Discovery of a new population of Rhithrogena delphinensis Sowa & Degrange, 1987, in the Arves Massif, and additions to the morphological description of the larva (Ephemeroptera, Heptageniidae). Rhithrogena delphinensis, described originally on the basis of four larvae from the Western Alps, south of the Arves Massif and from the northern flank of the Ecrins Massif, had not been captured again since 1986. Here, we report the discovery of a new population from river Arvan, whose drainage basin is located between the Grandes Rousses Massif and the northern flank of the Arves Massif. This newly discovered population seems abundant in numbers, and reveals the particular ecological requirements of the species as well as its dependence on glacier fed or nival streams. The morphological characteristics of the larvae are described in detail, and illustrated by photographs. The variability of some of the proposed identification criteria is discussed, and a key to the identification of the Rhithrogena species from the alpestris group of the Western Alps, to which R. delphinensis belongs, is provided. Finally, a portion of 658 base pairs of the COI gene of R. delphinensis is sequenced for the first time and compared to already existing data on the alpestris group in the Western Alps.
