Contributions to a revision of the Hylaeus brevicornis group (Hymenoptera, Anthophila, Colletidae)

The Hylaeus brevicornis group of the subgenus Dentigera is defined and revised. A total of 22 species are included in the group, their circumscription and identification using morphological characters is described and discussed, and their distribution is studied. Three species from Lebanon and Nepal are described as new to science. The hitherto unknown females of Hylaeus ( Dentigera ) alievi Dathe & Proshchalykin, 2021 and Hylaeus ( Dentigera ) biarmicus (Warncke, 1992) are described for the first time. An integrative revision of the group involving gene sequencing is being prepared.


Introduction
A revision of the species of the Hylaeus brevicornis group has been repeatedly demanded, but never realised. This was mainly due to an insufficient data base, in terms both of sample availability and geographic coverage. In recent years, more and more papers have been published in which several "white spots" on the map have been erased by new faunistic and taxonomic findings, so that in the meantime a more informed assessment of their biodiversity is possible. The most easterly occurrences of Dentigera are from central China (Hylaeus luna Chen & Xu, 2009) and eastern Mongolia. The joint work of Proshchalykin and Dathe , 2018, 2018 led to significant progress in the systematic recording of fauna from the Far East of Russia through Mongolia and northern China to the Caucasus. Dathe was able to gain substantial experience over larger parts of the northern Palaearctic, especially from Asia Minor, the Middle East and Central Asia, through further productive collaboration with partners in Israel (Mandelik and collaborators since 2007), Turkey (Özbek & Dathe 2020), Lebanon (Boustani et al. 2021), Morocco (Lhomme et al. 2021) and the Aegean Islands (Petanidou and collaborators since 2015). Furthermore, in the Himalayas a new species of the brevicornis group was discovered by Ikudome & Dathe (2021, Hylaeus kumari), as well as -described here -Hylaeus gutichauris spec. nov. and Hylaeus kaigaonis spec. nov.

Material and methods
The studies are based on my determinations of about 7,500 specimens studied during four decades. Species of the Hylaeus brevicornis group are mainly western Palaearctic, with a certain concentration around the eastern Mediterranean region (Greece, Lebanon, Israel, Cyprus), but in recent years more extensive collections from the Caucasus, Turkey and Central Asia have also been evaluated. The specimens were conventionally needle-pinned, and the terminalia of interesting and doubtful males were prepared and partly photographed. The majority of the specimens was returned to the senders and they are now in their collections; mostly only duplicates were retained in the SDEI collection as voucher specimens.
With the exception of Hylaeus (Dentigera) luna Chen & Xu, 2009, all types were examined and in males genitalized. The types of Förster (coll. ZSS Munich), Morawitz (ZISP St. Petersburg), Nylander (NHRM Stockholm), Alfken (ZMB Berlin), Nurse (NHML London) and Warncke (OLM Linz) were borrowed. Of particular value was the loan of specimens from the Natural History Museum Vienna, which have only recently been COI barcoded (Schoder 2018). Five species, Hylaeus brevicornis Nylander, H. gredleri Förster, H. imparilis Förster, H. intermedius Förster as well as (presumably) H. kahri Förster had been collected syntopically and comparatively studied. Thus, it was possible to apply baseline standards of morphological comparison in this study, correlated with these genetic data. Simultaneously, this also created a connection to the BOLD database (Ratnasingham & Hebert 2007). Major changes in our view of the mentioned taxa did not arise, even after the type comparisons.
In an early paper (Dathe 1980: 213) I had recommended that the terminalia should be routinely extracted from the anal aperture. This was not good advice, because proper dissection requires some experience and particular care. The sternites are so firmly attached to the tagmata that the metasoma is often stretched excessively when they are pulled out. The individual parts have to be cut off at specific sites. This is best done under the protection of water or gel, otherwise the elastic particles can spring away and are easily lost. The genital capsule is somewhat more robust, but the sternites are very sensitive. It is not recommended to try maceration with potassium hydroxide. Although this removes disturbing connective tissue and separates the parts well, they also deform considerably. In the process, they are discolored so that they can hardly be found again in the gel. They dry out ex situ anyway and can lose their functional structure as a result. Especially the extremely delicate bristles on the apical lobes are easily lost. In any case, being very small, they are at the limit of optical resolution, so that they are often barely perceptible even with a good microscope. This study therefore prefers to use drawings for the depiction of sterna 7 and 8, which also include an interpretation.
The morphological terminology of Michener (2007) is basically followed. Some terms that find special use in the genus Hylaeus are explained in detail in Özbek & Dathe (2020: 275 ff.). The following abbreviations are used in the description of the new species: N -Number of specimens examined; TL -total length; WL -wing length; HL:HW -head length to width ratio; UHW:LHW -upper face width to lower face width ratio; ScL:ScW -scape length to width ratio; CL:CW -clypeus length to width ratio; with indication of the range, in brackets the mean value. The tagmata of the metasoma, terga (T) and sterna (S), are named by their initial letters and the number of their sequence; T1 and S1, respectively, are the visible basal elements, which in comparative anatomical terms are known to correspond to tergite 2 and sternite 2. GC refers to the genital capsule. The specimens were studied with a stereomicroscope Olympus SZX12, the photos taken with a Leica camera and Leica Application Suite vs. 4.12. The stacks of digital images were processed using Helicon Focus 7.7.4.
In describing the geographic distribution of species, the following terms are used: Asia minor -Turkey (Asian part); Near East -Arab states (Syria, Lebanon, Jordan, Iraq), Iran and Israel; Middle East -Afghanistan, Pakistan; Central Asia -Kazakhstan, Uzbekistan, Turkmenistan, Kyrgyzstan, Tajikistan; Far East -Russian Far East. Mongolia, China and Japan are indicated separately.
The collections mentioned in the text are abbreviated as follows:

