Characterisation of four species of the genus Acrobeloides Cobb, 1924 (Rhabditida, Cephalobidae) from xeric areas in Spain, with revision of the genus and restoration of Paracephalobus Akhtar, 1962

New material of four Acrobeloides species (Acrobeloides arenicola, Acrobeloides bodenheimeri, Acrobeloides nanus and Acrobeloides tricornis) are redescribed from xeric areas in Spain. This species are morphological (based on light microscopy and scanning electron microscopy study) and morphometrically differentiated in two groups. One of them, including A. arenicola, A. nanus and A. tricornis (belonging to the buetschli-group), are characterised by having smaller body, lip region with labial probolae having rectangular base, pharynx with long isthmus, female genital system scarcely developed with vulva midventral, and usually males absent, these with small spicules. Other species group, including A. bodenheimeri (belonging to the maximus-group), is characterised by having longer body, lip region with triangular base, pharynx with very short isthmus, female genital system well developed with vulva usually located ventrolateral, and males frequent with large and robust spicules. This division is corroborate with molecular analyses based on 18S and 28S rDNA. According to this, the separation of the genus Acrobeloides in two different genera is proposed. As Paracephalobus litoralis is morphologically similar to the species of the maximus-group, the genus Paracephalobus is restored to name the species included in the maximus-group. An updated list of species for the genera Acrobeloides and Paracephalobus is included. In addition, Acrobeloides insignis, A. sexlineatus, A. thornei and A. varius are proposed new junior synonyms of A. tricornis.