Compared to females, the males have the abdomen with a lower number of apparent urites and a strong structural and morphological complexity of the hypopygium. Moreover, in the order we see a heterogeneous differentiation of organs that have functions more or less similar regardless of the homologies. The functional, morphological, and anatomical variability makes difficult the identification of homologies and this has led to formulate different hypothesis, even conflicting, and to adopt a varied and confusing terminology in the literature on Diptera (McAlpine, 1981; Zatwarnicki, 1996; Sinclair, 2000; Cumming & Wood, 2009).

Male terminalia of Peckia
Fig. 1 - Male terminalia of Peckia Robineau-Desvoidy, 1830 (Brachycera: Sarcophagidae).
Left: lateral view; right: details of the phallus.
b: basiphallus; d: distiphallus; C: cercus; Ep: epandrium (=periandrium); P: phallus (=aedeagus); Po: postgonite (=paramere sensu McAlpine, paraphyse sensu Griffiths); Pr: pregonite (=gonopod sensu McAlpine); S: surstylus (=gonostylus sensu Zatwarnicki); St: syntergosternite 7+8.
Authors: Karine Pinto e Vairo; Cátia Anunes de Mello-Patiu; Claudio J.N. de Carvalho
Original source
(License: Creative Commons BY-NC)

A substantial contribution to standardize the terminology comes once again from the Manual of Nearctic Diptera (McAlpine, 1981). Having said that, unlike other morphological regions, simpler than the male terminalia, the terminology proposed by the Manual of Nearctic Diptera is outdated in part, after further developments produced by various authors during the 90s, as reported by Sinclair (2000) in the Manual of Palaearctic Diptera. On the other hand there are, even today, many different and conflicting interpretations that makes still unsolved and controversial the discussion of this topic by dipterists (Cumming et al., 1995; Cumming & Sinclair, 1996; Griffiths, 1996; Zatwarnicki, 1996; Sinclair, 2000).

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In its basic structure, the male postabdomen is composed of urites 6-8, more or less modified, the hypopygium sensu stricto, and the rudiments of the last segment. As mentioned above, the development and shape of sclerites and appendages vary greatly throughout the order, with significant implications for the phylogeny and taxonomy. A further element of differentiation is the expression of two phenomena, recurring combined but with different degrees in most Diptera:

The rotation, which occurs by different ways, has anatomical and morphological implications. The rotation by 90° involves an asymmetric conformation of the hypopygium, because the genitalia are directed to one side of the median sagittal plane. The rotation by 180°, called inversion (McAlpine, 1981), has morphological reflexes because it reverses the positions of the epandrium, the hypandrium, and the genital appendages referring to he horizontal plane. The rotation by 360°, called circumversion (McAlpine, 1981), has substantial anatomical implications: in fact, the external structures retain their original position referring to the planes of symmetry, but the internal organs of the genitalia, the nervous system, and the tracheal system are spirally convoluted around the rectum.

The ventral bending, derived from a differential development of tergites 6-8 at the expense of the ventral plates, brings the terminalia under the abdomen, making it less visible from the dorsal view.

These changes are reflected on the behavior, because they affect the positions taken by the male and female during the mating.

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Urites 6-8

Male terminalia of Nematocera lateral view
Fig. 2 - Schematic drawing of the male hypopygium of a hypothetic nematocerous. Lateral view.
a ej: ejaculatory apodeme; a gx: gonocoxal apodeme; ce: cercus; ed: aedeagus; ep: epandrium; eprc: epiproct; gns: gonostylus; gnx: gonocoxite; ip: hypandrium; iprc: hypoproct; p gx: gonocoxal bridge; pm: paramere; v: sperm sac.
Author: Giancarlo Dessì
(License: Creative Commons BY-NC-SA)

From a morphological point of view, the location of urites 6-8 in the preabdomen or in the postabdomen of males is not clearly defined, because of the variability occurring throughout the order. These segments come before the terminalia sensu stricto, composed of the ninth urite, the vestiges of the following segments and the attached appendages. However, in the males of most Diptera there is a significant difference between the preabdomen composed of the visible urites and the section including the urites 6-8.

Usually, the literature does not refer to a morphological region as the pregenital section of female postabdomen, but this section is implicitly identified discerning between the first preabdominal segments (usually 1 to 5) and the following more or less modified. These morphological and structural adaptations of urites 6-8 are often combined with the rotation and the ventral flexion of the postabdomen or, rarely, with the dorsal flexion in some primitive families. In fact, the rotation may involve one or more segments before the hypopygium and the longitudinal flexion is caused by different development of their dorsal and ventral sclerites. The changes that involve these segments are the following:

In the nematocerous Diptera, the segments 6-8 are poorly differentiated compared with the previous, therefore the abdomen is oblong. Its shape is cylindrical or tapered if the last segments are progressively smaller (Ceratopogonidae, Mycetophiliformia, etc.). In some groups, the tergite 8 is shorter than the sternite causing the curvature of the abdomen and the dorsal flexion of the hypopygium (p.a. Blephariceridae, Deuterophlebiidae). The twisting of the abdomen is well described in all Culicidae: this occurs through the inversion of the urite 8, but with a partial dragging of preceding segments. Partial rotation or rotation by 180° are reported also in other nematocerous families, but in groups that are limited to genus, tribe or subfamily-group level, often not well documented. Eventual inversion in the urite 9 may drag preceding segments (McAlpine, 1981). Finally, in Bolitophilidae the seventh and eighth segments are telescoped and have strongly reduced tergites.

Male terminalia of Nematocera dorsal view
Male terminalia of Nematocera ventral view
Fig. 3 - Schematic drawing of the male hypopygium of a hypothetic nematocerous. (At the left: dorsal view - At the right: ventral view )
a gx: gonocoxal apodemes; ce: cerci; ed: aedeagus; ep: epandrium; eprc: epiproct; gns: gonostyli; gnx: gonocoxites; ip: hypandrium; iprc: hypoproct; p gx: gonocoxal bridge; pm: parameres.
Author: Giancarlo Dessì
(License: Creative Commons BY-NC-SA)

In the orthorrhaphous Brachycera the urites 6-8 are various in shape and size. In most of the families, these segments are well visible and give a cylindricas or tapering shape to the abdomen. In wide or small groups within various families, there is a tendency to decrease the number of apparent urites, due to the reduced size of segments 7-8 or, more often, of the eighth only (Xylophagidae, Stratiomyidae, Acroceridae, Bombyliidae, Dolichopodidae, etc.). In Dolichopodidae, the eighth urite is asymmetric due the rotation by 90°, with the tergite moved to the right side and the sternite to the left.