Characterisation of the Hylaeus brevicornis group
The species of Hylaeus subgenus Dentigera occur throughout Europe up to 64°N latitude and reach the Himalayan massif, Central Asia and central China in the east, via the Near East, Iran and Afghanistan. The vast majority of the known species can be easily assigned morphologically to two groups, which are primarily distinguished by the structure of their penis valves. The Hylaeus conformis group with eight species (in Michener 2000: 195 "brachycephalus group") has been studied in detail by Dathe (2006). In these animals, the penis valves appear in dorsal view as elongated with more or less curved keels with broad inward surfaces, which almost conceal the inner tooth which gives them their name (Dathe 2006: 76). The other large group, which we have named the Hylaeus brevicornis group, shows narrow curved valvae in dorsal view, which have a pointed inner tooth basally and remain separated distally, like an open horseshoe. These 22 species form the large majority in the subgenus. A recent study on the fauna of the Caucasus (Proshchalykin & Dathe 2021) even revealed a certain dominance of the group in the eastern high mountains. A separate development can be seen in the Himalayas. In the Mediterranean region, too, certain species are frequent and represented by larger numbers of individuals. The species exhibit a number of other very similar characters in both sexes, and neither is it easy to interpret levels of variability, so that the taxonomic study of the brevicornis group as a whole must be regarded as unfinished. A way out of this situation can only be provided by an integrative combination with molecular genetic studies, as already demonstrated by the resilient results of the first such studies (Schmidt et al. 2015, Schoder 2018. It would be futile here to present another classical morphological paper, if the best modern methods did not also require appropriate concepts. On the basis of data collection over several decades, obtained from thousands of animals studied, my experiences will be compiled here. A special role is played by the comparative study of such genital structures as have not so far been considered. Some species in the subgenus cannot be assigned to either group: Hylaeus rubicola Saunders, 1850 (close to conformis) as well as H. biarmicus (Warncke, 1992) and H. chukar (Warncke, 1992) (close to brevicornis).
The species of the Hylaeus brevicornis group are at most medium-sized, but usually small to very small, and hardly hairy. They are difficult to distinguish: often this is only possible with some certainty using complexes of characters. The males have differently expanded scapes, from slender to almost spherical in frontal view; the sternum 3 bears paired callosities, occasionally also a single strong process. The male terminalia are of special importance. All the species have a compact, round head with normal genae and a shallowly convex face, the clypeus is rarely depressed (Hylaeus glacialis Morawitz), the malae are narrow. The entire integument of the mesosoma and metasoma is transversely shagreen and always distinctly punctate, although the punctation is variable in strength and density. Males and females of the same species often differ in the size and density of the punctation and in the shininess of the basal metasomal tergites. A first glance at tergum 1 and the mesopleura usually leads in the right direction, but subsequently the determination becomes subtle. A useful character of females of the group is the tridentate mandibles; there are also such in species of other subgenera, for example in Lambdopsis, but together with the sculptural characters and the observation that the foveae faciales do not extend to the vertex (as in Paraprosopis species), the initial assignment is quite straightforward.

