THE SYSTEMATICS, PHYLOGENY, AND ZOOGEOGRAPHY OF SYMMERUS WALKER AND AUSTRALOSYMMERUS FREEMAN (DIPTERA: MYCETOPHILIDAE: DITOMYIINAE)

Abstract

The holarctic genus Symmerusand the transantarctic genus Australosymmerusare 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 are S. nepalensisfrom Nepal; S. uncatusand S. vockerothifrom eastern North America; A. acutus, A. magnificus, and A. truncatusfrom Mexico; and A. collessi, A. confusus, A. lobatus, A. maculatus, A. magellani, A. minutus, A. montorum, and A. peruensisfrom South America. Symmerus tristis(Loew) and S. dilutusFisher are considered synonyms of S. lautus(Loew), and a lectotype is designated for Plesiastina bifasciataWilliston. A subgeneric classification is proposed, in which two subgenera of Symmerusand eight subgenera of Australosymmerusare described. CrioniscaColless is considered to be a subgenus of Australosymmerus. The subgenera of Symmerusare Symmerus s. str. and the new subgenus Psilosymmerus(type-species: Symmerus coqulusGarrett). The subgenera of Australosymmerusare Australosymmerus s. str., Crionisca, and the new subgenera Ventrilobus(type-species: Centrocnemis fuscinervisEdwards), Araeostylus(type-species: Australomyia bivittataFreeman), Vellicocauda(type-species: Platyura insolitaWalker), Tantrus(type-species: Australosymmerus montorumMunroe), Melosymmerus(type-species: Centrocnemis bisetosaEdwards), and Calosymmerus(type-species: Plesiastina bifasciataWilliston). New combinations include A. 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), and A. 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 for S. coqulusGarrett, and some observations on the biology of S. vockerothiMunroe are also presented.The phylogeny of Symmerusand Australosymmeruswas 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 in Australosymmerusare 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 of Australosymmerusare 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 of Symmerusand Australosymmeruslived 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 to Australosymmerus. If Australosymmerusdid 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.