Table of contents
Ovipositor - Pregenital segments - Internal genitalia (Vagina - Spermathecae - Accessory glands) - Postgenital segments - Cerci - Bibliography - Web resources
c: cerci; ov: ovipositor; s6-8: sternites 6-8; t6-10: tergites 6-10.
Author: Giancarlo Dessì
(License: Creative Commons BY-NC-SA)
The female terminalia (also called hypopygium or postabdomen) are composed of posterior segments from the sixth or the seventh. The morphological and structural differentiation which occurs throughout the order makes difficult a general description without considering the specificity of various groups. First, under the structural and morphological point of view, the postabdomen can be divided into two sections:
- pregenital section: it is composed of the segments that precede the genital opening, usually the urites 7 and 8; in various groups of Cyclorrhaphous, the segment 6 becomes a morphological and functional part of the hypopygium in the pregenital section;
- postgenital section: it is composed of the segments following the genital opening, that is the urites 9 and 10 and the rudiments of the segment 11 (proctiger).
In dorsal to ventral order, the female hypopygium includes inside the ends of genitalia and hindgut (rectum). In all Diptera, as previously mentioned, the anal opening is set on the segment 11 (proctiger) and the genital opening of females (vulva) follows the sternite 8. Compared with other insects orders, the flies have not the typical gonapophyses of Orthopterous, composed of a pair of processes from the sternite 8 and two pairs from the sternite 9. The ninth sternite, finally, is often modified so it is a terminal part of the genitalia. Therefore, a general condition of Diptera is the lacking of the orthopteroid ovipositor. At the same, the flies of many groups have developed an organ not homologous, composed of the whole hypopygium and derived by the morphological, structural and functional adaptation of all posterior segments from the sixth or the seventh. Due the absence of homology, the term "ovipositor", as recurrent among the dipterists, is not appropriate (McAlpine, 1981; Kotrba, 2000).
Ovipositor
This species is featured by the sharp lenght of the ovipositor.
Author: Pest and Diseases Image Library, Bugwood.org (Insect Images, The University of Georgia
License: Creative Commons BY-NC)
The ovipositor, usually, is a slender, tubular, and telescopic organ composed of the posterior segments, that are rectrated into the segment 6 or, in part of Cyclorrhapha, the segment 5. The main adaptations consist of a relative elongation of pregenital segments, a significant simplification and reduction of postgenital urites, and, finally, an extensive development of intersegmental and pleural membranous areas at the expense of tergites and sternites. Those sclerites are reduced to small and slender dorsal and ventral plates or strips. With this conformation, at rest, each segment is retracted into the previous and the entire ovipositor is concealed within the segment 6 or 5.
Some groups have a strong sclerotization of distal sections, that make the ovipositor able to pierce, dig or scrape, depending on the substrate where the eggs are laid. A sclerotized ovipositor and a complex structure of the postabdomen are found in the groundplan of Tephritoidea. In this group, the ovipositor is composed of a proximal sheath and a distal aculeus. The sheath, retractile within the oviscape, is derived from the distal part of segment 7 and the intersegmental membrane between the segments 7 and 8. Its shape is cylindrical, more or less depressed in dorsal to ventral order, and it is membranous; two pairs of sclerotized strips (respectively dorsal and ventral) arise from the posterior edges of sternite and tergite 7. The posterior extremity of the sheath usually bears teeth, spicules, or other, able to scrape. The distal aculeus is the piercing section of the ovipositor and is composed of the urite 8, slender and sclerotized, but flexible, and the cerci, which are fused, pointed and strongly sclerotized. The entire ovipositor, at rest, retracts telescopically within the oviscape, a conical or tubular sclerotized sheath derived from the proximal part of segment 7.
Pregenital segments
c: cerci; mi: intersegmental membrane; pr: proctiger; s5-8: sternites 5-8; t5-8: tergites 5-8.
