches the margin of the wing. The second anal is relatively weak, incomplete, and concave.

The crossveins present as primitive condition are the following:

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Primary cells

Wing cells of ancestral Diptera
Fig. 6 - Cells of the wing in the groundplan of Diptera after McAlpine (1981).
a1-2: anal; bc: basal costal; bm: basal medial or second basal; br: basal radial or first basal; c: costal; cup: posterior cubital; cua1: anterior cubital; d: disc or discal; m1-3: medial; r1-5: radial; sc: subcostal.
Author: Giancarlo Dessì
(License: Creative Commons BY-NC-SA)

By convention, the nomenclature of the cells is based on the veins that surround them anteriorly, using the lowercase. Referring to the primary venation, in the ground-plan there are the following cells:

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Secondary modifications

Wing venation of Tanyderidae
Fig. 7 - Diagram of wing venation of Tanyderidae.
Veins. A1: anal; C: costa; CuA1-2: branches of anterior cubitus; CuP: posterior cubitus; h: humeral; M1-3: branches of media; m-cu: medial-cubital; m-m: medial; R1: anterior branch of radius; Rs: radial sector; R2-5: branches of radial sector; r-m: radial-medial; Sc: subcosta; sc-r: subcostal-radial.
Cells. bm: basal medial; br: basal radial; cup: posterior cubital; d: discal.
Author: Giancarlo Dessì
(License: Creative Commons BY-NC-SA)

Only a few groups of Nematocera approach at the ground-plan as described above. They are the most primitive, such as Tipulomorpha and Ptychopteromorpha. Other primitive characters recur in Culicomorpha and Psychodomorpha. In the rest of Diptera, the venation has evolved so that the specific morphology may deviate considerably from the primitive, sometimes making difficult to interpret the homologies. These adaptations involve substantially the improvement of the functionality of the wings.

In general, the number of divisions of longitudinal veins in the secondary specializations is more reduced than the ancestral condition. In particular, this trend involves the radial sector and the media. A more detailed overview on the most frequent adaptations is summarized in the following sections.

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The extension of the costa to the entire margin remains, as primitive character, in some groups of Nematocera and, rarely, in some Orthorrhapha, while in the generality of Diptera and, in particular, of Brachycera, the costa reaches the end of the radius or the media.

A character particularly important in taxonomy, which occurs manily in Cyclorrhapha, is the presence of 1-3 breaks of the costa, called costal breaks, corresponding to points of flexibility of the wing. Specific names for these breaks was first introduced by McAlpine (1981):

The presence and the number of costal break are uniform characters at level of single families or higher taxa, so they are useful in taxonomic diagnosis.

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Compared to the primitive type, the subcosta of most Diptera has different degrees of simplification and can lose the appearance of a longitudinal vein well formed. Among the changes most frequent there are the following:

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Wing venation of Tabanidae
Fig. 8 - Diagram of wing venation of Tabanidae.
Veins. A1-2: first and second anal; C: costa; CuA1-2: branches of anterior cubitus; h: humeral; M1-3: branches of media; m-cu: medial-cubital; m-m: medial; R1: anterior branch of radius; Rs: radial sector; R2-5: branches of radial sector; r-m: radial-medial; Sc: subcosta.
Cells. bm: basal medial; br: basal radial; cup: posterior cubital; d: discal.
Author: Giancarlo Dessì
(License: Creative Commons BY-NC-SA)

The primitive condition with the division into five branches, with the first from the anterior branch and the other from the radial sector, remains only in Tanyderidae and Psychodidae families. In all other Diptera, the branches of radial sector are reduced to 2-3, except for cases of extreme simplification of the venation. The anterior branch, however, remains present in allo groups, although is usually short, specially in Cyclorrhapha.

The nature of the reduction of the branches of the radial sector is uncertain and partly controversial. The examination of the venation within single groups, in fact, induces different interpretations based on the fusion of R2 and R3 or, alternatively, on the fusion of R4 with R5. For a more detailed analysis of possible homologies, see the discussion within the singole taxonomic groups. These conditions occur in Nematocera and Orthorrhapha, while in upper Brachycera, usually, there is the highes level of simplification, with loss of the second fork of radial sector. So, in these diptera, the radius divides only into three free veins: R1, R2+3 e R4+5.

The sectorial crossvein appears only in a few groups, but sometimes it is interpreted as blending of R2 on R1. Other special feature, recurring in some groups, is the presence of supernumerary crossveins or stumps.

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Regardless of different interpretations concerning the loss of M4, as mentioned above, throughout the order recurs the reduction in number of divisions of posterior media. The extreme condition, typical of Cyclorrhapha, is the presence of only one vein undivided, usually referred as "media" or sometimes M1 or M1+2.

