Musculature of the male genitalia of Xylophagus cinctus (De Geer) and relationships within the superfamily Xylophagoidea (Diptera, Brachycera)

There is no general agreement on relationships within Xylophagoidea (Diptera, Brachycera). The musculature of the male genitalia of Xylophagus cinctus (De Geer) (Xylophagidae, the most primitive family of Brachycera) is described and compared with that of some other Xylophagoidea: Exeretonevridae (Exeretonevra angustifrons Hardy), Coenomyiidae (Anacanthaspis biafasciata Röder), and Rhagionidae (Rhagio montanus Becker, Chrysopilus dives Loew, and Ch. helvolus Meigen) discussed earlier (Ovtshinnikova, 1989, 1998; Palmer et al., 2000). In spite of the differences in the structure of the genital sclerites, Xylophagidae possess all the muscles found in Coenomyiidae and Rhagionidae. The musculature of the male genitalia of Xylophagus cinctus includes two muscle pairs of the aedeagus sheath (M1 and M2); three muscle pairs of the ejaculatory complex (M30, M31, and M32); one muscle pair of the gonocoxites (M33); two muscle pairs of the gonostyli (M27 and M28); one muscle pair of the proctiger (M21), one muscle pair of the cerci (M29); two pairs of the tergosternal muscles (M5 ¹ and M5 ²); and two pairs of the pregenital muscles (M18 and M19). Muscles of the family Exeretonevridae are mostly the same, except for the muscles of the cerci M29, proctiger M29, and pregenital muscles M18 and M19, that are subdivided into two parts. This fact and also a different degree of the development of muscles M32 and M5 ² clearly distinguish Exeretonevridae from closely related families. The attachment places of the muscles of the aedeagus sheath M2 and of the gonostyli M28, as well as the split character of the tergosternal muscle pair M5 ¹ makes it possible to distinguish two sister groups, Xylophagidae plus Exeretonevridae, versus Coenomyiidae plus Rhagionidae. It should be noted that the muscles of the male genitalia of Xylophagidae, Exeretonevridae, Coenomyiidae, and Rhagionidae possess similar plesiomorphic characters, and these families should be united into the superfamily Xylophagoidea. This superfamily is the most primitive superfamily of Brachycera Orthorrhapha and possesses the most stable set and arrangement of male genital muscles within the entire suborder. An improved dendrogram of the phylogenetic relationships between the known groups of Xylophagoidea is proposed on the basis of the structure of male genital muscles.