Systematic studies of the subgenus Crewella of Ceratina (Hymenoptera, Apidae, Xylocopinae), with a revision of the species occurring in Argentina

: The subgenus Crewella Cockerell of Ceratina Latreille is found only in the Neotropical region, with maximal diversity in South America. A morphological study of the species in the subgenus led to the recognition of five species groups, based on several characteristics of the head, the first metasomal tergum, and the female sixth metasomal tergum. A key to the species-groups is presented. The following new species are described: C. dalyana from Colombia, C. amazonica , C. crassipunctata , and C. foveinasis from Peru, C. duplocarinata , C. guaranitica , and C. lobata from Argentina, C. acuminata from Brazil and Paraguay, and C. carbonaria from Argentina and Brazil. Lectotypes are designated for C. brunneipes Friese, C. diligens Smith, and C. vernoniae Schrottky. The following are new synonyms: C. brunneipes Friese, 1910 of C. maculifrons Smith, 1854, and C. gossypii var. asuncionis Strand, 1910 of C. gossypii Schrottky, 1907. A revision of the species in Argentina, with a key to species, is presented.