THE SYSTEMATICS, PHYLOGENY, AND ZOOGEOGRAPHY OFSYMMERUSWALKER ANDAUSTRALOSYMMERUSFREEMAN (DIPTERA: MYCETOPHILIDAE: DITOMYIINAE)
Authors: Munroe Donald Douglas 
Published: April 2011

The holarctic genusSymmerusand the transantarctic genusAustralosymmerusare revised, a reconstruction of their phylogeny is presented, and their zoogeography is discussed. Diagnoses or redescriptions of 31 of the 37 previously-known species and descriptions of 14 new species are presented. The new species areS.nepalensisfrom Nepal;S.uncatusandS.vockerothifrom eastern North America;A.acutus,A.magnificus, andA.truncatusfrom Mexico; andA.collessi,A.confusus,A.lobatus,A.maculatus,A.magellani,A.minutus,A.montorum, andA.peruensisfrom South America.Symmerus tristis(Loew) andS.dilutusFisher are considered synonyms ofS.lautus(Loew), and a lectotype is designated forPlesiastina bifasciataWilliston. A subgeneric classification is proposed, in which two subgenera ofSymmerusand eight subgenera ofAustralosymmerusare described.CrioniscaColless is considered to be a subgenus ofAustralosymmerus. The subgenera ofSymmerusareSymmerus s.str. and the new subgenusPsilosymmerus(type-species:Symmerus coqulusGarrett). The subgenera ofAustralosymmerusareAustralosymmerus s.str.,Crionisca, and the new subgeneraVentrilobus(type-species:Centrocnemis fuscinervisEdwards),Araeostylus(type-species:Australomyia bivittataFreeman),Vellicocauda(type-species:Platyura insolitaWalker),Tantrus(type-species:Australosymmerus montorumMunroe),Melosymmerus(type-species:Centrocnemis bisetosaEdwards), andCalosymmerus(type-species:Plesiastina bifasciataWilliston). New combinations includeA.aculeatus(Edwards),A.bisetosus(Edwards),A.mexicanus(Giglio-Tos),A.nitidus(Tonnoir),A.pediferus(Edwards),A.rieki(Colless),A.simplex(Freeman),A.tillyardi(Tonnoir),A.trivittatus(Edwards), andA.zonatus(Giglio-Tos). In addition to the six previously-described species which were not included in the study, five species remain unplaced in the subgeneric classification. Significant biological data are presented forS.coqulusGarrett, and some observations on the biology ofS.vockerothiMunroe are also presented.The phylogeny ofSymmerusandAustralosymmeruswas reconstructed by Hennig's system of "phylogenetic systematics" because there is no clearly-formulated alternative to the cladistic method for phylogenetic analysis. A number of logical and practical difficulties with the procedure are discussed. It was concluded that criteria of deviation from a basic plan and inferred adaptive significance were preferable to criteria of distribution of character states for the recognition and categorization of attributes as plesiomorphic or apomorphic, that the use of minute correspondence in structurally rich characters for the recognition of convergence and synapomorphy is not a reason for discarding phylogenetic methods for phenetic ones, as has been suggested, and that the necessity for tentative reference to a pre-existing phylogeny is neither a philosophical nor a practical problem.For each attribute used in the cladistic analysis, the inferred plesiomorphic and apomorphic state is described and the reason for the inference is stated. The monophyletic group described by each synapomorphy is identified; description of the monophyletic groups at each level specify the phylogenetic reconstruction, which is summarized by means of cladograms.Cladistic and zoogeographic evidence is presented which indicates that transantarctic relationships inAustralosymmerusare found in a complex of closely-related sister-groups forming a monophyletic group confined to the South Temperate region. Two of the three transantarctic relationships occur at the intra-subgeneric level; the remaining one occurs at the inter-subgeneric level. The remaining species ofAustralosymmerusare shown to form a monophyletic morphological-chorological progression from south to north across the equator in the New World. The evidence indicates that the ancestor ofSymmerusandAustralosymmeruslived in the northern hemisphere. One phyletic line dispersed to the southern hemisphere and reached what is now South America at least by the early Tertiary, but probably much earlier, and gave rise toAustralosymmerus. IfAustralosymmerusdid not cross large water gaps, it must have been present in Gondwanaland before the break-off of New Zealand (Lower Cretaceous).Australosymmerusis not present in South Africa, indicating that it did not exist in Gondwanaland before the break-off of South Africa, or that it was present and has subsequently become extinct in South Africa. The sister-group of the transantarctic group migrated northwards, probably reaching northern South America before the end of the Oligocene.