An updated checklist of ants (Hymenoptera, Formicidae) of Bulgaria, after 130 years of research

Abstract Background The Bulgarian myrmecofauna is one of the richest in the Balkans. This is a result of both the physicogeographical and paleoecological features of the area, as well as relatively well-studied fauna. The earliest myrmecological paper on Bulgarian fauna, listing 54 species, was published 130 years ago. The publication was later followed by numerous new faunistic records and three comprehensive reviews that significantly widened knowledge on the ant diversity from this country. The most recent checklist was released 12 years ago and considered 163 ant species from 40 genera. New information This work provides an updated checklist of 195 ant species from 43 genera occurring in Bulgaria. Since the last Bulgarian catalogue of ants, 44 species have been added, while 24 species have been synonymised or excluded after critical analysis of the last taxonomic revisions. Additionally, we discuss the status and distribution of 12 species described from Bulgaria, 23 species considered endemic and subendemic for this country, 19 species with conservation status and four non-native species.


Introduction
Bulgaria is amongst the Balkan countries with the richest ant fauna. There are several factors that favour the existence of more than 190 ant species. The country is located in the south-eastern part of the Balkan Peninsula, considered as an important hotspot of biodiversity in Europe, with 96 types of habitats referring to three biogeographical regions -Black Sea, Continental and Alpine (European Environment Agency 2022). The Balkans act as a connecting corridor between Europe and Asia. Due to its geographic location and paleoecological events, two major zoogeographical complexes can be distinguishednorthern (Holarctic-Eurosiberian) complex of cold-tolerant species and southern (Mediterranean-Central Asian) complex of thermophilic species (Hubenov 2008). The latter one includes a limited number of steppe elements (NW and NE Bulgaria), Anatolian and Iranian migrants (SE Bulgaria) and Pontian elements (eastern Bulgaria). In addition, the Bulgarian fauna includes a number of endemic and subendemic species and few exotic species. The high number of ant species (in comparison with other Balkan countries) is also due to the numerous studies on the Bulgarian myrmecofauna conducted in the last decade (see below).
The earliest paper on the myrmecofauna of Bulgaria was published 130 years ago, when Auguste Forel , a Swiss myrmecologist, recorded 54 ant species from various regions of the country and described three species as new to science (Forel 1892). Later, three other comprehensive reviews of the ant fauna in Bulgaria, made by Agosti and Collingwood (1987), Atanassov and Dlusskij (1992) and Lapeva-Gjonova et al. (2010), enriched knowledge on biodiversity of this country. In the list of the Balkan ants, Agosti and Collingwood (1987), based on literature and collection data, reported 112 species for Bulgaria. Exactly 100 years after the publication of the first paper on the ants of Bulgaria, Atanassov and Dlusskij (1992) presented data on the taxonomy, distribution and ecology of 111 ant species from 36 genera and four subfamilies, with identification keys to all taxa. The most recent review of 163 ant species from 40 genera (Lapeva-Gjonova et al. 2010) was prepared mainly based on published records and updated taxonomic status of taxa listed in papers preceding its publication.
Since the publication of the most recent catalogue, 44 more species have been added to the list. Some of them are new faunistic findings for the country, while others are new species mentioned for Bulgaria in taxonomic works covering also the Balkan myrmecofauna (e.g. Seifert 2012, Csősz et al. 2014, Seifert and Csősz 2015, Seifert 2016, Seifert and Galkowski 2016, Wagner et al. 2017, Steiner et al. 2018, Bračko et al. 2019, Seifert 2020. The high species diversity in the Balkan Peninsula is of considerable importance and has great conservation value as recognised by its hotspot status (Hewitt 2011). In recent years, the most significant progress in the study of the Balkan ant fauna has been made on Greek ants. Data on over 300 species (Salata and Borowiec 2018a), including their distribution and ecology, have been established. Additionally, a number of taxonomic revisions on specific groups of species and genera have been carried out. Important additions to the regional ant fauna of the Balkans were also made for Slovenia, Montenegro and the Republic of North Macedonia (Bračko et al. 2014a, Bračko et al. 2014b, Bračko et al. 2016).
The updated list of Bulgarian ants in the present study brings together the scattered information from numerous taxonomic and faunistic publications, justifies exclusion of some dubious and erroneous records and highlights the importance of such inventories for assessment and conservation of biological diversity.

