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The Palni Hills, part of the Western Ghats Mountain range in southern India, constitute a biodiversity hotspot, and their high elevation creates unique ecological conditions. Despite global recognition of the region as a biodiversity hotspot, many taxonomic groups remain poorly studied, including mayflies. We conducted a survey of mayflies in the Palni Hills to document the current diversity profile and analyse their altitudinal distribution patterns, ranging from 346 to 1685 m asl. Our sampling from April to October 2023 covered eleven sites, resulting in the collection of 788 individuals, representing 24 species, 16 genera, and six families. Four species are reported for the first time from Palni Hills. Notably, the families Baetidae, Heptageniidae, and Leptophlebiidae were the most species-rich, although individual population densities remained low. We have provided the updated taxonomic hierarchy, current diversity profile, and abundance of mayflies of Palni Hills. We have also highlighted the knowledge gaps in the taxonomy and distribution of Ephemeroptera within this study area

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Biomass is an important parameter in studying a variety of energetic processes in food webs, community structure, and composition of aquatic organisms. Biomass determinations are based on direct weighing of animals, biovolume determination, and length-weight conversion. Although direct weighing of individual organisms is the most accurate methodology, its application is not very common due to its time consuming nature. Length-weight regressions are the most widely used approach for estimating benthic invertebrate biomass because they are less time consuming and more precise than other methods. In this research, length-weight relationships are evaluated for the most common benthic invertebrates found in an Iranian mountain river in the Southern Caspian Sea Basin by fitting the power function (linearized by logarithmic transformation) to data of wet and dry weights against body length of aquatic invertebrates at both family and order level. A general predictive equation was also obtained for all individuals measured in this study. Regressions obtained were significant at a P value of < 0.05 and explained a high proportion of variation of the dependent variable, as expressed by the correlation coefficient (r= 0.82-0.99). Regression equations obtained in this study for three major orders of aquatic invertebrates were also compared to those in previous studies from different geographical locations. Relationships developed in this study, can be useful for future assessments of benthic community structure and for understanding the importance of these invertebrates in the energy flux of the river.