Hylaeus subtilis Förster, 1871: 1039 . Lectotype (here designated) ♀; Austria (MNHN, examined).

Hylaeus nigrifacies Bramson, 1879: 281 . Holotype ♀; Ukraine: Jekaterinoslaw [= Dnipropetrovsk] [Neotype ♀ designated by Warncke (1986); Ukraine: Cherson, ZSM, examined). New synonym.

Prosopis gracilicornis var. luteifrons Strand, 1909: 72 . Holotype ♂; Crimea: Sudagh (Holotype deposited at ZMHB and not at DEI as mentioned in Dathe (2023), examined). New synonym.

Prosopis moricella Bischoff in Stöckhert, 1954: 70. Holotype ♂; Hungary: Simontornya (ZMHB, examined). New synonym.

Prosopis moricella f. atricallosa Bischoff in Stöckhert, 1954: 72. Holotype ♂; Germany [not Hungary as mentioned in Dathe (2023)]: Rahnsdorf, Krumme lake (ZMHB, examined). New synonym.

Lectotype of H. subtilis Förster hereby designated (double slash separates different labels) (Fig. 8): nid de Pemph. lugubris // subtilis m. [Förster’s handwriting] // MUSEUM PARIS Coll. Giraud // LECTOTYPE Hylaeus subtilis Förster Le Divelec des. 2024 // MNHN, Paris EY38577.

Notes. Hylaeus subtilis Förster was described based on two Swiss specimens from Chur collected by Kriechbaumer and one Austrian specimen received from Giraud and reared from a nest of Pemphredon (Latreille, 1796) (Förster, 1871) . The two specimens of Kriechbaumer could not be located by Alfken (1904), who examined the collections of both Förster and Kriechbaumer (ZSM). We (RLD) could not find them either and we believe that the original identification labels on these syntypes may have been removed long ago or that the specimens themselves are lost. The syntype from Giraud’s collection (MNHN) has been located and is designated as the lectotype herein. It is in perfect condition and corresponds precisely with Förster’s original description. Alfken (1902, 1904) examined this specimen and, based on it, synonymised H. subtilis with H. angustatus (Schenck) . However, the lectotype is a typical specimen of H. nigrifacies Bramson. The names H. nigrifacies Bramson and H. moricei (Friese, 1898) have been used to refer to the same species in Europe. Their respective use has been subject to conflicting opinions (Ghisbain & Rosa et al. 2023). The authors of this work deliberately followed Warncke (1972, 1986, 1992a) because of the designation of a neotype for H. nigrifacies by Warncke (1986) who unambiguously assigned this name to the species referred to as H. moricei . Although Warncke’s neotype designation was somewhat arbitrary, it is nominally valid according to the ICZN (1985), as this third edition of the code allowed for the creation of neotype for names that were not in “general use”, and advocating now for the invalidation of the neotype could be argued to be equally arbitrary.

To address this nomenclatural ambiguity, we formally synonymize H. nigrifacies (and H. moricei) with H. subtilis, which is the oldest available name for this species. The use of the oldest available name, following the Principle of Priority, for the group which is represented by definitively located type material allows for a stable nomenclatural base, avoiding the controversial issue of Warncke’s neotype designation.