Two days of collecting (9^th August and 31^st October 2024) revealed a remarkably large population of the alpine carabid species Nebria casalei Giachino, 2013 (in our opinion belonging to Nebria cordicollis sstr.) in the exceptionally cool talus above the open blockfield area where D. sonjae sp. nov. is most common (Fig. 24). At the north face’s rocky edge the common Dichotrachelus rudeni Stierlin, 1853 was sifted from mosses. Trechus ceresai Binaghi, 1938 was rare in this environment, which consists of interspersed instable granite and is therefore not accommodative to cavity dwellers as this Trechus endemic to the northern ridge of Valle Grande eastwards to Monte Zeda (leg. Szallies). Notably, no Staphylinids were found and thus most strikingly not a single Leptusa specimen could be collected. In the vicinity between Toce and Lago Maggiore. Leptusa species seem to occur only on Monte Gridone in Switzerland, where Leptusa monacha (Fauvel, 1898), Leptusa monguzzii Pace, 1989 and Leptusa pilatensis Scheerpeltz, 1972 (nominate form from CH) could be collected (leg. Szallies), and the first two species were also recorded from Cima Laurasca (leg. Monguzzi, i.l.). Monte Gridone, on the other side, harbors Trechus piazzolii Focarile, 1950, the vicariant of T. ceresai, suggesting a different origin of the fauna of the Eastern Gridone aerea than that of the Western Toce region. As the widespread D. rudeni, D. lepontinus also occurs on Monte Gridone, westwards to Cima Laurasca. Apparently, no D. lepontinus was ever recorded from Valle Grande’s Togano ridge area (not mentioned by Giachino 2013), where D. sonjae sp. nov. was discovered.

Figures 24–27. 

24, 25. Habitat of D. sonjae in the scree slope north of Pizzo Nona at nearly 2000 m a.s.l. (corresponds to the line ending with the rock face above). The extraordinary coldness of this place is due to the extra cavity carved into the north face of Pizzo Nona, that can be seen in Fig. 24 above the middle in the picture, thus the habitat is deprived from direct sunlight most of the year; 26, 27. Last and first larval stages of D. sonjae collected in moss cushions growing on granite stones in the scree slope (Photos: C. Germann).

The distribution of the high alpine species Nebria cordicollis in the mountains of Val Grande may also be explained by its colonialization from the Gridone area. There it occurs in the talus of scree slopes below 2000 m a.s.l. as well as in the near-by mountain range to the north between rivers Toce and Maggia, where N. cordicollis ominously is rare, difficult to collect and thus neglected. Adults of N. cordicollis of Val Grande can be found in the watersprings from underground talus scree bodies and therefore they were assumed to belong to some cold stenothermous species of running water (Giachino 2013). Certainly, the occurrence of N. cordicollis in the Val Grande ridge is a glacial relic phenomenon as with the closely related Nebria heeri Daniel, 1909, which lives as glacial relic species in the Black Forest scree slopes even below 1000 m a.s.l. (Huber and Molenda 2004; Szallies and Huber 2013). In this environment the hygrophilous adults are totally absent at the surface in the night, when they are active. Similar seems to be the situation with N. cordicollis in suboptimal habitats on Cima Laurasca or Monte Zeda.

Our results suggest D. sonjae sp. nov. to be the sister taxon of D. lepontinus. Both share, quite unique among Dichotrachelus, the duplex lobal spurs on all their tibiae as well as a heavily sclerotized basal part of the penis with an adjacent distinct ostium with delineated basal borders. The similar D. meregallii, which shares the trait of duplex spurs of tibiae, shows an unsclerotized dorsal surface of the basal region of the aedeagus, hollow inside and thus lacks a true ostium, similar to most Dichotrachelus species of the Alps. Instead, these species possess a faintly sclerotized sheath covering the cavity of the dorsal surface of the aedeagus that is truncated to form a blunt ended orifice.

The duplex spurs at the base of tibiae (as well as the extra large single spur on the metatibiae of D. meregallii) might endow its carriers with an extra advantage to move and live in pure rocky habitats such as scree slopes and steep cliffs, while the common and more widespread species with small spurs regularly occur in less rocky habitats. There seems also some sexual selection involved, for the males having a bit more prominent spurs in all three species considered. Accordingly, tibiae of these three species with clear preference of extremely rocky habitats are longer than in the common ubiquitous species substituting them in less rocky habitats, as D. rudeni or D. stierlini. Most likely it is the humid microclimate in scree slopes that renders these extremely stony habitats particularly good habitats for Dichotrachelus species because their feeding plants, mosses, proliferate very well there.

