A taxonomic revision of species related to Mordellistenahirtipes Schilsky, 1895 is presented. Five species among the M.hirtipes complex are recognised: M.hirtipes Schilsky, 1895, M.pseudohirtipes Ermisch, 1965, M.purpurascens Costa, 1854, M.balearica Compte, 1985, and M.irritans Franciscolo, 1991. Descriptions, differential diagnoses, an identification key, and new distributional records are provided. Principal Component Analysis is performed for visualisation of differentiation between taxa. The following taxonomic acts are proposed: Mordellistenapodlussanyi Czető, 1990 and M.aegea Franciscolo, 1949 are proposed as junior subjective synonyms of M.hirtipes Schilsky, 1895; M.fageli Ermisch, 1969 and M.pseudohirtipeskrotosensis Czető, 1990 are proposed as junior subjective synonyms of M.pseudohirtipespseudohirtipes Ermisch, 1965; M.geronensis Ermisch, 1977 and M.istrica Ermisch, 1977 are proposed as junior subjective synonyms of M.purpurascens Costa, 1854.

Ancyronyx clisterisp. nov. (Coleoptera, Elmidae) a new spider riffle beetle discovered from northern Borneo (Brunei; Sabah and Sarawak, Malaysia) and the larva of Ancyronyx sarawacensis Jäch are described. Illustrations of the habitus and diagnostic characters of the new species and the similar and highly variable A. sarawacensis are presented. Differences to closely related species, based on DNA barcodes and morphological characters, are discussed. Association of the larva and the imago of A. sarawacensis, and the occurrence of Ancyronyx procerus Jäch in Peninsular Malaysia and Sabah are confirmed by using COI mtDNA sequences.

The current interpretation of two common European species, Mordellistena minima Costa, 1854 and M. pseudorhenana Ermisch, 1977, is based on misidentification. The confusion regarding the identity of the species is fixed based on the revised type material. Here, the species are redescribed, and diagnostic characters are provided. Mordellistena pseudorhenana is revalidated. Mordellistena emeryi Schilsky, 1895 is recognised as a new synonym of M. minima. Mordellistena sajoi Ermisch, 1977 is recognised as a new synonym of M. pseudorhenana. Lectotype and paralectotypes of M. emeryi are designated. Mordellistena pseudorhenana is reported for the first time from Bosnia and Herzegovina, Slovenia, and Switzerland. Two morphotypes of M. pseudorhenana differing in size and shape of the parameres are recognised. Morphological differences are quantified and displayed using principal component analysis. In addition, DNA barcodes have been used for the first time in family Mordellidae to examine the divergences between the species and to interpret the morphological variability observed in M. pseudorhenana. Low genetic divergences did not provide the evidence for considering the morphotypes as separate species. The discrepancy between the morphological and molecular evidence raises questions about the efficiency of the CO1 gene for Mordellidae identification and the stability of morphological traits conventionally used for species separation.

A complete list of the type material of Mordellidae described by Achille Costa is provided. Mordellistena picipes Costa, 1854 stat. restit. is restored as a valid species and a diagnostic description of the lectotype is given. Natirrica meridionalis Costa, 1854 is proposed as a new synonym of Mordellistena humeralis (Linnaeus, 1758). Lectotypes and paralectotypes are designated for the following taxa: Tomoxia bucephala Costa, 1854; Mordella coronata Costa, 1854 (= Variimorda villosa (Schrank, 1781)); M. interrupta Costa, 1854 (= Variimorda villosa (Schrank, 1781)); M. brevicauda Costa, 1854 (= M. brachyura Mulsant, 1856); M. perspicillata Costa, 1854 (= Mediimorda bipunctata Germar, 1827); M. viridescens Costa, 1859; Variimorda basalis (Costa, 1854); Mordellistena stricta Costa, 1854 (= M. pumila (Gyllenhal, 1810)); M. picipes Costa, 1854; M. confinis Costa, 1854; M. minima Costa, 1854; Natirrica meridionalis Costa, 1854 (= Mordellistena humeralis (Linnaeus, 1758)). The male genitalia of the Variimorda basalis lectotype are illustrated here for the first time. Photographs of the labels are provided for the type specimens.

A list of 22 new distributional records is presented for 16 Mordellidae species from the Western Palearctic: Variimorda caprai (Franciscolo, 1951) (Montenegro); V. mendax Méquignon, 1946 (Montenegro); Mordellistena falsoparvula Ermisch, 1956 (Bosnia and Herzegovina, Montenegro); M. olympica Ermisch, 1965 (Cyprus, Montenegro); M. kraatzi Emery, 1876 (Morocco); M. longicornis Mulsant, 1856 (Morocco); M. dives Emery, 1876 (Kazakhstan); M. krujanensis Ermisch, 1963 (Montenegro); M. tarsata Mulsant, 1856 (Cyprus, North Macedonia); M. michalki Ermisch, 1956 (Kyrgyzstan); M. thuringiaca Ermisch, 1963 (Bulgaria, Montenegro, Slovakia, Spain); M. koelleri Ermisch, 1956 (Italy, Montenegro); Mordellistenula longipalpis Ermisch, 1965 (Montenegro); Mordellochroa milleri (Emery, 1876) (Italy); Dellamora palposa Normand, 1916 (Italy). Information about the distributional range is summarised for each species, and notes on habitat and host plants are also provided.