TAXONOMY OF THE NEARCTIC SPECIES OF TWINNIA AND GYMNOPAIS (DIPTERA: SIMULIIDAE) AND A DISCUSSION OF THE ANCESTRY OF THE SIMULIIDAE

Abstract

Three species of Twinnia, including hirticornis n. sp., and five species of Gymnopais, including dichopticoides n. sp., holopticoides n. sp., and fimbriatus n. sp., are described, keyed, and illustrated. The ancestral simuliid is portrayed as possessing cephalic fans, because several related nematocerous families, viz., the Ptychopteridae, Dixidae, and Culicidae, also have similar and presumably homologous structures in the larval stage which are used in the same fashion to ingest food particles suspended in water The peculiar Twinnia–Gymnopais larval type, which lacks cephalic fans, is considered to have arisen simply by a neotenic retention of the fanless condition found in the first instar of Prosimulium. Phylogenetically, Twinnia and Gymnopais are therefore considered to have been derived from a Prosimulium-like ancestor. Twinnia is intermediate phylogenetically between Prosimulium and Gymnopais, and the latter is believed to be the most recently derived, possibly as a response to the development of arctic–alpine conditions in Beringia during the Tertiary.