The genital armature of the males
The terminalia or genital armature of males are generally composed of three parts, the genital capsule and sterna 7 and 8 (Figs 1, 2). The genital capsule of bees is a relatively large, well-chitinised pincer-like formation, retracted into the end of the body below the last visible tergites. It is stretched out for mating. Sometimes the genital capsule is called the actual copulatory apparatus, but this is not quite correct, because presumably the other two sternites, which lie in situ under the capsule ( Fig. 1), also have a special function in mating, which is however not known. An argument for this assumption is the fact that this "triad" is present in all bee taxa. During dissection, another very delicate structure ( Fig. 1: T8), which can be attributed to the terga, is regularly found on the underside of the last visible tergite, adjacent to the genital capsule. Excellent illustrations of genital capsules of bees, especially bumblebees, were given by Bertsch (2019) as electron microscopic 3D raster photos. In Hylaeus species, the genital capsule is comparatively reduced.
Here, it consists of a basal ring (gonobase) into which two movable gonoforcipes are inserted. Sometimes gonocoxite and gonostylus can be distinguished from the gonoforceps. The usual dorsal view shows a pair of penis valves between the gonoforcipes. The actual penis has a membranous structure and is not readily visible on specimens. Other structures like the paired volsellae and spatha inside the capsule are also barely visible; they are therefore of no importance for our present study. According to the illustrations in Méhelÿ (1935), who used transmitted light preparations as templates, one can also assume specific formation.
The last sternites of the males, called sternum 7 and sternum 8 ( Fig. 2: S7, S8), have received little attention in Hylaeus taxonomy. They are usually small and only weakly sclerotised. Sternum 7 consists of a basal pair of legs connected by a usually short shaft with paired basal and apical lobes. Sternum 8 is one-piece, sometimes only rhombic; distally various shapes of lobes may form. Two facts are remarkable about these sterna: in their design they appear to be unconstrained and not subjected to formative selection pressure, so that their appearance is not infrequently downright luxury; this also means they remain similar within kinship groups. How this morphological complex functions is not known in detail. A lock and key principle between partners of the same species cannot be substantiated, because on the female side there are no equivalents in the genital. The actual species recognition apparently occurs via chemical markers, which, according to previous observations, are mediated between the antennae of the females and sensory structures of the scape and the paraocular area of the males (Torchio 1984, Dathe 2009, Erbe 2011. Méhelÿ (1935) discussed in detail the possible functions of the parts of the copulatory apparatus. They are not inserted into the female anal aperture, but essentially only used for external fixation of the partner. Some male parts may serve to protect against the female´s sting apparatus, so that the membranous penis can be inserted and guided.
Bertsch & Titze (2019) describe a similar situation in bumblebees (Bombus): "Although adequate explanations for the morphological diversity of male genital capsule structure remain to be determined, they are obviously conservative characteristics, in which phylogenetically older adaptations have been preserved, probably because selection pressure is low." The authors know similar phenomena in botany: pollen grains show a variety of adaptations that cannot be explained functionally.