Materials and methods
The current checklist is based on the available taxonomic and faunistic literature concerning the Bulgarian myrmecofauna. Publications since the last Bulgarian catalogue of ants (Lapeva-Gjonova et al. 2010) till recently are considered. We make critical reviews on the taxonomic data on some species.
The genera in the list are arranged by subfamilies and tribes. The species are listed alphabetically and by subgenera (if available) as their actual names are generally agreed with the Online catalogue of the ants of the world by Bolton (2022) and the most recent publications. The changes in taxon names proposed by Ward et al. (2015) for social parasitic genera Anergates, Chalepoxenus, Myrmoxenus and Teleutomyrmex were not taken into account, based on ongoing discussions and arguments to maintain stability in names , Kiran et al. 2017. The excluded species from the last catalogue and subsequent articles are justified by relevant studies. The following abbreviations for the conservation status according to the IUCN Red List of Threatened Species (IUCN 2022), if any, have been used: Vulnerable (VU), Near Threatened (NT), Lower Risk (LR), Least Concern (LC) and Bulgarian Biodiversity Act (BBA). In the Notes section after the current species name, only the very first report for Bulgaria is given and if the species is endemic or subendemic.

Discussion
The current checklist contains 195 species of ants from Bulgaria belonging to six subfamilies and 43 genera. This places Bulgaria amongst the European countries with the highest richness of ant species after Greece (315), Spain (275), Italy (267) and France (215), despite its significantly smaller area (Janicki et al. 2016, Guénard et al. 2017, Salata and Borowiec 2018a,Schifani 2022. The distribution of species by subfamilies and genera is typical of European myrmecofauna. The richest in genera and species is the subfamily Myrmicinae, containing 23 genera and 106 species, followed by the subfamily Formicinae with 10 genera and 73 species. Thus, the two subfamilies represent 92% of the myrmecofauna in Bulgaria. The most speciose ant genera are Temnothorax (Myrmicinae) and Lasius (Formicinae) with 27 and 24 species, respectively. More than 10 species are also represented by Formica (18), Camponotus (16), Myrmica (15) and Tetramorium (11). Out of all the 43 genera, 26 contain one or two species only.
In this study, records for 24 previously reported species have been re-assessed following taxonomic revisions or reconsideration of available material. The list of excluded species from the current list with remarks and references is given in Table 1.

2015)
Temnothorax saxonicus (Seifert, 1995) (2000) checklist and in the IUCN Red List (except for Formica truncorum, which is absent from the latter). Additionally, Formica rufa is listed in Annexes 2 and 3 of the Bulgarian Biodiversity Act (2002) as protected on the entire Bulgarian territory. The vulnerable species are not currently endangered, but are in a high risk of endangerment in the near future due to threats to natural habitats, declining population, restriction in their area of occupancy or the number of locations. A recently published monitoring of the red wood ants in Bulgaria discussed the status of some of their populations (Antonova and Marinov 2021). However, further research is needed to study in more detail their population dynamics and threats.
An up-to-date assessment of the conservation status of the regional myrmecofauna is needed to reflect both status and taxonomic changes. Thus, potential candidates, such as Strongylognathus bulgaricus, S. huberi dalmaticus and Teleutomyrmex buschingeri, remain off the list for now.
So far, the presence of exotic ant species in Bulgaria is relatively low. These are four species -Linepithema humile, Lasius neglectus, Monomorium pharaonis and Hypoponera punctatissima. All of them are introduced, synanthrope species as L. humile and M. pharaonis are known only indoors and from greenhouses, while H. punctatissima may be found also outdoors in southern parts of Bulgaria (Atanassov and Dlusskij 1992). Lasius neglectus is an invasive urban species, but recently, its colonies have been declining in the country (Tartally et al. 2016).
Ant research in Bulgaria dates back to 1892, has continued with variable intensity over the decades and has resulted in 195 species at present (Fig. 1). After Forel's foundational paper with 54 species, more crucial progress in Bulgarian ant research occurred after World War II and with the work of Neno Atanasov. After the 1980s, a number of foreign scientists also contributed to the progress in myrmecological studies. During the last decade, important taxonomic revisions (which included materials from Bulgaria), as well as more intensive research in the southern territories of the country, led to a significant increase in the number of known species in Bulgaria, including descriptions of new ones. However, the number is expected to increase with upcoming surveys and taxonomic revisions. Number of newly-reported ant species from Bulgaria by decades (only currently valid species are considered).