Author: Giancarlo Dessì
(License: Creative Commons BY-NC-SA)
The pregenital section is included between the preabdomen and the genital opening, and usually is composed of the segments 7 and 8. Some Cyclorrhaphous have also the segment 6 in this region.
In females provided with ovipositor, the pregenital section is included in the ovipositor and retracts telescopically within the last segment of the preabdomen (urite 5 or 6). The shape is adapted to this feature, therefore the pregenital segments are more slender and membranous than the preabdominal, with tergites and sternites reduced.
In females without ovipositor, the pregenital section is gradually joined to the preabdomen and the segments 7 and 8 are similar to the anterior urites and poorly differentiated. This condition occurs in most Nematocera and part of Orthorrhapha. However, in various orthorrhaphous groups there is a significant reduction in the diameter of pregenital segments as a tendency to thinning of the postabdomen throughout the Brachycera and the development of a telescopic ovipositor (McAlpine, 1981).
In some families of Acalyptratae, the tergite and sternite 7 are fused to form a sclerotized ring not retractile, called oviscape. It is conical or bulbous, more or less elongated, or flattened with a triangular shape. At rest, the ovipositor is telescoped into the oviscape. This conformation of the segment 7 is found in some families of Nerioidea (Cypselosomatidae) and Opomyzoidea (Agromyzidae) and is the groundplan of Tephritoidea. However, in Tephritoidea, the females of the family Piophilidae differ in the structure of the oviscape, because the tergite and sternite 7 are separated by a membrane.
Another different character is the shift of the abdominal spiracle from the segment 7 to 6, so the sixth urite bear two pairs of spiracles. This feature occurs in various Calyptratae.
The segment 8, in most Diptera, have not significant external changes, except for the possible transformation into a piercing stick (p.e. Tephritoidea) and, especially, for the close relation with the genitalia of ectodermal origin. Within the segment 8 there is the vagina, a canal or cavity that opens behind the posterior edge of the sternite. In some lower Diptera (p.e. Tipulidae), the segment 8 is called also gynium and its sclerites epyginium (tergite 8) and hypogynium (sternite 8). In these primitive diptera, two symmetrical appendages arise from the posterior edge of the hypogynium and they may be homologous of the first appendages of the orthopteroid ovipositor (McAlpine, 1981). These appendages was called with various names by different Authors (hypogynial valves, hypogynal valves, hypovalves, ovopositor lobes, sternal valves).
Genitalia
As primitive condition of Diptera, the genitalia are composed of following:
- a pair of ovaria (gonads);
- a pair of lateral oviducts (gonoducts);
- a common oviduct (common gonoduct);
- a vagina or genital chamber (distal adaptation of the common oviduct);
- three spermathecae (dorsal receptacles derived from the vagina);
- two accessory glands (collaterial glands).
Vagina, spermathecae and accessory glands are by ectodermal derivation (McAlpine, 1981). The basilar structure of the female system is homologous with the anatomy of most insects, except for the number of spermathecae, that in other orders is usually reduced to one. The organs of most interest by taxonomic and phylogenetic point of view are the spermatecae and the vagina, due the wide differentiation throghout the order.
Vagina
The genital chamber or vagina is derived from the ectodermal section of the common oviduct and is homologous with the ejaculatory duct of the male. Its function is associated with the copulation and the oviposition. This chamber is closely associated with the sternite 8 and, dorsally, the sternite 9; the distal end opens into the vulva. The efferent ducts of spermathecae and accessory glands flow into the vagina by the dorsal wall.
The development and the conformation change according to the groups, from a poorly differentiated canal to a more or less expanded cavity, with singular specialization in some parts. The main differentiation found in the order are the following.
- Uterus or ovisac. Found in viviparous or ovoviviparous Calyptratae, it is generally adapted to mantain the eggs during the development and, in viviparous, the larvae. The uterus is an extension derived from the proximal section of the genital chamber and is present in the superfamily Hippoboscoidea and in part of families Sarcophagidae, Oestridae, and Tachinidae.