In cyclorrhaphous Brachycera there is a broad uniformity of the pregenital region, with the reduction of the number of apparent segments, the circumversion of abdomen, and the ventral flexion. Regardless of the many differences in the single details found within this large group, strong changes occur in the seventh and eighth segments, whereas the sixth is similar to the fifth. The tergites 5 and 6 are usually large and rather convex, whereas the sternites are poorly developed, specially the sixth, because they are involved in the ventral flexion of the abdomen. In the Cyclorrhaphous, these features give an ovoidal shape to the male abdomen, more or less oblong, compared to that of lower Diptera, which is often cylindrical. Therefore the sixth segment appears as a preabdominal segment smaller than the preceding ones. The size, shape and structure of the seventh and eighth segments, however, are strongly differentiated. Recurrent changes are the following:

Male terminalia of lower Brachycera lateral view
Male terminalia of lower Brachycera dorsal view
Fig. 4 - Schematic drawing of the male hypopygium of a hypothetic lower brachycerous belonging to a primitive lineage. (At the left: lateral view. - At the right: dorsal view. )
a gx: gonocoxal apodeme; a ej: ejaculatory apodeme (=aedeagal apodeme sensu McAlpine); ce: cercus; en p: endoaedeagal process; ep: epandrium; gns: gonostylus; gnx: gonocoxite; g pm: parameral sheath; ip: hypandrium; iprc: hypoproct; p ej: lateral ejaculatory process; sb m: subepandrial membrane; v: sperm sac.
Author: Giancarlo Dessì
(License: Creative Commons BY-NC-SA)
Male terminalia of lower Brachycera posterior view
Male terminalia of lower Brachycera ventral view
Fig. 5 - Schematic drawing of the male hypopygium of a hypothetic lower brachycerous belonging to a primitive lineage. (At the left: posterior view - At the right: ventral view )
an: anus; a gx: gonocoxal apodeme; a ej: ejaculatory apodeme (=aedeagal apodeme sensu McAlpine); ce: cercus; ed: aedeagus; en p: endoaedeagal process; ep: epandrium; gns: gonostylus; gnx: gonocoxite; g pm: parameral sheath; ip: hypandrium; iprc: hypoproct; p ej: lateral ejaculatory process; sb m: subepandrial membrane; v: sperm sac.
Author: Giancarlo Dessì
(License: Creative Commons BY-NC-SA)

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The hypopygium sensu stricto is composed of the urite 9, called also andrium, and the rudiments of posterior segments (urites 10 and 11). In the groundplan of Diptera, it includes the copulatory organ (aedeagus) and some accessory appendages involved in the mating (gonopods and parameres). The structure and morphology of these appendages are very important in the taxonomic diagnosis due the variety throughout the order, but the origin and the identity, referring to the homologies with other order of insects, are uncertain and controversial. This makes confused and various the terminology in the literature: the same name adopted by different authors may refer to different organs due to disputed interpretation of homologies, specially within the Brachycera.

In attempt to define the homologies and standardize the terminology applied to brachycerous Diptera, a substantial contribution was given by the Manual of Nearctic Diptera, but the issue by McAlpine (1981) is partially obsolete after more recent studies published in the Nineties. In this context, important milestones are the works given by Wood (1991) on Nematocera, Sinclair et al. (1994) on lower Brachycera, and, finally, Cumming et al. (1995), Cumming & Sinclair (1996), Griffiths (1972, 1996), Zatwarnicki (1996) on Eremoneura.

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Theories and disputes on evolution of the hypopygium of higher Diptera

Male postabdomen of Schizophora
Fig. 6 - Shape of male postabdomen in a generic schizophorous fly. Lateral view.
ce: cercus; df: distiphallus; ef: epiphallus; ep: epandrium (=periandrium); ip: hypandrium; iprc: hypoproct (=sternite 10 sensu Zatwarnicki); m i: intersegmental membrane; m p: pleural membrane; pr: proctiger; preg: pregonite (=gonopod sensu McAlpine); pstg: postgonite (=paramere sensu McAlpine, paraphyse sensu Griffiths); s6: sternite 6; sin7+8: syntergosternite 7+8; ss: surstylus (=gonostylus sensu Zatwarnicki); t6: tergite 6.
Author: Giancarlo Dessì
(License: Creative Commons BY-NC-SA)

At present there is no consensus about the interpretation of some homologies and sometimes Authors use different terms in certain systematic groups. Within the lower Diptera, specially the Nematocerous, a substanzial uniformity of terminology exists for the easy identification of the hypopigial structures, although in some groups a different terminology occurs due the morphological and functional specificity (p.a. Culicidae). The context is complicated when refers to Cyclorrhapha and some lineages of lower Brachycerous: the strong morphostructural changes of the hypopygium make difficult the recognition of homologies, causing uncertainties and academic disputes still unsolved.

This issue has not secondary importance because the male postabdomen is often determinant in cladistic analysis to identificate the phylogenetic relations on which the systematic tree is based. Suffice it to say that the taxonomic tree of Cyclorrhaphous built by Griffiths (1972) is based on the structure of the male hypopygium and it differs considerably from the ones proposed from the 70s to today due to a substantial divergence in the interpretation of homologies.

Since the end of the nineteenth century, the Dipterists tried to reconstruct the dynamics by which the male hypopygium has evolved throughout the lineages differentiated during the Cretaceous and the early Cenozoic, which now represent the largest part of the order. Ten hypothesis has been developed and proposed. They are more or less complex and often they contradict each other. A detailed comparative study of these theories was given by Zatwarnicki (1996), the last Author in chronological order who has developed a theoretical model. Apart from the overcoming of the hypothesis historically made​​, the dispute revolves around three theories, which differ substantially in the interpretation of the strong changes of the clasping system during the evolutionary process of the male terminalia.

During coupling of lower Diptera, male and female assume their positions with opposite directions ("tail to tail"). The clasping function is done by lobes of the ventral part of the hypopygium and they move on a horizontal plane. During coupling of higher Diptera, by virtue of the circumversion and flexion of hypopygium, the male take position over the female and face in the same direction. The clasping function is done by lobes of the dorsal part of hypopygium and they move on a vertical plane. Between these behaviors, which require deep adaptations, several cases occur both in the functionality and morphological and anatomical structure throughout the lower Brachycerous. The difficulty of finding unequivocal primary adaptations, typical of the ground plan, and the secondary ones, which may be associated with homoplasies, makes difficult the reconstruction of the ground plan of Brachycerous and the determination of homologies and phylogenetic relationships. The setting of different analysis criteria and, sometimes, real postulates also, has led to the origin and development of controversial hypothesis.

