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.

Tephritoidea , Platystomatidae , male genitalia, sclerites, musculature, body size

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predominantly in the Paleotropics, with a few genera and species in the Holarctic and Neotropic regions (V.A.Korneyev, unpublished data).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).

The male postabdomen in Platystomatidae consists of modified segments 6–8 (pregenutal segments) and derivatives of segments 9–11 forming the genitalia and cerci.

In the superfamily Tephritoidea, the musculature of the male genitalia has been studied in representatives, shown in Table 1.

There are different views on the homology of some genital sclerites in the superfamily Tephritoidea. Different terminology is therefore used for some structures in a few cases, where morphology of Platystomatidae genitalia is considered: outer and inner surstyli (McAlpine 1973, 1999) or lateral and medial surstyli (White et al. 1999, Korneyev 2001); lateral sclerites of hypandrium (McAlpine 1973, White et al. 1999), gonites and vanes of fultella (Korneyev 1987), gonocoxites and vanes of phallapodeme (Korneyev 1999); decasternum or 10^th sternite (Korneyev 1987), ventral plate of proctiger, derivate of 10^th sternum (Ovtshinnikova 1989, 1994, 2000), subepandrial sclerite (Cumming et al. 1995) etc. Here, we provisionally follow terminology of White et al. (1999), with some reservations.

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 1998; Galinskaya 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.

The terminology of the genital sclerites mainly follows White et al. (1999), Kameneva (2000), Galinskaya (2012), and Sinclair (2000).

Musculature of the male genitalia was studied by manually dissecting material (preserved fresh in 70% alcohol) with microknives in water under a Leica MZ9⁵ stereomicroscope. The illustrations were obtained using the image capture function of the Leica MZ9⁵ 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).

List of abbreviations: cerc – cerci; epand – epandrium; bph – basiphallus; hypd – hypandrium; l scl – lateral sclerite of hypandrium; l sur – lateral surstylus; m sur – median surstylus; phapod – phallapodeme; sbepand scl – subepandrial sclerite; sec scl – secondary sclerotisation of hypandrial membrane; sur – surstylus; 8 stgst – 8 syntergosternite; M1–M19 – muscles.

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.

For useful criticism and many interesting discussions we are grateful to A.I. Shatalkin (ZMMU). We thank P.N. Petrov (Department of Entomology, Faculty of Biology, Lomonosov Moscow State University) for improuving the English of the manuscript.

Authors are very grateful to V.A.Korneyev for reviewing, greatly improuving the manuscript and many valuable comments. Authors are also grateful to B. Sinclair for reviewing the manuscript.

The work of T.V. Galinskaya was financially supported by the Russian Science Foundation (14-14-00208); that of O.G. Ovtshinnikova, was performed in the scopes of the state research project No. 01201351183 and by the Russian Foundation for Basic Research, project No. 15-04-03457.