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An annotated checklist with complementary information for leptophlebiid mayflies from Tocantins State, Brazil is provided. Additionally, we describe a new species of Thraulodes Ulmer, 1920 based on male adults. New reports are based on collections conducted between August 2017 and August 2019 in 13 localities of 10 municipalities. A total of 436 adults distributed in 11 genera, 16 species and five morphospecies treated as undescribed species were collected. Of the total species examined, 11 are new records.

In stream ecosystems, waterfalls intercept the fluvial continuum and limit the dispersion of aquatic organisms. However, the barrier effect may be minimized or absent for insects whose life cycle is not restricted to the aquatic environment (i.e., winged adults), such as the orders Ephemeroptera, Plecoptera and Trichoptera (EPT). In this study, we investigated EPT diversity (abundance, richness, and composition) in savanna headwater streams (Cerrado Domain, Brazil) to investigate the hypothesis that large waterfalls have little effect on instream assemblage structure. During 2017 and 2018, we sampled EPTs and measured environmental conditions at reaches adjacent (downstream and upstream) to waterfalls (5 to 70 m high). We collected 2532 immature specimens of 33 genera, including the first record of 14 genera of Ephemeroptera, 11 Trichoptera, and three Plecoptera for the Tocantins State. Hydro-geomorphic and streambed conditions varied among sites, but they were not related to the presence of waterfalls. Assemblage structure was similar between downstream and upstream reaches, with no significant difference in taxa richness, abundance, and composition. This pattern was consistently observed considering all data combined, and each order separately. These results support the hypothesis that physical barriers have weak effects on EPT assemblages at small spatial scales, where strong dispersal and habitat similarity probably homogenizes the structure of local assemblages in stream corridors adjacent to large waterfalls. The study provides important information about factors driving the distribution of biodiversity in savanna streams, which are relevant to support conservation and management plans in the Brazilian Cerrado.

There are seven genera and eight species of mayflies recorded for Tocantins state, Brazil, but only three belonging to the family Leptophlebiidae. Based on male imagoes, Simothraulopsis primus sp. nov. is described from Tocantins state, Northern Brazil. The species is characterized by the male genitalia being short, lanceolate, with a straight spine and by the "U" shape between the penis lobes. Other distinguishing features are the brown thorax and yellowish brown abdomen.