Holotype worker: SLOVENIA: Mt. Velika Milanja | MSS | Volovja reber | Ilirska Bistrica, SLO | 45.593N, 14.313E, 1060 m | 23.05.2003, leg. S. Polak (MNHW, holotype no. CASENT0872099).

Paratypes: BULGARIA: 5 workers (CASENT0872100-CASENT0872104): Maleshevska Mt., Strumyani distr., Dobri Laki vill., 41.58484N, 22.98138E, 650 m, soil traps, along Lebnitsa river, beech and alder trees, 30.07.-20.08.2002, leg. S. Lazarov, T. Ljubomirov (BFUS); 1 worker (CASENT0872105): Belasitsa Mt., Petrch district, Belasitsa hut, 41.370N, 23.187E, 690 m, beech forest, 28.03.2009, leg. R. Bekchiev (BFUS); 15 workers (CASENT0872106-CASENT0872120): Belasitsa Mt., Petrch district, Kamena vill., 41.360N, 23.074E, 500 m, beech forest, along Kamenishka river, soil traps, June 2009, leg. R. Kostova; 02.05.2013, direct sampling, leg. A. Lapeva-Gjonova (BFUS, DBET); 6 workers (CASENT0872121-CASENT0872126): Slavyanka Mt., Sandanski district, Goleshovo vill., 41.42139N, 23.625N, 1094 m, 16.08.2014, leg. A. Lapeva-Gjonova (BFUS); CROATIA: 9 workers (CASENT0872127-CASENT0872135): Oltari, Mt. Senjsko bilo, 7 km NW of Krasno, 44.84604N, 15.00298E, 02.06.1992, leg. A. Schulz, K. Vock (DBET, PW); GREECE: 3 workers (CASENT0872136- CASENT0872138): [Macedonia] Kerkini Mts., Ano Poroia, 41.28563N, 23.03598E, 28.5.1984, V. Vohralik lgt. (PW, DBET); NORTH MACEDONIA: 4 workers (CASENT0872139-CASENT0872142): Golešnica Mts., 2 km S of Aldinci, 41.80189N, 21.42848E, 9.7.2010, 1420 m, V. Vohralik lgt. (DBET, PW); SLOVENIA: 1 worker (CASENT0872143): Mt. Velika Milanja, MSS, Volovja reber, Ilirska Bistrica, SLO, 45.593N, 14.313E, 1060 m, 23.05.2003, leg. S. Polak (DBET); 2 workers (CASENT0872144-CASENT0872145): Mt. Velika Milanja, MSS, Volovja reber, Ilirska Bistrica, SLO, 45.593N, 14.313E, 1060 m, 05.10.2018, leg. G. Bračko (BFUL).

The sculpture of head and mesosoma, head shape, scape length, and length of funicular segments place this species into the Aphaenogaster subterranea species group. Aphaenogaster illyrica differs from other members of this group in the combination of the following features: mesonotum clearly raised above the surface of pronotum, long and thin propodeal spines, as long as or longer than 0.7 length of the first segment of funiculus, elongated mesosoma, large body size (ML more than 1.64 mm, HW more than 1.02 mm), anterolateral sides of pronotum regularly convex, without setose angulations or tubercles, and yellowish brown to rusty brown body colour. In most of the other members of the group (i.e., A. lesbica Forel, 1913 from Lesbos, A. maculifrons Kiran & Aktaç, 2008 from the western Turkey, A. subterranea (Latreille, 1798)), pronotum and mesonotum form a regular convexity, without mesonotum raised above the surface of pronotum, propodeal spines are shorter, not longer than half length of the first segment of antennal funiculus, ML is less than 1.60 mm, and HW less than 1.0 mm.

Aphaenogaster illyrica most closely resembles A. graeca Schulz, 1994 from Mount Olympus (see Table 1) in morphometric data and general body shape. The new species differs form A. graeca in having a brighter and more uniform body colouration (yellowish brown to rusty brown vs. dark brown), weaker head sculpture, which fades laterad, less distinctly sculptured pronotum especially at sides, propodeum smooth and lacking longitudinal rugae on almost of whole lateral surface, and absence of long rugae at the base of the first gaster tergite (Figs 9–12).

Stout members of the A. splendida species group, i.e., A. festae Emery, 1915 and its relatives with the mesonotum raised clearly above the surface of pronotum, clearly differ in the yellowish body, short propodeal spines directed distinctly upwards, and elongate segments 2–4 of antennal funiculus, always 1.5 times or more longer than wide.

We also recognise several yet undescribed members of the A. subterranea group, which will be a subject for further, more advanced studies. Aphaenogaster illyrica is most similar to an undescribed species collected on the island of Cephalonia, especially in its long propodeal spines and mesonotum slightly raised above the surface of pronotum, but the undescribed form differs in having a distinctly microreticulated and dull dorsal and occipital parts of the head surface and dorsum of pronotum, as well as in the anterolateral corners of pronotum bearing setose tubercles.

Measurements: see Table 1.

