Ant genus Strongylognathus (Hymenoptera, Formicidae) in Bulgaria: a preliminary review

Abstract Background Strongylognathus Mayr, 1853 is a Palaearctic genus, comprising 25 ant species and one subspecies, all permanent social parasites, infesting colonies of various species of Tetramorium Mayr, 1855. They have patchy distribution throughout their areas and most of them are very rare and listed as vulnerable. The taxonomy of the Strongylognathus huberi group needs thorough revision and the results presented below can be considered as preliminary. New information Four species of the socially parasitic ant genus Strongylognathus (S. karawajewi Pisarski, 1966, S. huberi dalmaticus Baroni Urbani, 1969, S. afer Emery, 1884 and S. italicus Finzi, 1924) are recorded for the first time from Bulgaria and, together with the previously-known S. testaceus and S. bulgaricus stat. rev., their total number reaches six. The taxonomic position and geographic distribution of all species are discussed and a Key for their identification, based on worker caste, is compiled.


Introduction
. Thereby, the currently-known Strongylognathus species to the country has increased to six. Their taxonomic position and a Key for the identification of Bulgarian Strongylognathus species, based on the worker caste, are provided.
The taxonomy of the Strongylognathus huberi group needs thorough revision and it is simply impossible to unambiguously identify most of the West Palaearctic species and the results presented below can be considered as preliminary.

Materials and methods
Strongylognathus species were collected during a myrmecological survey in Bulgaria by the first co-author of the paper (ALG). In recent years, special attention has been paid to the southern regions of the country (Eastern Rhodopes, Thracian plain, Strandzha, Slavyanka and Maleshevska Mountains), where the ant fauna is most diverse, but has not been properly investigated yet in detail. Collected material is preserved at the Biological Faculty, University of Sofia, Bulgaria (BFUS). The examined type specimens of S. karawajewi Pisarski, 1966 are preserved in the collections of the Schmalhausen Institute of Zoology NAS of Ukraine, Kiev (SIZK) and the Museum and Institute of Zoology PAS, Warsaw (MIIZ). All Tetramorium nests, infested by Strongylognathus, were located in the ground under stones. Photos of some collection sites are shown in Fig. 1. Since there are no modern keys for identification of the European Strongylognathus, we used data from various publications for their identification (Pisarski 1966, Baroni Urbani 1969, Radchenko 1985, Radchenko 1991, Dlussky et al. 1990, Sanetra et al. 1999, Sanetra and Güsten 2001, Schulz and Sanetra 2002, Borowiec and Salata 2013, Seifert 2018, comparative material from SIZK, MIIZ, Zoological Museum of the Moscow State University, Zoological Institute RAS, St-Petersburg and Petr Werner's collection, Prague, as well as the original description of all taxa and images of the type and non-type specimens on the AntWeb (2021) website. The specimens for scanning electron microscopy (SEM) were gold-coated in a vacuum unit and then images were taken using the microscope LYRA/TESCAN 5007, operating at 10 kV.
Six measurements of specimens (accurate to 0.01 mm) were taken and used to calculate four indices:  Strongylognathus karawajewi has been known as a social parasite of T. caespitum (s. l.), T. sulcinode Santschi, 1927, T. inerme Mayr, 1877, T. ferox Ruzsky, 1903 and T. feroxoide Dlussky & Zabelin, 1985(Pisarski 1966, Radchenko 1991, Radchenko 2016 and we found it in the nests of T. hungaricum and T. chefketi. This species inhabits extremely xerothermic sites in Bulgaria at an altitude below 660 m. One of them is located in the Besaparski Hills in the Thracian plain -low calcareous ridges with typical steppe-like vegetation (Fig. 1a), one in the southern foothills of the Stara Planina Mts. (Fig. 1b) and five of them in the Eastern Rhodopes (Fig. 1c). The Eastern Rhodopes are quite low with an average altitude of 320 m and with hilly slopes. The climate in this area is mild continental-Mediterranean with an average annual temperature 12°C.
Two species of the testaceus species-group ( S. testaceus and S. karawajewi) are known from the West Palaearctic and their separation is usually straightforward. The head dorsum in workers of S. karawajewi is usually completely smooth and shiny, fine striation may be present only on its sides, while at least frons and genae, but often whole head dorsum, is with well developed longitudinal rugosity in S. testaceus. The sculpture on the sides of mesosoma in males of S. karawajewi is strongly reduced, but it is at least partly coarsely rugulose and shagreened in S. testaceus (Radchenko 1991, Radchenko 2016).