These are considered phylogenetically conservative and are therefore suitable for the taxonomic classification of genera. Even in Hylaeus species, which are morphologically rather monotonous, phylogenetic relationships are apparently preserved in the structure of the terminalia, on the basis of which Méhelÿ (1935) Constantinescu (1973Constantinescu ( , 1974 also recommended using the genital armatures of Hylaeus species in taxonomic studies. Popov (1939) gave formal expression to Méhelÿ's system according to the international nomenclatural rules, the main features of which remain virtually unchanged now, 80 years later.   1. Hylaeus (Dentigera) acer Dathe, 1980 Hylaeus (Dentigera) acer Dathe, 1980: 80-81, 86  Description of the female Fig. 51 Diagnosis: The males were described from Azerbaijan, but in the meantime both sexes have become known from Iran too, so that the female can be added here. The Hylaeus alievi female is conspicuous because of its complete lightyellow mask, and the yellow apical spot on the black scapes. The mesopleura are strongly but shallowly punctate. Similarly, unusual is the finely shagreened and only very sparsely punctate sculpture of the T1. These peculiarities make the species unmistakable.
Female new: N = 3. TL 5.2-5.8 (5.52) mm, WL 3.3-3.6 (3.47) mm. Head: Proportions HL:HW 0.83-0.89 (0.86), UHW:LHW 1.38-1.45 (1.42), outline transversal-trapezoid. Pilosity short and sparse. Scapes black, at the tip with a yellow spot extending downwards; flagella blackened above, yellow below, terminal segment dark brown. Mask complete, pale yellow, dull, lateral spots extended to foveae faciales, well beyond scape bases. Foveae faciales long, reaching the vertex. Clypeus CL:CW 0.97-1.00 (0.99), flat; surface finely shagreen, matt, with shallow, sparse punctation, anterior margin, seams und tentorial pits black; supraclypeal area likewise completely yellow. Frons and vertex without gloss, surface streaky-wrinkled with rows of flat, close punctation; vertex coarsely and densely punctate. Genae finely longitudinally striped, with moderately scattered punctation; occiput rounded; malae narrow. Labrum black, with flat horseshoe-shaped hump; mandibles trifid, black, in front of the tip with brown ring. Mesosoma normal, somewhat depressed; pilosity reduced, with erect, short, white hairs, also on underside. Coloration black, pronotum with yellow band, calli totally, tegulae partly yellow. Mesonotum and scutellum shagreen, scarcely glossy, punctation strong, close; mesopleura with punctation somewhat coarser than mesonotum, but remarkably shallow; anterior margin (omaulus) rounded. Legs from the tip of the femur yellow, tibiae with black spot, all basitarsi yellow-white, remaining tarsomeres blackened; wings slightly brownish, venation light brown, subcosta dark. Propodeum evenly rounded, only the terminal area ventrolaterally sharpedged; medial area basally with mesh row, distally with sharp longitudinal wrinkles; lateral areas finely meshed, matt; terminal area shagreen, surface in the middle somewhat impressed, propodeal furrow narrow with sharp margins. Metasoma spindle-shaped, coloration black. T1 finely shagreen, with fine, very scattered punctation, silky shiny; T2 and following terga more finely and densely punctate; lateral parts of terga with narrow ciliar fringes. End fringe yellowish.   Characters: Hylaeus gredleri und H. brevicornis were long considered a reason for the indivisibility of the superspecies "H. brevicornis auctorum" due to their morphological similarity and comparable distribution. Both species are common and often live syntopically. The difference, however, becomes clear when they are compared without the aforementioned prejudice: H. gredleri has coarser integument sculpture, which appears correspondingly dull, for example on the frons, whereas these areas are only sparsely punctate and shiny in H. brevicornis. This applies to both sexes. The scapes of the males are similarly expanded and spherical, index ScL:ScW 1.4-1.6, but have a different coloration: the pale marking is at the tip of the scape in H. gredleri, in H. brevicornis at the sides. The light coloration, for example of the pronotum, tends to be more pronounced in H. gredleri. These features are by no means present "with all transitions". On the contrary, they prove to be quite stable, especially considering the generally small differences in the brevicornis group. Incidentally, these taxa were widely separated in the recent molecular genetic analyses: they are not even, as might have been suspected, sister species (BOLD 2011, Schmidt et al. 2015, Schoder 2018