Another species preferring rocky habitats in a high alpine environment is the already mentioned D. augusti sharing the slender and long tibiae, with short, singular, but very pointed spurs on its tibiae. Different populations of D. augusti and D. meregallii occur besides each other scattered in the wider Grand St. Bernard area north of Aosta (Germann 2011), while in the core Monte Rosa area (I and CH) only D. augusti was found and D. lepontinus solely in the lower regions towards the Toce valley. The population of D. sonjae sp. nov. in the exceptionally cold scree slope of the north face of Pizzo Nona of Valle Grande might be a singular relict one. It is not unlikely, however, that further populations exist in the core Monte Rosa area or in more distant places as the isolated population of D. meregallii in the Ortles region might imply.

Genetic data confirm the close relationship of D. sonjae sp. nov. and D. lepontinus, with only 6% divergence between them, whilst all the other Dichotrachelus, including D. meregallii exhibit up to 20% difference towards each other, leaving little space for further chance to accumulate and thus maybe obscuring the discernment of actual phylogenetic lineages. Remarkably, only D. rudeni and D. imhoffi exhibit a similar close relationship, with only 5% sequence divergence (Meregalli et al. 2018).

It is puzzling to state that the separation of two disjunct populations of D. meregallii (Fig. 28) must be considered a most recent event (about 2.5% divergence according to Meregalli et al. 2021), whilst the separation of D. lepontinus and D. sonjae sp. nov. must have happened considerably before. The huge spatial distributional gap between populations of D. meregallii may be due (at least in part) to the competition of D. lepontinus and D. sonjae sp. nov. in between, as well as of D. augusti, an additional species capable to compete with the other three species (personal observations). In its eastern distribution area, D. meregallii predominantly lives in the high alpine moraine block fields of the large glaciers of the Ortler system, but is also present on high altitude ridges and cliffs, above 3000 m a.s.l. In blockfields around 2500 m a.s.l. and on more grassy terrain, D. stierlini dominates everywhere and Holdhaus (1954) outlined that he had found the species in large numbers in Martelltal even above 3000 m a.s.l. It is noteworthy then, that the fortunate absence of D. stierlini from the Sulden Valley allowed for the discovery of D. meregallii by Gert Müller in the shallow debris fields there, an otherwise precluded biotope dominated by D. stierlini. If there is any species that might be eligible for a nunatak survivor species, D. meregallii should be considered prime candidate, since the Ortler region must be considered one of the most devastated and most poorly recolonized of the Alps with no native fauna (Holdhaus 1954). Closer investigation of the Ortler system, however, has revealed a more nuanced picture, with a record of Trechus tristiculus Daniel, 1898 in the Serottini range and the nearby Val Carrè. It appears likely, that the Ortler system did in fact harbor a low altitudinal zone of general survival, a massif de refuge, most likely north of Edolo, where D. meregallii could as well have persisted the last glacial maximum alongside this Trechus, that has its next populations south of the Adamello group and in the Brenta. The population of D. meregallii in the West (Fig. 28) might have persisted the glaciation in the region of the lower river La Iserè, from which it might have reached all present localities within the same time, in particular the locality near By (Aosta) via the low pass of Petit Bernard.

Figure 28. 

Distribution of D. lepontinus (blue dots), D. meregallii (green dots) – note the strong disjunction of populations – and the single locality of D. sonjae (red dot) based on our studied specimens (Suppl. material 1; map simplemapper).

All former reasoning, however, does in no way account for the vast gap between the two populations of D. meregallii in a relatively short time frame and the filling of the gap with a complex variety of populations of high alpine species competing for the same extreme habitat. The current situation of knowledge about these Dichotrachelus species seems to warrant Winston Churchill’s famous dictum from 1939 of the riddle, wrapped in a mystery, inside an enigma. Further research on high alpine beetles and their biogeography and phylogeny is urgently needed.

Dichotrachelus sonjae sp. nov. adds another one to the short list of endemic species between river Toce and Lago Maggiore: the Carabidae Amara constantini Binaghi, 1946 and Trechus ceresai (Taglianti et al. 1998).