Taxonomic revision of 14 south-western European species of Isotomurus (Collembola, Isotomidae), with description of four new species and the designation of the neotype for I-palustris

A detailed study of morphological characters in populations of the collembolan genus Isotomurus from southern Europe is presented. Isotomurus is a cosmopolitan genus in which species diagnosis primarily relies on pigmentation patterns and chaetotaxy. Correct diagnosis is often complicated by the high variability of these characters. This has created confusion in the systematics of the genus and has led to descriptions of several varieties, colour forms and subspecies, the taxonomic value of which needs revision. Complete screening of pigmentation patterns and the study of sensorial chaetotaxy in specimens collected in 37 localities across south-western France, Italy and Switzerland have enabled 14 species to be distinguished; one taxon is elevated to the rank of species (Isotomurus prasinus) and four new species are described: I. hadriaticus, I. aetnensis, I. gallicus and I. pseudopalustris. The neotype for I. palustris is designated in order to clarify the taxonomic status and the type locality for this species whose topotypic material is lost. A redescription of previously known species on the basis of the newly introduced characters is also presented, together with a key to the south-western European species of Isotomurus. Following this study, the two species I. indipendente and I. italicus came to be junior synonyms of I. palustris and I. fucicolus, respectively. A cladistic analysis of 14 morphological characters provides a phylogenetic reconstruction which is largely concordant with previous taxonomic works.