Taxon discussion
The previous records of the species in Bulgaria are from northern Bulgaria -Veliko Tarnovo, Preobrazhenski Monastery (10 km from Veliko Tarnovo), Dryanovo, Veliki Preslav and one (Silistar) is on the southern Black Sea coast (Viehmeyer 1922, Atanassov andDlussky 1992, Lapeva-Gjonova and Kiran 2012). All northern Bulgarian sites are located in the Predbalkan geographic region, in a hilly and lowland area with an average altitude of 360 m. Viehmeyer (1922) described S. huberi subsp. rehbinderi var. bulgaricus, based on all three castes from Veliko Tarnovo (northern Bulgaria), but this name is unavailable (quadrinomen). Pisarski (1966) used the first available name for this species, S. rehbinderi subsp. bulgaricus and considered it as a senior synonym of S. kratochvili Šilhavý, 1937, but the latter name has priority. It was later recorded for the country under the name S. bulgaricus by Atanassov and Dlussky (1992) and under the name S. kratochvili by Lapeva-Gjonova et al. (2010).
Recently, Seifert (2018) noted, without comments, that S. bulgaricus is not a synonym of S. kratochvili, but is a junior synonym of S. christophi. In our opinion, the proposed synonymy seems doubtful: the sculpture on the head dorsum in S. christophi is much coarser, the head is relatively shorter (CI ≤ 1.10), the antennal scape is longer (SI > 0.70), the petiolar node with widely rounded dorsum and the propodeal dents are directed mostly backwards at an angle of ca. 45 . On the contrary, the sculpture on the head dorsum in S. bulgaricus is strongly reduced, its head is relatively longer (CI > 1.16), the antennal scape is shorter (SI < 0.70), the petiolar node with much more narrowly rounded dorsum and the propodeal dents are directed almost upwards (Table  1 and  the propodeal dents etc.), but S. kratochvili differs from S. bulgaricus by noticeably larger body size (it is one of the largest Strongylognathus species, as Šilhavý has already emphasised). We agree with the proposed separation of these species (see Seifert 2018), but consider S. bulgaricus (at least tentatively) a good species.
It is no coincidence that this species is found in the Eastern Rhodopes, where the influence of the warmer Mediterranean climate is stronger and xerothermic plant communities are present. The collecting site near the village of Meden Buk is located in the valley of the Byala Reka River near the Greek border and it is one of the southernmost points of Bulgaria.

Taxon discussion
Finzi (1924a) described S. italicus, based on a single queen from the Island of Elba and then Sanetra et al. (1999) recorded it from the same Island and from Italian mainland (Florence Province), but unfortunately, they did not indicate whether workers of this species were found.
In the same year, Finzi (1924b) described S. alboini, based on workers from Mt. Nanos (now Slovenia) and later, Baroni Urbani (1969) re-described its workers and described queens and males, based on the material collected by Kutter in southern Switzerland (Roveredo, Canton Cicino). Recently, Seifert (2018) compared morphometrically and subjectively the holotype queen of S. italicus with the queens of S. alboinii from Roveredo and concluded that they belong to the same species; in addition, workers of S. italicus from Roveredo are identical to syntype workers of S. alboini from Mt. Nanos. Consequently, he considered S. alboinii as junior synonym of S. italicus.
Strongylognathus italicus differs from other Bulgarian species of the huberi-group by the coarser sculpture on the head dorsum and somewhat longer antennal scape. In Bulgaria, it was found only once on a southern slope of xerothermic grassland situated in an oak forest (Fig. 1c) at an altitude of ca. 650 m in a nest of Tetramorium chefketi. It is interesting to note that, in the same site, we found S. karawajewi and the very rare social parasite of Tetramorium -Teleutomyrmex buschingeri Lapeva-Gjonova, 2017. Strongylognathus italicus Finzi, 1924, workers: a -head, dorsal view;b -body, lateral view.