Body colouration. Head, mesosoma, petiole and postpetiole yellowish brown to rusty brown, frons and area lateral of frontal carinae darker brown. Gaster from yellowish to mostly brown, first tergite yellowish anteriorly and yellowish brown posteriorly, but without distinct border between paler and darker parts, or completely brown. Mandibles yellowish-brown, legs yellow, antennal scapes ochraceous brown with yellowish apex, funiculus ochraceous-yellow (Figs 1, 2). Head. Approximately 1.2 times as long as wide, lateral margins in frontal view almost parallel behind eyes and evenly rounded at the posterior cephalic corners, posterior margin straight (Fig. 3). Anterior margin of clypeus shallowly emarginated. Eyes small, approximately 0.16 times as long as lateral margin of head, placed in the middle of lateral margin of head (Fig. 4). Scape approximately 1.2 times as long as head width, at base twice narrower than at apex, then gradually widened, without preapical constriction. Funiculus approximately 1.4 times as long as scape, first segment elongated, 2.6 times as long as wide at apex, 0.9 times as long as two subsequent segments combined, segments 2–6 short, 1.2–1.4 times as long as wide, segments 8–10 approximately 1.6 times as long as wide, last 4 segments forming an indistinct club, as long as basal funicular segments 1–7 combined. Mandibles elongate, with distinct striation and with some elongate punctures but shiny, masticatory margin with 7–9 teeth. Clypeus in the middle microreticulated, with short and thin median keel and few indistinct, longitudinal rugae, laterally with distinct longitudinal rugae. Frontal carinae moderately elongate, not reaching half-length of head, subparallel, frontal triangle with median keel and smooth laterally. Frons along the middle with single elevated keel, on sides with 2–3 longitudinal rugae, interspaces microreticulated, moderately shiny. Antennal cavities margined by regular, circular rugae. Central part of head dorsum between eyes with mostly sparse, partly longitudinal and partly irregular rugae, extending to 2/3 length of head, area between rugae microreticulated and moderately dull. Posterior part of head dorsum microreticulated, slightly dull, occiput smooth and shiny. Antennal scape with thin, longitudinal rugae. Mesosoma. Distinctly elongate. Promesonotum in dorsal view approximately 1.7 times as long as wide, pronotum strongly convex in profile. Anterolateral sides of pronotum convex, setose angulations or tubercles absent. Anterior part of mesonotum angulate or bituberculate, protruding distinctly above the level of posterior part of pronotum, thus promesonotal outline with distinct emargination in profile. Propodeum elongate, approximately 1.26 times as long as wide. Propodeal spines long, thin, at base only twice wider than at apex, acute apically, run only slightly upwards (Fig. 2). Dorsal part of pronotum with diffused microreticulation and only with median line smooth and shiny to mostly smooth and shiny, sides of pronotum with sparse, thin, mostly longitudinal rugae and diffused microreticulation between rugosities but appear shiny. Elevated part of mesonotum dorsally shiny with diffused microreticulation, laterally microreticulated with few rugae, posterior part of mesonotum rugose dorsally and granulate laterally (Fig. 2). Anterior surface of propodeum with short longitudinal rugae, laterally at least anteriorly smooth and shiny, posteriorly with few longitudinal rugae, dorsally with transverse or more or less longitudinal and around spiracles with irregular rugae. Petiole. Elongate with long peduncle, its anterior face deeply concave, node subangulate. Ventral margin of petiole in the middle straight, shallowly concave before apex, without spine or angulation. In dorsal view, petiole constricted at base then weakly divergent, almost parallel before petiolar node, then slightly globular. Base and ventral side distinctly microreticulated but without rugae, on sides and dorsally with diffused microreticulation to smooth and shiny, posterior faces with few rugae. Postpetiole. In lateral view rounded or slightly depressed at apex, in dorsal view approximately as long as wide with regularly rounded sides (Fig. 1). Base and ventral side distinctly microreticulated but without rugae, on sides and dorsally with diffused microreticulation to smooth and shiny, posterior faces with few rugae. Gaster. Shiny, with indistinct, diffused microreticulation, basal part of first tergite without or with very short longitudinal rugae. Setosity. Head in frontal view with short, light yellow, sparse setae. Entire dorsum of mesosoma and anterior margins of pronotum with sparse, short to moderately long, erect setae, the longest setae from shorter to approximately as long as propodeal spines. Petiolar node, postpetiole and gaster with short standing pilosity, the longest setae in large specimens shorter and in small specimens as long as propodeal spines.

Figures 1, 2. 

Aphaenogaster illyrica, holotype 1 dorsal 2 lateral. Scale bars: 1 mm.

Figures 3, 4. 

Aphaenogaster illyrica, holotype 3 head and antennae 4 head sculpture. Scale bars: 1 mm (3), 0.5 mm (4).

Figures 5, 6. 

Aphaenogaster illyrica, paratype from Kamena, Bulgaria 5 dorsal 6 lateral. Scale bars: 1 mm.