The divergence between various theories can be summarized in a dispute which reached its peak in the mid-nineties and involved three different ideas.

Epandrial hypothesis

This is the oldest interpretation, outlined at the end of the nineteenth century (Zatwarnicki, 1996) and yet formulated in detail by Crampton (1936). It was supported with various changes by several Authors from 60s to 90 years. The last review ("revised epandrial hypothesis") was given by Cumming et al. (1995) and Cumming & Sinclair (1996). This theory asserts that the clasping lobes of higher Diptera, often called "surstyli", have originated from the dorsal sclerite of hypopygium and substitute the primitive function performed, in lower Diptera, by the forceps associated with the ventral sclerite and usually called "gonopods". According to the revised epandrial hypothesis (Cumming et al., 1995; Cumming & Sinclair, 1996), the surstyli are neomorphic structures, whereas the gonopods are reduced to gonocoxites fused with the ventral sclerite, involving the anatomic and morphologic structures related to the copulatory organ.

Periandrial hypothesis

This is the interpretation given by Griffiths (1972), then adapted by the same Author in subsequent publications from eighties to '90 years, cause new knowledges and for reply to interpretations and critiques from epandrialists. The last contribution, by (Griffiths (1996), replied to Cumming et al. (1995). This theory asserts that, in the evolutionary process of higher Diptera, the epandrium has reduced until the loss and has been replaced by the dorsal migration of gonopodial complex. Therefore the dorsal sclerite of higher Diptera, called "periandrium" by Griffiths, would have derived by the fusion of gonocoxites and the associated clasping lobes (called "telomeres") would be homologous to gonostili.

Hinge hypothesis

This interpretation has been proposed by Zatwarnicki (1996) at the height of the dispute between the revised epandrial and periandrial hypothesis. He started from a comparative analysis of various former theories, which has highlighted critical aspects and weakness. According to this hypothesis, the gonocoxal apodemes separated from gonocoxites and fused with each other to form a transverse structure, called "transandrium", attached to hypandrium and placed over the basis of copulatory organ. The gonocoxites has moved dorsally and fused with each other to form a sclerotized structure, called "medandrium", placed between the ventral complex and the epandrium and forming a double articulation with the transandrium and the clasping lobes. The clasping lobes could be homologous to gonostyli and no neomorphic structure as hypothesized by Cumming et al. (1995).

Different formulations given by Authors of these theories cannot allow a resolution of dispute in absence of further studies. Cumming et al. base their hypothesis on morphological and structural criteria, Griffiths and Zatwarnicki on criteria of functional continuity. In each theory, critics have found weaknesses or incongruities also, so that they should need revisions, changes, further studies, but the dispute stopped during the second half of '90s without any progress. The epandrialists rejected the periandrial hypothesis with the publication of Cumming et al. (1995) revision with the change by Cumming & Sinclair (1996). Griffiths (1994) replied to Wood, Cumming, and Sinclair clinching the existence in the epandrial hypothesis of incongruities with empirical facts. Zatwarnicki (1996) pointed out the weakness and incongruences of the epandrial and periandrial hypothesis, while recognizing the validity of some concepts, then proposed his theory. In turn Sinclair (2000) defended the relevancy of the epandrial hypothesis confuting Griffiths' and Zatwarnicki's concepts. Since then the debate has stopped with the interlocutors remaining in their positions, but no further contributions in the research. After 1996 neither Griffiths nor Zatwarnicki published other works on male terminalia: Griffiths' career was broken by his death in 2009, due to a cancer diagnosed in 2006, while the work of Zatwarnicki has focused mainly in the continuation of his studies on Ephydridae.

Should be also noted that the coordination of manuals on Palaearctic Diptera (Sinclair, 2000) and Central American Diptera (Cumming & Wood, 2009) uses a glossary based on concepts proposed by Wood, Sinclair, and Cumming during the first half of '90s. However this tendency does not state the final promotion of the epandrial hypothesis, lacking further empirical research, after '90s, supporting the superiority of one thesis over the other (Zatwarnicki, 2012, personal communication).

In the following sections I present the main anatomical and morphological structures of hypopygium explaining in broad terms the divergences most significant about the terminology and the homologies.

Hypopygium of Asilidae male in dorsal view
Hypopygium of Asilidae male in ventral view
Fig. 7 - Schematic drawing of the male hypopygium of a hypothetic robber fly (Brachycera: Asilidae) as recurrent in the subfamily Asilinae. (At the left: dorsal view - At the right: ventral view )
a ej: ejaculatory apodeme (=aedeagal apodeme sensu McAlpine); a gx: gonocoxal apodeme; ce: cercus; ep: epandrium; f: phallus (=aedeagus); gns: gonostylus; gnx: gonocoxite; g pm: parameral sheat; ip: hypandrium; iprc: hypoproct.
Author: Giancarlo Dessì
(License: Creative Commons BY-NC-SA)

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Epandrium and hypandrium

The basic structure of the hypopygium is composed of the sclerites of segment 9. In the terminology related to Diptera, these sclerites are called epandrium and hypandrium (Crampton, 1923), referring to the specific name given to this segment (andrium). In the ground plan of Diptera these sclerites are fused, as found in most Nematocera. In the higher Diptera the sclerites are separated, but during the evolutionary process they are involved, with other hypopygial structures, in changes more or less strong, as to make the relevance of same terms used for lower Diptera uncertain and controversial from the phylogenetic point of view. For this reason, in literature Authors use sometimes other names which imply a general reference regardless the homologies.

The epandrium sensu stricto is the dorsal plate (tergite) of segment 9, connected laterally to the hypandrium and posteriorly to the rudiments of the tergite 10, with which can be fused. It is usually well developed and its structure is based on two different forms:

throughout the Eremoneura, the name "epandrium", which is widely recurrent in literature, prescinds from the real interpretation of its origin. For this reason, sometimes other names was used, as neutral or interpretative, to refer to a complex origin of epandrium in higher Diptera. Therefore this sclerite was also called "saddle-shaped sclerite" (Zatwarnicki, 1996) and "periandrium" (Griffiths, 1972). Authors cited also the usage in literature of other names as "ninth tergite" (Zatwarnicki, 1996; Sinclair, 2000; Cumming & Wood, 2009) and "dorsal sclerite" (Sinclair, 2000; Cumming & Wood, 2009), without specify the context (authors and taxonomic ambits).