Characters:
Male scapes strongly expanded, ScL:ScW 1.3-1.4, with curved lateral edges; flagellum recessed decentrally, forming a free, deeply depressed area inwards; outer margin of scape with yellow markings, triangularly tapering proximally. S3 in normal-sized specimens with strong triangular cusp. Integument in both sexes shagreen with moderate to strong, close punctation, but largely silky-glossy. Females are also richly pale-marked, with extensive yellow-white spots on the paraocular areas and often also on the clypeus. Male terminalia (Figs 12, 28,

Diagnosis:
The males of the new species are characterised by completely black scapes with strong punctation. This is particularly remarkable in that these are usually richly pale-marked in species of the region with strongly thickened scapes (Hylaeus kahri, H. punctus, H. syriacus).
The mask is ivory-white, and in females the clypeus and often the base of the supraclypeal area are also pale-spotted.

Etymology:
The new species is dedicated to Mira Boustani (University of Mons-Hainaut/Belgium) in recognition of her contributions to the knowledge of the bee fauna of her home country, first published in a critical compilation in 2021 (Boustani et al. 2021).

Distribution:
The species is widespread in the area of the eastern Mediterranean countries and seems to be by no means rare. Until now, however, it has been misinterpreted; I had incorrectly determined it as "syriacus". It was possible to revise records from Israel, which are listed here: Israel:

Characters:
The species is hard to mistake because of its distinctive sculpture and characteristic mask, at least in the male. Its mask is usually intensely yellow and appears three-pointed because of the wedge-shaped clypeus macula below; scapes strongly expanded, ScL:ScW 1.3-1.4; sternum 3 with strong, anteriorly shallowly hollowed cusp; metasoma dorsally with coarse contiguous punctation. Male terminalia (Figs 17,33,49): apical lobes of S7 triangular, margin with few protruding bristles, S8 apical with broad apex, spiculum elongate; GC with narrowed gonoforcipes, penis valves very narrow with only slight interspace. The integument of females is correspondingly strongly sculptured, the paraocular area largely filled, and the clypeus mostly yellow-spotted.

Distribution: Lebanon, Azerbaijan (Nakhichevan AR).
Remarks: It does not seem to be completely excluded that H. syriacus and H. kahri belong to the same species. However, in Shakhbuz, Zarnatun, both forms live side by side, but no transitional forms occur between them. Genetic sequencing of such specimens would be useful.

The species of the Himalayas
The following is a compilation of the Dentigera species that have been described from the Kashmir region and Nepal. This group shows a certain exclusivity, which probably has to do with the special conditions in high mountains. It is striking that a whole series of species are so far only known from single specimens and that the sexes cannot always be associated with certainty. The taxonomic evaluation of many specimens is unusually difficult, especially in the case of the very similar females. Their colour characters are quite uniform; the propodeum is equally rounded, with the basal area only wrinkled at the base and the remaining parts with fine sculpture; tergum 1 has white lateral fringes, it is smooth and shiny, hardly chagreen, the punctation fine and rather scattered. The males possess a slightly larger number of usable morphological characters, but they too give the impression of continuing radiative development. Finally, hybridisation between local forms cannot be excluded, so that the findings recorded here can only be preliminary. Above all, they may serve as an orientation for further investigations, also supported by molecular genetics. So far, four species have been identified; one of them, Hylaeus kumari, was described only recently (Ikudome & Dathe 2021), two others are newly introduced here. The longest known is Hylaeus kashmirensis (Nurse, 1903). This species is easily recognisable by its slender black scape and broad head, and it also seems to be more common. The other rarer species have males with thickened black scapes and differ only in subtle details: the shape of the scapes and abdominal humps, in color characters, and in the punctation of the integument.
Recent collections of small series by Creutzburg (Jena) have created doubts about its status. It could well be a brightly marked specimen of Hylaeus kashmirensis, as individual specimens of this species also tend towards having a spotted clypeus. Furthermore, the specimen fits H. kashmirensis in size and has the characteristic broad head (HL:HW 0.86). On the other hand, the yellow coloration of the mask speaks against their conspecifity.
Hylaeus vetustus also resembles the females of Hylaeus syriacus and Hylaeus alievi in having an extended yellow mask However, it differs from both by the rhombic shape of the supraclypeal area, which has parallel sides in the latter. Tergum 1 of H. vetustus resembles H. alievi, for the punctation in both is only very fine and scattered; the integument is smooth and shiny, though in H. alievi distinctly shagreen and less lustrous. H. syriacus has moderately close punctation. The differences are subtle, but in view of the generally very small differences in the brevicornis group, the combination of characters seems to be very special in each case. -Nurse's (1903) statement "Nearest to P. strenua. " is incorrect.
Description of the female: Fig. 53 Diagnosis: The assignment of a female to Hylaeus kumari made here is not without problems, because the specimen was taken from another locality which does not directly correspond with those of the two known males. However, it shows similarities with them, and it differs from the females of the other species in certain characters. Future observations should pay attention to whether the sexes occur together. Female new: N = 1. TL 5.15 mm, WL 3.84 mm. Head: Proportions HL:HW 0.89, UHW:LHW 1.48, outline rounded-trapezoid. Scapes black; flagella blackened above, yellow below. Paraocular area ivorywhite filled to above antennal sockets, dorsally oblique to foveae faciales, shiny; foveae faciales long, reaching the vertex. Clypeus with white fleck; CL:CW 1.11, flat; surface finely shagreen, matt, with indistinct shallow punctation; supraclypeal area rhombic, with protruding lateral corners. Frons and vertex shiny, moderately densely punctate. Genae finely longitudinally striped, with moderate scattered punctation; occiput rounded; malae narrow. Labrum black, with flat horseshoe hump; mandibles trifid, black, in front of the tip with brown ring. Mesosoma normal, rounded; pilosity with erect white hairs laterally and on underside. Coloration black, pronotum with two long yellow stripes, calli and tegulae pale. Pronotum not expanded, anterior margin medially narrow, dorsolateral angles rounded. Mesonotum, scutellum and mesopleura shagreen, silky-shiny, punctation moderately dense; omaulus rounded. Legs black, only tibia bases yellow; wings clear, venation black, costa dark. Propodeum evenly rounded; basal area with irregular mesh ridges, laterally not demarcated; lateral areas setose, finely shagreen, matt; terminal area with finely sculptured surface, propodeal furrow narrow below, dorsally widely broadening. Metasoma spindle-shaped, coloration black. T1 smooth and shiny, with punctation fine, sparse, apically somewhat denser, with white lateral fringe patches; T2 and following terga more finely and densely punctate; distal margins blanched, with fine ciliary bands. End fringe pale.