Taxon discussion
Strongylognathus afer was described by Emery (1884), based on a single queen from Algeria, workers and males being later described from Algeria and Tunisia by Forel (1900) and Santschi (1910), respectively; finally, Sanetra and Güsten (2001) recorded this species in many localities in Algeria, Tunisia and Morocco. Strongylognathus afer, Figure 7.
Strongylognathus afer Emery, 1884, workers: a. head, dorsal view;b. body, lateral view. in all studied sites, infested colonies of Tetramorium semilaeve Andre, 1883 (Sanetra and Güsten 2001). Workers of this species are very small, the propodeum bearing small and blunt tubercles instead of sharp dents and the head dorsum and mesosoma are generally smooth and shiny (Forel 1900, Sanetra andGüsten 2001).
One nest sample of workers, together with the host species T. hungaricum, was collected in the Eastern Rhodopes on the border of light oak forest and a pasture with a southern exposure (Fig. 4) at an altitude about 550 m. Collected workers morphologically fit well with the main characteristic features of S. afer, but are even smaller than the specimens from Algeria and Morocco, as well as workers of S. minutus Radchenko, 1991 and, apparently, are the smallest known workers of the huberi species-group (compare Table 1

Key for identification of Strongylognathus species of Bulgaria (workers)
1 Occipital margin of head strongly concave (seen from above), posterio-lateral corners of head strongly prominent (seen from sides) ( Fig. 2a and Fig. 3a) 2 -Occipital margin of head straight or, at most, very shallowly concave (seen from above), posterio-lateral corners of head rounded and not prominent (seen from sides) (Fig. 4a, Fig. 5a, Fig. 6a and Fig. 7a) 3 2 Whole head dorsum usually smooth and shiny, fine striation may be present only on the sides of head dorsum (Fig. 3a) S. karawajewi -At least frons and genae (often whole head dorsum) with well developed longitudinal rugosity (Fig. 2a)

Discussion
The record of S. karawajewi in Bulgaria is quite consistent with the zoogeographic data and today represents the westernmost edge of the range of this East Tethyan species. At first glance, it may seem that the finds of the north-west African or west Mediterranean species (e.g. S. afer and S. italicus) in Bulgaria are unlikely, but this is not entirely true. As mentioned above, at present the taxonomic situation in the huberi species-group, especially in the West Palaearctic (i.e. west of Yenisei River and the Tien Shan Mts.; see Radchenko and Elmes 2010), is very complicated, often rather confusing and most of the problems have not yet been finally resolved and require thorough revision.
Although S. afer is formally recorded only from the north-western Africa and its relationships with three other Iberian and Italian species is not fully resolved, their conspecificity appears quite possible. Sanetra and Güsten (2001) have shown that the difference between the holotype queen of S. caeciliae Forel, 1897 from the Iberian Peninsula and Algerian queens of S. afer is the same as the difference observed between samples of the latter species collected in Algeria and Tunisia; Italian S. destefanii Emery, 1915 obviously differs from S. afer only by its somewhat larger size; finally, S. insularis Baroni Urbani, 1968 from Malta almost certainly should be a synonym of S. afer and/or S. destefanii. A somewhat similar situation applies to S. italicus as indicated by Seifert (2018).
Previously, Bolton (1976) on p. 305 wrote: "… many of the species-level names in the genus may merely be localized populations and I am convinced that further collections and study will reduce the number of species in Strongylognathus to a much lower figure". We can agree with this opinion and many modern nominal species in the end may turn out to be only separate populations of widespread species, as, for example, S. afer or S. italicus (or its putative senior synonyms).
In addition, it should be emphasised that Strongylognathus fauna is very poorly understood in the former Yugoslavian countries and this territory appears a "blind spot" between Italy on the west and Bulgaria in the east. Thus (excluding S. alboini and S. huberi dalmaticus with the type localities in Slovenia and Croatia), only one more species, the common S. testaceus, was recorded from Serbia, Croatia and Slovenia (Petrov and Collingwood 1992, Bračko 2006, Bračko 2007, Vesnić 2013; similarly, four Strongylognathus species were previously recorded for Greece (Buschinger 1989Legakis 2011, Borowiec and Salata 2012, Borowiec and Salata 2013, but this number was recently reduced to three (Salata and Borowiec 2018). Considering that about ten species are known now in Italy, six in Bulgaria, five in Ukraine (Radchenko 2016) and eight in Turkey (Kiran et al. 2014, Kiran andKaraman 2020), the number of Balkan Strongylognathus species is definitely underestimated and further research may close the "blind spot" between Italy and Bulgaria.