The hypandrium is the ventral plate (sternite 9), connected laterally to the epandrium and posteriorly to genital appendages. Size and shape of this sclerite are usually similar to one of the following forms:

Regardless of the interpretation of homologies, the name "hypandrium" is generally used by all Authors. However it is also cited the usage in literature of other names, as "ninth sternite" and "ventral sclerite" (Sinclair, 2000; Cumming & Wood, 2009).

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Hypopygium of Asilidae male
Fig. 8 - Schematic drawing of the male hypopygium of a hypothetic robber fly (Brachycera: Asilidae) as recurrent in the subfamily Asilinae (epandrium bilobed, trifid phallus, gonostylus articulated to the base of gonocoxite). Lateral view.
a ej: ejaculatory apodeme (=aedeagal apodeme sensu McAlpine); a gx: gonocoxal apodeme; ce: cercus; ep: epandrium; f: phallus (=aedeagus); gns: gonostylus; gnx: gonocoxite; ip: hypandrium; iprc: hypoproct; s sb: subepandrial sclerite.
Author: Giancarlo Dessì
(License: Creative Commons BY-NC-SA)

In comparative anatomy, the word gonopod occurs to refer to abdominal appendages of males, paired or unpaired, used as copulatory organs or involved in the mating. This name is even applied to Arthropods to refer to abdominal appendages with functions more or less homologues; in the ground plan of Insects, the primitive gonopods with a clasping function are found in Thysanura (Insecta: Apterygota) and are identified with the lateral pair of the genital arms, bisegmented and associated to the urite 9. In the Endopterygota, the presence of appendages homologues to gonopods of Thysanura is disputed (McAlpine, 1981). According to some Authors, the Endopterygota would lose the primitive gonopods and the clasping function would be performed by a secondary adaptation of other appendages, the parameres. The extension of this hypothesis to all Endopterygota, however, does not find consensus and several Authors support the homology of gonopods of Thysanura and Panorpoidea (Diptera included) and Hymenoptera.

The identification of these appendages as homologues of the primitive gonopods is supported by the Manual of Nearctic Diptera (McAlpine, 1981), but the studies published during the next two decades have a more complex approach. This results in a different terminology when the object is represented by the higher Diptera. The gonopods, as homologous or analogous to those of Thysanura, are evident in most lower Diptera. In these insects, the gonopods are paired and symmetrical appendages joined to posterolateral margin of the hypandrium and are composed of two segments: the proximal is named gonocoxite or basistylus, the distal is named gonostylus or dististylus. Gonopods are used by the male as clasping organs during the mating, so they are also named claspers, specially the gonostyli.

The conformation of gonopods is varied and sometimes they are not easily recognizable for controversial interpretation about their destiny throughout the evolution. It is believed that in the ground plan of Diptera, as primitive condition, the gonocoxites are separated from each other and they are distinct from the hypandrium. This condition occurs in most Panorpoidea, except for Mecoptera, the most primitive order, where the gonocoxites are fused (Sinclair, 2000).

In Nematocera, the primitive condition is found only in some lineages: in most of Culicomorpha and in some groups of Tipulomorpha and Bibionomorpha (McAlpine, 1981; Wood, 1991; Sinclair, 2000). In remaining Nematocera, the gonocoxites are ventrally fused each other and often they are fused with the hypandrium (Sinclair, 2000). The gonostyli appears as appendages simple or lobate, usually articulated at the distal end of gonocoxites. Often they bear setulae or spines with sensory functions and move horizontally as plesiomorphic condition (Sinclair, 2000), however they move obliquely in some groups (Anisopodidae and Culicomorpha in part).

Among the lower Brachycera, we must separate the superfamily Empidoidea, closely related to Cyclorrhapha, from the rest of Orthorrhapha. The disjunction of a gonocoxite from each other and gonocoxites from the hypandrium are primitive conditions even in the ground plan of Brachycera (Sinclair et al., 1994). This feature occurs only in a few groups of some families (Asilidae, Bombyliidae, Therevidae, Rhagionidae, Acroceridae). The presence of gonopods is clear, but often the gonocoxites are fused each other and the basis of gonocoxites may be fused with the hypandrium. The gonostyli are usually present, but in some families they are more or less fused with gonocoxites or missing (Apioceridae, Acroceridae, Asilidae, Bombyliidae, Scenopinidae). The horizontal articulation of gonostyli is a plesiomorphic condition as in Nematocera (Sinclair, 2000), but it does not occur in orthorrhaphous not eremoneuran of superfamilies Nemestrinoidea and Asiloidea, where the gonostyli have oblique to vertical movement referred to the epandrium (Sinclair, 2000).

In the large clade of Eremoneura, which includes all the Cyclorrhapha and the orthorrhaphous families belonging to Empidoidea, the definition of the structure and homologies of the ventral sclerites is complicated by different interpretations. This fact has led, for most Eremoneura and, specially, for Cyclorrhapha, to adopt specific terminologies to refer to hypopygial structures which have uncertain origin. Therefore it may overshadow the names which refer to gonopods sensu stricto (gonocoxite and gonostylus). Indeed the gonopods have evolved by different way according to three alternative hypothesis:

We cite also the old hypothesis proposed by McAlpine in the Manual of Nearctic Diptera (McAlpine, 1981): he stated that the ventral sclerite has derived by the fusion of hypandrium with the basis of gonocoxited, whereas the "gonopodial lobes" (often referred as pregonites in literature) have derived by the fusione of the remaining part of gonocoxites with the gonostyli. This reconstruction defines an uniformity throughout the Brachycerous, because brings back the differences between lower Brachycera and Eremoneura to a substantial structural simplification. The suggestion of McAlpine (1981) is now outmoded, either in terminology and interpretation of homologies, by subsequent hypothesis proposed or repeated during '90s.

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Gonocoxal apodemes

The gonocoxal apodemes, sometimes referred by other names, are two processes joined to the gonocoxites and proiected forward and upward within the hypopygium, visible after removing the epandrium. These structures are present in the lower Diptera only.

In nematocerous Diptera, the presence of gonocoxal apodemes is well reported in several families of Culicomorpha and Bibionomorpha. In other nematocerous groups they are absent or rudimentary (Sinclair, 2000; Cumming & Wood, 2009), therefore the literature lacks any references. Usually the gonocoxal apodemes diverge dorsally, but they are apically fused in Chironomidae (McAlpine, 1981; Sinclair, 2000) and some Ceratopogonidae (McAlpine, 1981).

