Abstract The mayflies are insects widely known as indicators of aquatic quality in freshwater systems, they are abundant and diverse in tropical streams. In this study, it was determined the influence of physical, chemical and bacteriological characteristics on the Ephemeroptera assembly in a period of low precipitation in the Ocoa river of Meta department. There were 5,332 nymphs belonging to 4 families, 10 genera and 3 species. Three new genera records and one species are presented for Meta: Cloeodes, Zelusia, Lumahyphes and Americabaetis alphus. The genera Camelobaetidius and Varipes were found associated with conditions of contamination by organic matter and water mineralization. Likewise, Americabaetis, Thraulodes, Lumahyphes and Tricorythodes were associated with contamination conditions, especially with nitrites and nitrates. Nanomis, Cloeodes and Zelusia showed greater sensitivity to the contamination condition and were related to high percentages of dissolved oxygen in the water and they were in the sampling stations of the upper part in the river. It is important to mention that there were not Ephemeroptera in most of the stations associated with the urban area. That reflects their high sensitivity to polluting conditions at low precipitation period.

Resumen Objetivo: Registrar por primera vez para Colombia la ninfa de Fittkaulus amazonicus, ampliando su rango de distribución en la cuenca del río Orinoco. Metodología: La determinación taxonómica se realizó con material colectado en el departamento del Meta. Alcance: Se destaca el primer registro de asociación de la especie con Eriocaulon sp.

Abstract Objective: To register for the first time in Colombia the nymph of Fittkaulus amazonicus, expanding its range of distribution in the Orinoco river basin. Methodology: The taxonomic determination was made with material collected in the Department of Meta. Scope: The first record of association of the species wiTheriocaulon sp is highlighted.

Resumen En la región andina Colombiana, se han realizado diversas investigaciones en taxonomía y ecología de las comunidades de Ephemeroptera. Sin embargo, otras regiones, como la Orinoquia, se encuentran poco estudiadas. En esta región, se han reportado cuatro especies: Varipes lasiobrachius Lugo-Ortiz & McCafferty, Coryphorus aquilus Peters, Miroculis (Atroari) colombiensis Savage & Peters y Tricorythopsis rondoniensis (Dias, Cruz & Ferreira). El objetivo principal de este estudio fue reportar por primera vez para la región las especies: Mayobaetis ellenae (Mayo), Hydrosmylodon primanus (Eaton), Haplohyphes baritu Domínguez, Camelobaetidius edmundsi Dominique, Mathuriau & Thomasy Nanomis galera Lugo-Ortiz & McCafferty.

Abstract There is extensive research of the Ephemeroptera communities taxonomy and ecology in the Andean region of Colombia. However, other regions such as the Orinoquia have been insufficiently studied. From this region, in the Meta department, four species have been registered: Varipes lasiobrachius Lugo-Ortiz & McCafferty, Coryphorus aquilus Peters, Miroculis (Atroari) colombiensis Savage & Peters and Tricorythopsis rondoniensis (Dias, Cruz & Ferreira). The main objective of this study is to report for the first time for this region the species: Mayobaetis ellenae (Mayo), Hydrosmylodon primanus (Eaton), Haplohyphes baritu Domínguez, Camelobaetidus edmundsi Dominique, Mathuriau & Thomas and Nanomis galera Lugo-Ortiz & McCafferty.