Diagnosis:
The male of the new species has strongly expanded, black scapes with a tiny apical spot; the mask is ivory-white. The female has oval spots in the paraocular area.

Diagnosis:
The male of the new species has strongly expanded, completely black scapes with a flat impression frontally; the mask is ivory-white. In females, the paraocular area has wedge-shaped spots.

Etymology:
The species is named after the Nepalese municipality Kaigaon (Kaigaun) in the district of Dolpa, where the specimens were collected.

Annex
The following two species do not strictly belong to the Hylaeus brevicornis group, nor to any other: each stands alone. But they are added here for the sake of completeness, because Hylaeus rubicola Saunders, 1850 was presented earlier as a member of the subgenus Dentigera in connection with the Hylaeus conformis group (Dathe 2006: 101-103).
Hylaeus (Dentigera) biarmicus (Warncke, 1992) Prosopis (Nesoprosopis) biarmica Warncke, 1992: 770, 799 . Egypt: Sakkarah. Holotype OLM Linz. Hylaeus (Dentigera) biarmicus (Warncke, 1992) -Lhomme et al. 2020 Diagnosis: The author of the species knew only two males, one from Egypt (1897, coll. Schmiedeknecht) and one from Spain (Zamora). He compared these in detail with Hylaeus pictipes (by which he probably meant Hylaeus taeniolatus), then concluded that the species is closely related to H. brevicornis. At least he gave an illustration of the face, but this did not improve the recognizability of the species (compare Fig. 60a). However, the decisive character of this very small, gracile species is, in its entirety sharply margined terminal area of both sexes, in connection with a smooth, shiny integument of T1. The penis valves show an open cleft in dorsal view, but these parts are not curved but run parallel (Fig. 61c). The inner tooth is bluntly rounded and not visible from above, which does not correspond to the characters of the brevicornis group. However, the mandibles of the female are clearly tridentate, so that the classification into the subgenus Dentigera is at least not completely unjustified.
Distribution: Egypt: Sakkara. Spain: Zamora (Warncke 1992 Characters: Small species with short, transversely oval head (female HL:HW 0.87). The most important diagnostic character of both sexes is the almost circularly expanded supraclypeal area, which merges flatly and broadly above into the frons. Setae on the clypeus are long, dense and upright. The propodeum is rounded, the basal area rimmed and has radially arranged fine longitudinal ribs, thus it merges into the terminal area in a rounded manner, as do the lateral areas. T1 very fine shagreen, shiny, with shallow fine sparse punctation. Male scape slender with pale apical spot; the mask white.
Male terminalia (Fig. 62); apical lobes of sternum 7 laced, at the outer edge with long fine bristles; sternum 8 short; the penis valves only slightly bent, the space between them narrow.