- Bursa inseminalis. The meaning of this term is uncertain, because in literature was used referring to various anatomic structures, not homologous, involved in the copulation and the insemination. The bursa seminalis sensu stricto is a blind diverticulum derived from the segment 9 and opening in the dorsal wall of the proximal section of the vagina. It is a feature of some primitive groups of lower Diptera (Culicidae, Tipulidae), but according to some Authors, also primitive groups of lower Brachycera, p.e. the Asilidae (McAlpine, 1981). McAlpine (1981) reports as recurrent synonyms the terms bursa and bursa copulatrix, but he highlights the substantial uncertainty about the presence of these structures throughout the order. Kotrba (2000) discerns between the terms bursa inseminalis and the others, attributing the meaning sensu stricto to the bursa inseminalis of lower Diptera, whereas bursa and bursa copulatrix must used to not homologous organs found in some brachycerous Diptera.
- Ventral receptacle. Present in some families of Acalyptratae, it is a blind diverticulum joined to the anteroventral part of the genital chamber and has different appearance: as a tubule, a spiral, a pouch, uni- or plurilocular, membranous or slightly sclerotized. Due its variety, the ventral receptacle is therefore an important feature in taxonomic diagnosis. Its presence is usually combined with the reduction of the spermathecae, because it serves as insemination chamber and to store the sperm after mating.
Spermathecae
a: anus; g: accessory glands; ov: common oviduct; r: rectum; rv: ventral receptacle; spm: spermathecae; st: sternite 8; v: vulva; va: vagina.
Author: Giancarlo Dessì
(License: Creative Commons BY-NC-SA)
The spermathecae are seminal receptacles derived from the urite 8 and serve to store and feed the spermatozoa during the time between the insemination and the fertilization. Each spermatheca is composed of a capsule, provided with a glandular epithelium, and an efferent duct, own or shared with other spermathecae. In the groundplan of Diptera there are three spermathecae, but their number may be reduced to 2 or 1 or they may be lacking. Rarely there are four spermathecae (Chamaemyiidae in part).
The shape and consistence of spermathecae vary throughout the order. Usually they are spheroidal or cylindrical and slightly sclerotized, but may be also tubular or helicoidal or may have various more or less sclerotized. The external surfaces is smooth or may be covered by various reliefs. The variety of appearance makes the spermathecae as important in taxonomic diagnosis of female specimens.
As primary condition, the efferent ducts of spermathecae are free from the rest of genitalia and they open directly to outside through the sternite 9. This condition is found in some primitive families of Nematocera, but, in most Diptera, the spermathecae are shifted further inside and their ducts flow in the dorsal wall of the genital chamber, behind the opening of the common oviduct. As primary condition, finally, each spermathecae has its own duct, but throughout the order there is the tendency to merge two or more ducts in a single common.
Accessory glands
The accessory or collaterial glands are ectodermal and unsclerotized organs derived from the urite 9. Most Diptera have two glands, a few groups have a single gland. Only the females of Bombyliidae are provided with an additional pair of tubular glands (Kotrba, 2000).
The shape changes throughout the order, from spheroidal to pyriform, from cylindrical to tubular, etc. Each gland has an efferent duct which opens near the openings of spermathecal ducts, in the dorsal wall of the genital chamber or, in some primitive Diptera, in the sternite 9.
The secretion of the accessory glands, usually, serves as glue for the eggs. In some families of Hippoboscoidea it serves as nutrient fluid for the larvae.
Postgenital segments
c: fused cerci; g: retractile sheath of the ovipositor; mi: intersegmental membrane; os: oviscape; spm: spermathecae; s6-8: sternites 1-8; t6-8: tergites 3-8; u7: urite 7.
Author: Giancarlo Dessì
(License: Creative Commons BY-NC-SA)
The postgenital segments follow the genital opening; they are the urites 9 and 10 and the remains of the urite 11. In most Diptera, these segments are more or less reduced or lacking and only in Nematocera and lower Brachycera the primary segmentation is still discernible.