In lower Brachycera, the gonocoxal apodemes are present in most families of all infraorders, except to Muscomorpha: within this infraorder, the gonocoxal apodemes are absent in most Bombyliidae and all Cyclorrhapha. Within the Empidoidea they are present only in the primitive groups with the postabdomen no permanently rotated, like the Atelestidae and part of Empididae (Sinclair, 2000; Sinclair & Cumming, 2006). In basal lineages of Empidoidea and at the same time of analogues or homologues structures in basal groups of Cyclorrhapha, the presence of gonocoxal apodemes lead to different hypothesis about their transformation in the evolution of Eremoneura:

Postabdomen of male of Dolichopodidae lateral view left side
Postabdomen of male of Dolichopodidae lateral view right side
Fig. 9 - Approximate drawing of male postabdomen in Dolichopodidae (Brachycera: Dolichopodidae), asymmetric cause of 90° rotation. Lateral view. (At the left: left side - At the right: right side )
ce: cercus; ep: epandrium (=periandrium); f: phallus (=aedeagus); ip: hypandrium; l ep: epandrial lobe; pstg: postgonite (=paramere sensu McAlpine, paraphyse sensu Griffiths); ss: surstyli (=gonostyli sensu Zatwarnicki); st5-8: sternites 5-8; t5-7: tergites 5-7.
Author: Giancarlo Dessì
(License: Creative Commons BY-NC-SA)

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The parameres, also called in literatura with other names (paraphyses, hypophyses, dorsal plate), are appendages present in the ground plan of the order and well defined in lower Diptera. Related to the copulatory organ, the parameres would protect it when the aedeagus is in repose and would guide it during the mating (McAlpine, 1981). In basal groups they appears as paired, symmetrical, and medial appendages, separated from each other and placed between the gonopods andthe copulatory organ. They are articulated to the basis of aedeagus and, laterally, to gonocoxites (McAlpine, 1981) or the gonocoxal apodemes (Sinclair, 2000, Cumming & Wood, 2009). Free parameres can be found in some groups of Nematocerous (i.e. Tipulomorpha), but they are often fused to each other and form a structure more complex. In Nematocerous, the fusion of parameres on the medial side is a secondary adaptation which could be part of the ground plan of Axymyiomorpha, Blephariceromorpha, Bibionomorpha, and Pyschodomorpha, therefore it occurs only in some clades of these groups (Wood, 1991). This secondary character appears as a plate which connects the gonopods to each other over the aedeagus, from which the recurrent name "dorsal plate" derives.

In Brachycerous the destiny of these structures is uncertain and controversial and usually in literature there are alternative names that do no consider the homologies. In the Manual of Nearctic Diptera, McAlpine (1981) accepts as parameres al pairs of medial appendages placed between the gonopods and the copulatory organ, so he supports a great morphostructural uniformity throughout the order, regardless of the fusion to each other; this interpretation originates from the presumed homologie of the primitives parameres of lower Diptera to the appendanges that usually Authors refer as postgonites in higher Brachycerous. In general the supporters of the gonopodial origin of surstyli assume the postgonites as homologous to primitive parameres (Griffiths, 1972; Zatwarnicki, 1996), whereas, regardless the interpretation of McAlpine (1981), the recent supporters of the epandrial origin of surstyli speculate a conjunct evolution of primitive parameres and the aedeagus, that form a phallic complex in the Cyclorrhaphous.

According to works published by Wood, Cumming, and Sinclair during the nineties, the fusion of parameres is a condition extended to entire suborder, although the appearance and the structure changes through the Brachycerous from the Orthorrhaphous to Cyclorrhaphous. In most lower Brachycerous within Tabanomorpha and Xylophagomorpha, the parameres are fused and form a conical case which wraps the apex of aedeaugus on the dorsal side (parameral sheath); on the ventral side, the fusion is broken by a weakly sclerotized suture. In Stratiomyomorpha and Muscomorpha, the parameres disappears because the parameral sheath is fused with the apex of aedeagus making a structure called phallus (Sinclair et al., 1994; Sinclair, 2000).

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Postabdomen of male of Dolichopodidae dorsal view
Fig. 10 - Approximate drawing of male postabdomen in Dolichopodidae (Brachycera: Dolichopodidae), asymmetric cause of 90° rotation. Dorsal view.
ce: cerci; ep: epandrium (=periandrium); eprc: epiproct; ip: hypandrium; pstg: postgonites (=parameres sensu McAlpine, paraphyses sensu Griffiths); st8: sternite 8; t5-7: tergites 5-7.
Author: Giancarlo Dessì
(License: Creative Commons BY-NC-SA)

The gonites are posteroventral lobes typical of higher Brachycera, absent in lower Diptera. They consist of two pairs, called pregonites and postgonites. The uncertain and controversial origin of these structure led to usage of various names in literature, often taken from names of analogues structures of lower Diptera or event of other groups of Insects. The specific terms - "pregonites" and "postgonites" - were proposed by Crampton (1944) and are widely used by Authors of last decades, also because they are independent from the homologies. Sometimes they are reduced to a single pair and the possible presence in some lower Brachycerous does not attest the homology with the gonites sensu stricto of Cyclorrhaphous.

The pregonites are ventral processes present in several groups of Schizophora but totally absent throughout the lower Diptera. They are a pair of lobes joined to the ventral sclerite (hypandrium sensu lato), anterior and ventral to postgonites. The lack of appendages that are unequivocally referable as gonopodial structures, in higher Diptera, led some Authors, also recent, to identify the pregonites as homologous to gonopods else, more specifically, to structures derived by them. This hypothesis was supported also by McAlpine (1981), who considered the name "pregonite" as synonym of gonopod. However, since the '40s, other hypothesis of alternative origin are given. Among these there are Griffiths (1972), who referred the pregonites as a pair of additional paraphyses, Cumming et al. (1995), as derived from processes of the hypandrial-gonocoxal complex, Zatwarnicki (1996), as derived from ventral processes of the hypandrium.

Also the origin of postgonites is controversial. Their homologies is closely related to destiny of gonopods within the evolution of eremoneuran hypopygium. The postgonites are appendages articulated at the basis of the phallus, strechted along the dorsal margins of the hypandrium and goin beyond its posterior margin (Sinclair, 2000). Their function could be sensory and closely related to working of the phallus during the mating (Sinclair, 2000). McAlpine (1981) assume them as homologous of parameres of lower Diptera. Cumming et al. (1995) first suggested their origin from the gonostyli of lower Brachycerous, but in the following year Cumming & Sinclair (1996) stated the origin of postgonites de novo as structures differentiated from the gonocoxal portion of the hypandrial-gonocoxal complex. This hypothesis was definitively adopted by the same Authors in their last works (Sinclair, 2000; Sinclair & Cumming, 2006; Cumming & Wood, 2009), to support the loss of gonostyli and the epandrial origin of surstyli in higher Diptera. The supporters of the gonostylar origin of surstyli, instead, identify the postgonites as homologous to paraphyses (parameres sensu McAlpine, 1981) of lower Diptera (Griffiths, 1972; Zatwarnicki, 1996).