Discussion
The eponymous species of the Hylaeus brevicornis group is itself the biggest problem case in the group. To date, more than 30 synonyms have been assigned to the taxon. The current concept of the Nylander species was only recently stabilised by Erlandsson (1981), who designated a lectotype. One may well understand this "diversity" as a comfortable capitulation of the old authors in the face of the unusual difficulties posed by this species complex. The laudable exception is Förster (1871), who in turn went too far in his differentiation of taxa based on morphological details. Based on the rather small amount of material available to him, he could hardly evaluate intraspecific and interspecific variability, yet we nevertheless owe to him a relatively solid taxonomic basis. Most later authors have hardly exerted themselves to get to the bottom of the problem. The proposal that Hylaeus brevicornis should be treated as a collective species goes back to Alfken (1905), who, with clear criticism of Förster's meticulousness, placed no less than 16 names as synonyms of H. brevicornis [with "Prosopis atrata" he meant Hylaeus atratulus]. Meade-Waldo (1923) adopted his complete list of synonyms, with the addition of further names. Warncke (1972), who did not know the latter paper, also adopted the complete Alfken list, but subdivided the nominal species into four subspecies with more or less geographical reference. However, he later retreated from this position as not "gerecht" [just, but meaning probably justified]: only a single species was involved, he concluded (Warncke 1981: 158). He extended the list of "known synonyms" to about 30 names, not infrequently by an "applicable description" (sic!). Finally, Warncke (1992) described "Prosopis brevicornis" as a polymorphic species, which at most varies geographically. He even defended this view vehemently towards dissenting opinions with personal insults, which cannot be tolerated in scientific articles. Some previous authors were already much further ahead. Méhelÿ (1935) shows on his plate XIX clear differences in the terminalia of the brevicornis variants imparilis, pallidicornis and kahri. In the text (p. 152) kahri formally appears as a subspecies, but in the legends (p. 205) all four forms are handled as species, although he certainly did not mean to define their status in a strict taxonomic sense. Móczár (1960: 9.17 , 9.29 ) in Fauna Hungariae also listed imparilis, ambigua and kahri as separate taxa, but as varieties of brevicornis. Finally, de Beaumont (1958: 166) expressed a quite decisive viewpoint; he recognised three forms in his Swiss material that "most probably represent three species", Prosopis brevicornis s. str., P. kahri and P. gredleri. The estate of Blüthgen (1880Blüthgen ( -1967, which he bequeathed to the Museum für Naturkunde Berlin, also contains sketches from around 1958 that could have put an end to the lumping. Unfortunately, none of them appeared in print. However, his findings were included in the cautious modern revision by Dathe (1980a). The striking proof that Hylaeus brevicornis is a complex of species was provided by Schoder (2018). She found a unique situation on the site of Vienna's Nordbahnhof, which the designer of an experiment could not have arranged better to clearly disprove the notion of a single species: four (possibly even five) of the "synonyms" or "varieties" live syntopically side by side here, Hylaeus brevicornis, H. gredleri, H. intermedius and H. imparilis (possibly also H. kahri), and all of them could be reliably distinguished from each other genetically. The syntopic occurrence of five very similar species naturally also gives rise to the thought that we see a polymorphic species. A comparable situation exists with certainty also at other places; so Wilhelm Grünwaldt (in litt.) reported me already in the past sixties from his studies in Greece (Florina, Kalavryta). In fact, the intraspecific variation is comparatively large, so that overlaps can occur. This refers on the one hand to geographical variations, because quite a few species are widespread. They vary especially among the colonizers of islands, namely on the Aegean archipelago, where one can find just in some cases characteristic island forms. On the other hand, allometric relations play a role. In larger males, for example, the characteristic features such as expanded scapes, the ventral callosities on S3, and other sculpture characters are much more pronounced. However, Schoder suspects ambiguity in Hylaeus intermedius, for which she found two clearly separated groups whose barcodes appeared next to those of H. gredleri on the one hand, and next to H. kahri on the other. For the latter, she assumes another cryptic species. In my morphologically based opinion, her "intermedius 2" corresponds to the true H. kahri, which was not included in her program. After all, it would be the fifth species at that locality and thus the full spectrum -actually a sensation. All taxa considered proved to be well defined, which was confirmed in connected morphometric examinations. Our approach has been fully confirmed in these studies (Schmidt et al. 2015, Schoder 2018. Contrary to what Warncke (1992: 754) claimed, who assumed that intraspecific variability is the decisive factor "die typisch ist für die primitiven Prosopis-Arten", they are probably recently evolved forms that are still diversifying and spreading. This is indicated by their omnipresence in the western Palaearctic, their ecological adaptability and their high abundance in many places. Species development is obviously still in full swing, while morphological differences have not yet become pronounced. We can look forward to the results of the next phases of comprehensive gene sequencing.