The segment 9 follows the vulva and part of it is closely associated with the female genitalia, due the shape and position of sternite 9. In most Nematocera and Orthorrhapha, the sternite 9 is reduced to a V-shaped or Y-shaped sclerite and is called genital fork or vaginal apodeme or furca. This denomination comes from the retracted position of sternite 9 into the urite 8, closely the dorsal wall of the vagina and with the anterior limbs near the opening of the efferent ducts of spermathecae. In the Cyclorrhapha, whereas, the sternite 9 is lacking or not distinguished McAlpine, 1981). The tergite 9 is visible as dorsal plate only in lower Diptera (Nematocera, most Orthorrhapha, and Aschiza in part); in Muscomorpha it is fused with the sclerites of segment 10 to form a posterior segment reduced and poorly differentiated. In higher Diptera there are two plates behind the posterior edges of the segment 8: the ventral one is called postgenital lobe or subanal sclerite, the dorsal one supranal sclerite. Several Authors believe that these plates originate from the sclerites of urite 9 (McAlpine, 1981; Kotrba, 2000).
The segment 10 is still evident in lower Diptera but only to the tergum. In various Orthorrhapha, the tergite is divided into two lateral plates bearing spines, used for digging during the oviposition. These plates are called acanthophorites in some groups. The sternite 10 is not evident in all Diptera because is lacking or fused with the remains of urite 11 (McAlpine, 1981).
The segment 11, called proctiger, is present as rudiment not identified as true urite and consists of a reduced area bearing the anal opening and the base of cerci. Some Author call proctiger the entire posterior section of postabdomen of higher Diptera, originated from the fusion of segments 10 and 11, but this denomination is in fact inappropriate (McAlpine, 1981; Kotrba, 2000).
Cerci
The cerci are paired and symmetrical appendage of the proctiger, two-segmented as primary condition in both Nematocera and Brachycera. Throughout the order, however, this appendages reduced to a single segment in several groups. The cerci are well developed and elongated in some lower Diptera, but in most of the order they are reduced to a small lobes subspherical or oblong.
In some groups whose the females are provided with the ovipositor, the cerci may be sclerotized and integrate the aculeus (Tephritoidea and Cecidomyiidae in part).
Bibliography
- Hennig, W. (1973) Imagines: 141-236. In Diptera (Zweiflüger), Work cited.
- Kotrba, M. (2000) 1.3. Morphology and terminology of the female postabdomen: 75-84. In Papp, L. & Darvas, B. (eds.) Contributions to a Manual of Palaearctic Diptera. Volume 1. General and Applied Dipterology, Work cited.
- McAlpine, J.F. (1981) Morphology and terminology - Adults: 9-63. In McAlpine, J.F.; Peterson, B.V.; Shewell, G.E.; Teskey, H.J.; Vockeroth, J.R. & Wood, D.M. (eds.) Manual of Nearctic Diptera. Volume 1, Work cited.
- Merz, B. & Haenni, J.P. (2000) 1.1 Morphology and terminology of adult Diptera (other than terminalia): 21-51. In Papp, L. & Darvas, B. (eds.) Contributions to a Manual of Palaearctic Diptera. Volume 1. General and Applied Dipterology, Work cited.
- Servadei, A.; Zangheri, S. & Masutti, L. (1972) Entomologia generale ed applicata, Work cited.
- Tremblay, E. (1985) Morfologia: 14-24. In Entomologia applicata. Volume Primo: Generalità e mezzi di controllo, Work cited.
- Tremblay, E. (1991) Ordine Diptera (Ditteri): 11-20. In Entomologia applicata. Volume III Parte I, Work cited.
Web resources
- Yeates, D.K.; Hastings, A.; Hamilton, J.; Colless, D.H.; Lambkin, C.L.; Bickel, D.J.; McAlpine, D.K.; Schneider, M.A.; Daniels, G. & Cranston, P.S. Anatomical Atlas of Flies. In CSIRO Entomology. CSIRO, Commonwealth Scientific and Industrial Research Organisation. Last access: 28 May 2019.
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