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Copulatory organ

Male genitalia in lower Diptera
Fig. 11 - Structure of male genitalia in lower Diptera.
a ej: ejaculatory apodeme (=aedeagal apodeme sensu McAlpine); d ej: ejaculatory duct; e: aedeagus; gp: primary gonopore; pm: parameres; p sp: sperm pump; t: testicles; vd: deferent ducts; v sem: seminal vesicles; v sp: sperm sac.
Author: Giancarlo Dessì
(License: Creative Commons BY-NC-SA)

This is a genital appendage which during the mating is introduced into the female genital opening to allow the internal insemination. In entomology, this organ is usually referred as phallus, penis, and aedeagus, but the terminology is often affected by the complexity and the morphoanatomical relation with other parts of postabdomen. Throughout the order we observe a strong differentiation, of the penis, both in structure and in shape and it is not possible a general description extended to all Diptera (McAlpine, 1981; Sinclair, 2000). A detailed discussion must refer to single systematic groups, therefore we limit herein to basic concepts on the main lineages and some problems about the homologies and the terminology.

The primitive copulatory organ of Diptera is a tubular and evertable appendage. The proximal portion works as ejaculatory organ and is named sperm pump. It is composed of the sperm sac, a vesicle where the sperm is stored, and the ejaculatory apodeme, a rod-like sclerite moved by one or more muscles. The sperm sac is connected to the endophallus, a duct which runs through the ejaculatory organ and opens at the distal end (phallotrema). This structure allows the type of insemination most common among the Diptera: the male introduces the distal part of the penis into the vagina of the female and pours the semen pushed by the sperm pump.

In a few Diptera, included as primary condition in the infraorder Culicomorpha (McAlpine, 1981; Sinclair, 2000), the sperm is transferred in the genital chambre of females enveloped within preformed gelatinous vesicles called spermatophores. In these Diptera, the copulatory organ loses its primitive tubular structure and changes into a membranous plate, more or less wide and not evertable. Contextually to the absence of the ejaculatory function, the sperm pump regresses and the ejaculatory apodeme disappears.

In the rest of Diptera, recurrent characters are an ejaculatory apodeme well developed related to the sperm sac and a copulatory organ more or less elongated, named phallus or aedeagus according to different interpretations. The term aedeagus, widely used in literature, is generally accepted by Authors when it refers to lower Diptera. About Cyclorrhapha and Orthorrhapha there are different concepts on the homology of the phallico complex of these groups with the aedeagus of lower Diptera.

As descrive above, in Tabanomorpha and Xylophagomorpha, the parameral sheath wraps the aedeagus without fusing with it, therefore these appendages reain their structural identity. Wood (1991) proposed the concept of phallus as a structure derived from the fusion of the parameral sheath with the aedeagus. The hypothesis of fusion was subsequently supported by Sinclair et al. (1994), Cumming et al. (1995), and Sinclair (2000) as general condition throughout the Brachycerous, except to some basal lineages of Orthorrhaphous. An analogous structures can be found also in Nematocera, within the Bibionomorpha (Sinclair, 2000). Therefore, in the Manual of Palaearctic Diptera, Sinclair (2000) stated that the terms "aedeagus" and "phallus" should be applied to different systematic groups, because they refer to structures that are only partially homologues.

The concept of phallus sensu Wood (1990) is in contrast with the thesis of retention of parameres (or paraphyses according to some Authors) of lower Diptera, which is supported albeit with different theories by Griffiths (1972), McAlpine (1981), and Zatwarnicki (1996). According to these Authors, the extension of the terms "aedeagus" to higher Diptera is therefore pertinent.

The structural complexity of the copulatory organ in higher Diptera is reflected in the recurrent distinction into two parts: the proximal one, called basiphallus, and the distal one, called distiphallus Sinclair, 2000).

The basiphallus was referred by authors even as phallobase (McAlpine, 1981; Sinclair, 2000). This word, however, should be used carefully throughout the Diptera, due to the ambiguity inherent in the interpretation of homologies: in fact, in entomology, the term phallobase refers to the proximal section of the penis of Insects and is distinct from the aedeagus sensu stricto, whereas the term aedeagus applied widely by Authors to higher Diptera (Griffiths, 1972; McAlpine, 1981; Zatwarnicki, 1996), assumes implicitly that the basiphallus is an integral part of the aedeagus. The basiphallus is articulated to the basis of the phallapodeme and its distal end is prolonged by the distiphallus. Often, the basiphallus bears a dorsal lobe called epiphallus.

The distiphallus sensu stricto is the distal section of the phallus within the Schizophora (Sinclair, 2000). McAlpine (1981) assumed that this name would be used even to indicate the distal end of the aedeagus within the lower Diptera. The same author also applied the term distiphallus to lower Brachycera when referred to the subdivision into three distal filaments, feature that recurs in Syrphoidea, Asilidae, and Empididae (McAlpine, 1981).

Both the distiphallus of higher Diptera and the aedeagus of lower Diptera usually end with a single genital opening (gonopore or phallotrema), but in several families three separated orifices may occur, related to the terminal subdivision into three branches or filaments. This conformation would be related to the number of efferent ducts of the spermathecae opening in the common oviduct of the female genitalia (McAlpine, 1981).

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Apodemes of the copulatory organ

Male genitalia of Cyclorrhapha
Fig. 12 - Structure of male genitalia in cyclorrhaphous Diptera.
a ej: ejaculatory apodeme; bf: basiphallus; d ej: ejaculatory duct; df: distiphallus; d sp: sperm duct; ef: endophallus; f ap: phallapodeme; ft: phallotrema; gp: primary gonopore; g pm: parameral sheat; p sp: sperm pump; t: testicles; vd: deferent ducts; v sem: seminal vesicles; v sp: sperm sac.
Author: Giancarlo Dessì
(License: Creative Commons BY-NC-SA)

Another cause of different terminologies is represented by the internal sclerites (apodemes) placed near the copulatory organ. The ejaculation and movements of the penis depend on these, therefore the terminology adopted by authors refers to these functions. Three names are often used in literature, but they are applied with different concepts in distinct taxonomic fields: ejaculatory apodeme, aedeagal apodeme, and phallapodeme.

