This readme file describes the data file "Nuclear_SSRs_Leys_et_al_MolEcol.txt" accompanying the above publication. For any further queries please contact marie.leys@gmail.com. - The table has 800 rows and 20 columns (excluding the column headers). - Each row correspond to a individual. - A describition is given for each column header: column 1 ("ID"): identifier of Baetis larvae column 2 ("Site"): sampling sites column 3 ("Year"): year when sampling was done. column 4 ("Month"): month when sampling was done. column 5 ("Sampling_date"): sampling date column 6 ("Sex"): F = female, M = male, nd = means that the sex is undetermined (Baetis alpinus sex was determined when the total larval lenght was larger than 3.5 mm). column 7 ("Size"): corresponds to total larval body lenght from the tip of the head untill the end of the abdomen (mm). column 8 ("Last_instar"): 0 = early or intermediate instar, LI = Last-instar (i.e., when larvae have black wingpads). column 9 ("Species_lineage"): Species (B. alpinus = Baetis alpinus, B. melanonyx = Baetis melanonyx). For Baetis alpinus, lineage A and lineage B are indicated as _A and _B, respectively. column 10 ("Cohort"): indicates the larval sizecohort for which the larvae was assigned to, for a specific site. Columns 11 to 20 correspond to nuclear microsatellite loci data (first allele and second alleles at the same loci are shown in the same columns separated by "/"). - Missing values are represented as NA. - Additional information and spatial coordinates of the sampling sites are provided in the Additional file 1 (Table S1) of the following publication: Leys, M., I. Keller, et al. (2016). "Distribution and population genetic variation of cryptic species of the Alpine mayfly Baetis alpinus (Ephemeroptera: Baetidae) in the Central Alps." BMC Evolutionary Biology 16: 77.