The probable loss of M3 and the probable merge of M1 and M2 have important implications on the morphology of the membrane in the discal area. The presence of a single cell called generically "disc", in the middle of blade, recurs throghout the Diptera, but its genesis differs depending on the systematic group:

Under the system proposed by Comstock (1918) and adopted by McAlpine (1981), d and dm cells can not be homologous: the true discal is delimited by two primary branches of posterior media (M1+2 anteriorly and M3 posteriorly) and is distally closed by the medial crossvein (m-m); the medial discal cell, however, is delimited anteriorly by the media and posteriorly by the cubitus (CuA1) and is distally closed by the discal medial-cubital crossvein. Finally, both cells are distal to the basal media cell, but the proximal limit of the true discal is given by the fork of the posterior media, while the proximal limit of the discal medial is given by a crossvein, the basal medial-cubital, that connects the media to the cubitus.

A morphological detail wich appears in some groups of Cyclorrhaphous is the partial development or the total disappearance of the basal medial-cubital crossvein, so the basal medial cell is merged with the discal medial making a single closed cell.

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Wing venation of genus Musca
Fig. 9 - Diagram of wing venation of genus Musca (Brachycera: Muscidae).
Costal breaks. cb: costagial; hb: humeral; sb: subcostal
Veins. A1-2: first and second anal; bm-cu: basal medial-cubital; C: costa; CuA1-2: branches of anterior cubitus; dm-cu: discal medial-cubital; h: humeral; M: media; R1: anterior branch of radius; Rs: radial sector; R2-5: branches of radial sector; r-m: radial-medial; Sc: subcosta.
Cells. bm: basal medial; br: basal radial; cup: posterior cubital; dm: discal medial.
Author: Giancarlo Dessì
(License: Creative Commons BY-NC-SA)

About the posterior cubitus (CuP) no relevant differences occurs throghout the Diptera compared with the ancestral condition: this vein is weak, untrachead, and incomplete in all Diptera. The only difference that appears is the intensification of the character in upper Diptera, which have the posterior cubitus very weak.

The anterior cubitus has rather a strong secondary differentiation. In the primitive flies, this vein gives rise to two branches that reach the posterior margin with a convex profile. This condition recurs in all Nematocerous, except in cases of reduced venation with the loss of one o two branches of cubitus.

In brachycerous Diptera, the vein CuA2 is strobly curved and tends to converge on the first anal. In some Orthorrhaphous, the CuA2 vein is still a free vein and reaches the margin close to anal. In other Orthorrhaphous and the generality of Cyclorrhaphous, CuA2 bends sharply and converge on A1 until it merges with this vein to form a terminal common section indicates as A1+CuA2. This vein may reach or not the edge of wing.

The morphology of CuA2 vein is reflected in the appearance of the posterior cells:

The differentiation in shape of this veins and the cup cell is very important as element of taxonomic diagnosis within the generality of Brachycerous.

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In the ground-plan of Diptera wings, the two anal veins are are complete and free, so reach the margin of the wing delimiting two open cells well defined. This condition remains only in most Tipulomorpha, while in the rest of Diptera recurs a substantial simplification of the venation in the anal lobe.

The first anal is usually weak but well developed and complete. In many groups it is often incomplete and in the generality of Brachycera is merged with the CuA2 to form a common distal section. Whe the venation is reduced, the first anal is vestigial or missing.

The second anal, in lower Diptera, is usually weak, poorly developed and incomplete, while in most of Brachycera is a rudimentary vein confined to the base of anal lobe and close to alular incision.

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The metathoracic wings of Diptera lose their original function and are transformed into the halters, which have function in stabilization of the flight with the perception and the control of the balance of the body. These organs appear as a pedunculated bulb, where we distinguish three parts: scabellum (basal), pedicellum (intermediate) and capitulum (distal and bulbous).

The scabellum is an expansion articulated on the thorax, rich in sensilla proprioceptors, sensory organs that detect the balance. The pedicellum and the capitulum, due their shapes, adjust the balance of body and stabilize the position during the flight, by vibrations. The pedicellum bears rows of setulae homologous to those of the costa in the forewing.

Due the substantial morphological uniformity, the halteres have limited interest in taxonomy. Any diagnostic elements that are considered are generally limited to the color.

[1] According to certain Authors, this condition recurs even in Diptera of family Tanyderidae and, in part, of infraorder Tipulomorpha, among the most primitives. However, the prevailing is to consider the branch Sc2 as a crossvein which connects the subcosta to the anterior branch of radius.
[2] No enough references there are in literature about of presence of subcostal-radial vein, which effectively separates two adjacent cells. On the other hand, in these cases the subcostal cell is poorly developed and has limited interest in taxonomic diagnosis.

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