In Nematocerous (except to Culicomorpha) and most Orthorrhaphous, the male have a sclerite whose basic function is to compress the sperm sac and start the ejaculation (McAlpine, 1989); the close connection of the sperm sac with the aedeagus makes this sclerite can also act as a lever by transmitting movements to the copulatory organ (McAlpine, 1989; Sinclair, 2000). In literature, when Authors refer to Nematocerous and lower Brachycerous, usually they call this sclerite ejaculatory apodeme (Zatwarnicki, 1996; Sinclair, 2000). In the first and third volumes of the Manual of Nearctic Diptera, McAlpine uses the name aedeagal apodeme, because he considered incorrect the term "ejaculatory" (McAlpine, 1981).

The Cyclorrhaphous have a strong difference in the anatomy of the proximal part of the copulatory organ: the sperm sac is separed from the basiphallus by the interposition of an ejaculatory duct. At the same two apodemes are present. The first one is associated to the sperm sac and works as rod of the sperm pump. The second one is articulated with the basiphallus and acts as a lever by giving movements to phallus.

In literature these apodemes are called with different names, which can make confusion if they are not related to the systematic groups and the Authors. The apparent confusion is caused by various attempts to the origin of these apodemes. The question was treated by sereral Authors from seventies to nineties, as Hennig, Griffiths, McAlpine, Wood, Cumming, Sinclair, and Zatwarnicki (McAlpine, 1989; Sinclair, 2000).

Griffiths throughout his studies proposed some different interpretations. According to the ontogenetic differentiation of the apodema associated with the copulatory organ, Griffiths (1972) presumed its homology with the apodeme of lower Diptera and therefore he considered as neomorphic the sclerite associated to the sperm pump. In a subsequent instance, during the '80s, he changed his interpretation (McAlpine, 1989; Zatwarnicki, 1996; Sinclair, 2000) and stated that the apodeme related to the copulatory organ is a neomorphism originated by the fusion of the gonocoxal apodemes. The last hypothesis was rejected by McAlpine (1989), who considered it incongruous with the periandrial hypothesis of Griffiths.

Hennig (1976) formulated three alternative interpretations on the possible origin of these structures:

McAlpine (1981) suggested a terminology based on functional criterion and proposed the name "ejaculatory apodeme" referring to the sclerite associated with the sperm pump and "aedeagal apodeme" for that one related to the copulatory organ. This terminology was independent of homologies, In fact, in the third volume of the Manual of Nearctic Diptera, McAlpine (1989) discussed the Hennig's and Griffiths' thesis and stated that the ejaculatory apodeme of Cyclorrhaphous is homologous to the apodeme of lower Diptera, while the aedeagal apodeme is a neomorphic structure derived from the hypandrium. The same term, "aedeagal apodeme", is used by McAlpine to refer to analogous structures and not homologous in lower Diptera and Cyclorrhaphous.

Cumming et al. (1995) assume a condition which, such as the concept of McAlpine (1989), is similar to the first hypothesis of Hennig (1976): the apodeme joined to the sperm pump could be homologous to the apodeme of lower Diptera, while the sclerite joined to the copulatory organ is a neomorphism, derived from the hypandrium. However, unlike McAlpine, the authors of the revised epandrial hypothesis use a specific terminology based on the criterion of homology: therefore they call "ejaculatory apodeme" both the sclerite of lower Diptera and the apodeme joined to the sperm pump in Cyclorrhaphous, while call "phallapodeme" the neomorphic structure associated with the basis of cyclorrhaphous phallus (Sinclair, 2000).

Zatwarnicki (1996), finally, assumes a more complex hypothesis about the homologies of the cyclorrhaphous apodemes. He treats only the differentiation of the internal sclerites of the ventral complex within the Eremoneura, without explicit references to Nematocera. The name "ejaculatory apodeme" is reserved exclusively to the Empidoidea, as homologous to the ejaculatory apodeme of lower Brachycera. About the apodemes of Cyclorrhaphous, it suggests a terminology which presumes a neomorphism for the sclerite related to the sperm pump ad a partial homology for the sclerite joined to the aedeagus. He names the first one (the ejaculatory apodeme sensu Cumming et al., 1995) as "ejacapodeme". Zatwarnicki (1996) stated that the ejacapodeme is a neomorphic structure, without specify its origin. According to him, the ejacapodeme could be differentiated by a sclerotization of the sperm pump or the phallapodeme. The sclerite associated with the copulatory organ is referred by Zatwarnicki as "phallapodeme", using the same name proposed by Cumming et al. (1995), but he does not agree about the homology: in fact, the phallapodeme could be a sclerite derived in part from the ejaculory apodeme of Empidoidea and in part from the hypandrium.

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Intersegmental membrane and sclerotized derivatives

The ninth segment of males of Brachiceri has an intersegmental structure, more or less membranous and associated to sclerites, placed between the dorsal structures (epandrium and proctiger) and those ventral (hypandrial complex and copulatory organ). It closes ventrally the subepandrial space and is dorsal to the hypandrial-phallic complex.

In literature there are various terms which refer to these structures, due to morphological and structural differences throughout the Brachycera and the divergences about the homologies. There are also different ascriptions related to the segments: in the past this intersegmental structure was associated to the tenth segment. However the latest interpretations assume that it belongs to ninth segment (Cumming et al., 1995; Zatwarnicki, 1996).

In lower Brachycera, the subepandrial membrane appears as a concave pouch which connect to the anterior margin of the ventral sclerite of the proctiger (referred as hypoproct or tenth sternite according various Authors) and to anterodorsal margins of the parameral sheath. In several families, the subepandrial membrane differentiates one o two sclerites.

In the Apsilocephalidae family and most Eremoneura, the subepandrial membrane is sclerotized along its length and the subepandrial sclerite articulates with the anterodorsal side of the copulatory organ. The most important morphoanatomical feature is the differentiation from the subepandrial sclerite of two elongated thickenings, called bacilliform sclerites, articulated to anterodorsal side of the copulatory organ and extending to posterolateral margin of the epandrium or the medial margin of surstyli. In several Schizophora, the subepandrial sclerite is strongly reduced and replaced by the bacilliform sclerites, that are articulated to posterior processes of the hypandrial complex.

