Musculature of the male genitalia in Rivellia (Diptera: Platystomatidae)

Abstract The musculature of male genitalia was studied hitherto only in two species of Tephritidae, one species of Platystomatidae, one species of Pallopteridae, and three species of Ulidiidae of the superfamily Tephritoidea. The split of the hypandrium from one structure into three (the hypandrium and two lateral sclerites) is traced. The hypandrial origin of the lateral sclerites of the hypandrial complex is shown based on the localization of muscle attachment sites. The subepandrial origin of the inner lobes of the surstyli is also confirmed.


Introduction
The signal flies (Tephritoidea: Platystomatidae) include nearly 1300 described species of more than 120 genera occurring predominantly in the Paleotropics, with a few genera and species in the Holarctic and Neotropic regions (V.A. Korneyev, unpublished data). They are a sister group to the Pyrgotidae + Tephritidae lineage, sharing with them numerous synapomorphies, including the structures of male genitalia, such as the phallus stored in rest in a membranous pocket under the 5 th tergite of abdomen, glans clearly separated from the remaining part of distiphallus, lateral (outer) surstyli fused to epandrium, phallapodeme and adjusting structures of hypandrium forming a "fultella", etc. (Korneyev 1999).
Study of the musculature is helpful not only in specifying the functions of genital sclerites, but also for revealing homology of some poorly traced structures (Ovtshinnikova 1989, 1993, Ovtshinnikova and Yeates 1998Galinskaya and Ovtshinnikova 2015). Recently, the musculature of the male genitalia was described for three species of the tribe Ulidiini (Galinskaya and Ovtshinnikova 2015) of the family Ulidiidae, which is a basal group to Platystomatidae + Pyrgotidae + Tephritidae lineage (Korneyev 1999).
In this paper, the musculature of male genitalia is described in Rivellia (Platystomatidae), continuing comparative study of morphology of the Tephritoidea.
Musculature of the male genitalia was studied by manually dissecting material (preserved fresh in 70% alcohol) with microknives in water under a Leica MZ9 5 stereomicroscope. The illustrations were obtained using the image capture function of the Leica MZ9 5 trinocular head and subsequently processed.
The male genital muscles of Tephritoidea were classified into several groups: muscles of the epandrial complex, muscles of the hypandrial complex, tergosternal muscles, and pregenital muscles. The muscles are designated by numbers following the classification previously accepted by Ovtshinnikova (1989).
Platystomatidae have same set of muscles as in Ulidiidae, differing from them by the degree of development, shape, and their attachment sites (Figures 2, 3).
Muscles of the hypandrial complex: M1, M2, and M23. Strong, wide, and short phallapodeme retractors M1 connect the anterior part of hypandrium with grooves on the Y-shaped phallapodeme arms. Strong, wide, and long phallapodeme protractors M2 are attached to the distal half of the lateral surfaces of the unpaired phallapodeme lobe and to the inner surface of hypandrium at left and to the lateral sclerite at right side.
Ejaculator compressors M23 strong and long, surrounding ejaculator apodeme. Their contraction pumps semen into the phallus.
Tergosternal muscles. Tergosternal muscles M5 long, fan-shaped, connecting lateral parts of the epandrium anterior margin with lateral parts of anterior margin of  hypandrium (in contrast to Ulidiidae, in which these muscles are attached to the distal margin of the hypandrium). These muscles draw hypandrium to epandrium. During copulation, epandrium clasps female's ovipositor while the hypandrium is retracted into the male's abdomen by contraction of these muscles.
Muscles of the epandrial complex: M3, M4, M7, and M25. Subepandrial sclerite adductors M3 strong, connecting the inner surface of the epandrium (occupying a considerable part of it) to the inner surface of anterior part of the subepandrial sclerite as a wide bundle.
Adductors of surstyli M4 short, fine, poorly visible, extending from the middle of long posterolateral lobe of the subepandrial sclerite (medial surstyli) to the middle of lateral part of at inner surface of the (lateral) surstylus. Cercus retractors M7 short, fine, extending from the inner surface of the basal part of the epandrium to the basal cercal lobes.
The long and fine poorly visible retractors of anal integument M25 connect the median part of medial surstyli with the basal part of the rectum.
Pregenital muscles: M18 2 and M19. The unpaired adductor of the hypandrium M18 2 extends from the distal part of syntergosternite 8 to the left hypandrial arm. The strong fan-shaped unpaired epandrial retractor M19 obliquely extends from the distal part of syntergosternite 8 to the left part of the basal margin of the epandrium.

Discussion and Conclusions
Our results were compared with musculature of male genitalia of Rivellia basilaris (Wiedemann, 1830) studied by Sueyoshi (2006). In this species he studied muscles of epandrial complex and tergosternal muscles and revealed three pair of muscles: M42+43 (=M4 sensu Ovtshinnikova), M31 (=M3), M34 (=M5). We confirmed his results and expanded the area of the study.
Comparisons of descriptions revealed homologies and the following correspondence between numbers of homologous muscles (Galinskaya and Ovtshinnikova 2014) (Table 2). Analysis of the attachment sites of muscles has shown that in all studied families paired phallapodeme muscles M2 are attached with one end to the distal half of the lateral surface of the unpaired lobe of phallapodeme, and with the other end it is attached either to the the inner surface of the hypandrial arms (in Ulidiidae; only on the left side, in Rivellia ); they are attached to the lateral sclerites in some Tephritidae and (only on the right side) in Rivellia. Thus, the attachment of M2 muscles to the lateral sclerites confirms their hypandrial origin.
It can be noted that the attachment sites of the subepandrial sclerite adductors M3 and surstyli adductors M4 are constant and these muscles are thus clearly distinguished from each other. These muscles are synergistic, and when they contract during copulation, the surstyli grasp and hold the female ovipositor, as they do in most other cyclorrhaphan flies, including those considerably different in the structure of the surstyli and subepandrial sclerite.
Comparative analysis shows that studied Rivellia displays similar sets of muscles of the male genitalia, close to the plan of structure fundamental for Cyclorrhapha, possibly as a result of reduction of or lacking some of the muscles (Ovtshinnikova 1989), and differs from this fundamental plan in the split of muscle M3-4 into two pairs and presence of muscles of the anal integument M25, which is also typical of the family Syrphidae. We have also noted asymmetry in the muscles of genitalia.
In this paper we confirmed the hypandrial origin of lateral sclerite and or medial surstylus is a lobe derived from the bacilliform sclerite.