Figures 7, 8. 

Aphaenogaster illyrica, paratype from Kamena, Bulgaria 7 head and antennae 8 head sculpture. Scale bars: 1 mm (7), 0.5 mm (8).

Figures 9, 10. 

Aphaenogaster graeca, worker from Mt. Olympus 9 dorsal 10 lateral. Scale bars: 1 mm.

Figures 11, 12. 

Aphaenogaster graeca, worker from Mt. Olympus 11 head and antennae 12 head sculpture. Scale bars: 1 mm (11), 0.5 mm (12).

Figures 13, 14. 

Habitat of A. illyrica 13 locality Velika Milanja, Slovenia 14 locality Kamena, Bulgaria.

Unknown.

Variability within the geographic populations of the new species A. illyrica is mostly in size of propodeal spines and distinctness of microreticulation of head occiput, dorsal part of pronotum, and mesopleuron. Variability between geographically distant populations is more distinct but features overlap. Specimens from Slovenia (terra typica) have the stoutest head (largest HI) while in samples from Croatia and Bulgaria head is less stout. Microreticulation on the dorsal surface of the head and on the dorsal part of the pronotum in specimens from Slovenia and Croatia is more distinct than in those from Bulgaria, and similarly the northern populations have more distinct longitudinal rugae on the sides of the pronotum. In contrast, reticulation of mesopleuron in Bulgarian samples is distinct on the whole surface while in some specimens from Croatia and Slovenia reticulation of the mesopleuron is partly diffused. In specimens of similar sizes, the propodeal spines are shorter and directed more or less upwards in northern populations, while in Bulgarian populations they are longer and almost in the prolongation of the upper edge of the propodeum, not or very slightly directed upwards.

Named after Illyria, a historical region in the western part of the Balkan Peninsula inhabited by the Illyrians and the ancient Roman Prefecture of Illyricum. All localities of Aphaenogaster illyrica are within the area of this region.

All known records of Aphaenogaster illyrica are restricted to the mountainous areas of the Balkan Peninsula, from the altitudes of 500 m to 1420 m a.s.l. Its range stretches from the Dinaric Alps in southern Slovenia and western Croatia to Osogovo-Belasica Massif in southwestern Bulgaria and the adjacent Kerkini Mts. in Greece and to Golešnica Mt. in North Macedonia. This distribution area is much larger compared to the area of the sister species Aphaenogaster graeca, whose distribution range is limited to the massif of Mount Olympus and adjacent mountain ranges (Fig. 15).

Figure 15. 

Distribution of Aphaenogaster illyrica (red circles) and Aphaenogaster graeca (black circles).

Details on the new species habitat are available only from the Bulgarian and Slovenian records. In Bulgaria, A. illyrica was mostly collected in beech forests in wet sites, close to streams, on silicate (Belasitsa and Maleshevska Mts.) and limestone (Slavyanka Mt.) rocks. This differs quite dramatically from the Slovenian site, where the ants were found in a large karstic depression (karstic doline) situated in the sub-montane karst grassland, party covered with sparse trees and shrubs. This area is characterised by harsh winters and relatively wet summers. Due to the strong and almost permanent winds, the upper part of the soil is often dry. The specimens collected in 2003 were found in subterranean pitfall traps set in soil at the depth of 30–50 cm among the limestone rocks in the so-called Superficial Subterranean Habitat (SSH) or “Milieu Souterrain Superficiel” (MSS), as originally described (Juberthie et al. 1980; 1981). SSH is a hypogean environment, generally formed by the fragmentation of the bedrock and accumulation of debris, which contains a wide network of air-filled epikartsic spaces, small voids and fissures (Culver and Pipan 2009; Giachino and Vailati 2010) and represents a transition zone between surface soils and deeper subterranean habitats such as caves (Culver and Pipan 2009). A presence of a rare species A. cardenai Espadaler, 1981, was already reported from SSH in the Iberian Peninsula (Ortuño et al. 2014). In 2018, we found few scattered workers at the same site while digging in the stony ground to the depth of approximately 50 cm.

Aphaenogaster illyrica can be characterised as a ground-dwelling species. The records of A. illyrica well above 1000 m a.s.l. or those from beech forests at lower altitudes indicate that it tolerates lower temperatures, which is relatively rare in other species of the genus.

Comments. Recently published papers (Borowiec and Salata 2017, Alicata and Schifani 2019) indicate that the A. subterranea group is very diverse and comprise several undescribed taxa. The Balkans appears to be the most species-rich region and is in need of further investigation. Results presented in this publication are a preliminary attempt to systematise our knowledge about this group, and Aphaenogaster illyrica and A. graeca compose a distinct complex within the A. subterranea group. Therefore, we decided to describe the new species in a separate paper. Other undescribed forms, mentioned in the publication, will be a subject of further study. Because the A. subterranea group consists of mixture of species of uncertain taxonomic status and several undescribed morphotaxa, we can provide only a generic key to the subterranea group with features focused on the graeca complex.