Describing this structure McAlpine (1981), Zatwarnicki (1996) , and Sinclair (2000) cite various terms, recurrent in literature, which refers to different contexts:

a) the hypothetical derivation from the tenth segment: "sternite 10", "ventral lamella of proctiger", "ventral proctigeral sclerite", "decasternum";

b) the conformation of a membranous plate more or less sclerotized: "plate", "membrane", "sclerite". In this case there is an adjective related to the substantive which refers to the position with respect to the dorsal hypopygial complex, as "subepandrial", "intraepandrial", "intergonopodial", "interperiandrial";

c) the differentiation of two elongated sclerotized thicknenings running from the hypandrium to the dorsal structures: "processus longi", "bacilliform sclerites";

d) the single origin of a part, variously shaped and structured, which is typical of the ground plan of Eremoneura and is not homologous to the subepandrial membrane of lower Brachycera: "medandrium".

From the '70 years, the reference of this structure as derived from the segment 10 of lower Diptera was adopted by Ulrich (1972, 1975), Hennig (1976), and McAlpine (1981, 1989). The Manual of Nearctic Diptera generally uses, throughout the monographs of second volume (McAlpine et al., 1987), the criterion proposed by McAlpine (1981), with the exception of some families of Calyptratae, in which references as made to the bacilliform sclerites.

However, the literature after seventies uses a specific terminology which exclude the origin of the subepandrial organ from the tenth segment. In any way there are divergent interpretations about the homology between the subepandrial structures of Eremoneura and lower Brachycera.

Griffiths (1972) presumed the origin of bacilliform sclerites of Calyptratae from the basis of gonostyli (surstyli sensu Cumming et al., 1995). First he used the term "interparameral sclerite", but subsequently he adopted the term "intergonopodial sclerite" (Zatwarnicki, 1996; Sinclair, 2000).

Cumming et al. (1995) support the homology between the elongated sclerites of higher Brachycera with the subepandrial membrane or the subepandrial sclerite of lower Brachycera and consider the sclerotization of the membrane along its entire length as part of the ground plan of Eremoneura. Unlike other epandrialists, however, they presume the origin of this structure from the ninth segment. The terms adopted by Cumming et al. refer only to morphological and structural aspects: "subepandrial membrane", "subepandrial sclerite", "bacilliform sclerites". The bacilliform sclerites are considered two thicknenings therefore no closely homologous to the membrane.

Zatwarnicki (1996) assumes this structures as the main component of his hypothesis and adopts a new terms, "medandrium", which excludes the homology between the subepandrial structures of Eremoneura and lower Brachycera. According to Zatwarnicki, the subepandrial sclerite and the bacilliform sclerites are homologous structures originated by the disjunction of gonocoxites from the gonocoxal apodemes, their fusion to each other, and their articulation with the gonocoxal apodemes fused, referred as transandrium.

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Hypopygium of male of Calyptratae
Fig. 13 - Schematic drawing of the male hypopygium of a hypothetic higher fly of section Calyptratae. Lateral view.
a ej: ejaculatory apodeme; af: acrophallus; bf: basiphallus; ce: cercus; df: distiphallus; d sp: spermatic duct; ef: epiphallus; ep: epandrium (=periandrium); f ap: phallapodeme (=aedeagal apodeme sensu McAlpine); ip: hypandrium; preg: pregonite (=gonopod sensu McAlpine); pstg: postgonite (paramere sensu McAlpine, paraphyse sensu Griffiths); sb: bacilliform sclerite (=medandrium); ss: surstylus (=gonostylus sensu Zatwarnicki); v: sperm sac.
Author: Giancarlo Dessì
(License: Creative Commons BY-NC-SA)

The surstyli are a pair of posterolateral appendages articulated to epandrium and used as clasping organ during the mating. They are present only in the Eremoneura lineage and in the family Apsilocephalidae (Zatwarnicki, 1996; Sinclair, 2000). The name "surstyli" was proposed first by Crampton (1923) referring to clasping lobes associated with the ninth tergite of Mecoptera and some lower Diptera, but subsequently it was adopted to refer to analogous organs of Cyclorrhapha (Zatwarnicki, 1996).

Regardless the presence of dorsal clasping lobes in Apsilocephalidae, that are probable homoplasy, the origin of these organs is the main subject of dispute about the evolution of eremoneuran hypopygium. According to Zatwarnicki (1996) this question could mean the fundamental key to interpret the deep changes of the male hypopygium through the evolution of higher Diptera. Since the end of 19th century, over a hundred years, Authors proposed ten different hypothesis about the origin of surstyli (Zatwarnicki, 1996), most of which are outdated or integrated by the latest theories proposed during the last decades of the 20th century. Referring only to latest hypothesis, the clasping lobes of higher Diptera have two possible alternative origins:

Regardless the real origin of these lobes, the surstyli play the primitive clasping function of gonostyli of lower Diptera. In basal basal Eremoneura (in general the Empidoidea) they appear as lobes poorly developed and weakly articulated with the epandrium, whereas in Cyclorrhapha they acquire full functionality because well developed and articulated with the ninth tergite.

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The term proctiger refers to the ensemple of morphological accessories placed behind the hypopygium sensu stricto and belonging to male terminalia. The homology of these structures referred to the ancestral segmentation (segments 10 and 11) is uncertain and controversial: in the proctiger of lower Brachycera there are two small plaques more or less sclerotized, dorsal and ventral, often called epiproct and hypoproct, a pair of cerci and, finally, the anus.

If present, the epiproct appears like a small dorsal lamina placed between the cerci. In Acroceridae and most Asiloidea and Eremoneura, the epiproct is absent and the hypoproct appears as a small plate with a subtriangular or oval shape placed under the cerci and behind the subepandrial membrane or sclerites (Sinclair, 2000).

Authors do not agree about the origin and names of these structures. McAlpine (1981, 1989) presuming the homology of subepandrial sclerites to the tenth sternite, stated the origin of epiproct and hypoproct from the segment 11, whereas Griffiths (1972), Cumming et al. (1995), and Zatwarnicki (1996) consider them as derived from the segment 10. About the terminology, the name "hypoproct" is used by McAlpine (1981, 1989) and Cumming et al. (1995), instead of Griffiths (1972) and Zatwarnicki (1996), who use the name "sternite 10".

The cerci are a pair of unsegmented lobes variously shaped but often poorly developed. In some primitive groups of Nematocera (Tipulomorpha and Trichoceridae), they seem absent(Sinclair, 2000), whereas in other groups, both among Nematocerous and Brachycerous, are secondary developed to assume also a clasping function (Sinclair, 2000).

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I thanks to Prof. Zatwarnicki for his relevant criticism to previous version of this page and for his encouragement and suggestions provided to develop a more neutral and less confusing description of this topic.

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