A key for the determination of European species of Eosentomon Berlese, 1909 (Protura, Eosentomata, Eosentomidae)

Abstract European species of Eosentomon are examined. A taxonomic key to identification of 61 Eosentomon species is provided based on body chaetotaxy, shape, and position of sensilla on the foretarsus and shape of sensilla on the maxillary palps. Biogeographically, 13 of the known European Eosentomon species are known only from their type localities.

A key for the identification of European Eosentomon species was created by Nosek (1973) for 14 species, in which the author differentiated species into four groups according to the shape of female squama genitalis. This approach followed Tuxen (1960), who divided worldwide Eosentomon into 11 groups based on the squama genitalis. Szeptycki (1984Szeptycki ( , 1985aSzeptycki ( , 1985bSzeptycki ( , 1986) wrote a series of papers with keys to identification of four groups of Polish Eosentomon species, in which cephalic chaetotaxy was used for group separation (Szeptycki, 1986). Both Nosek and Szeptycki frequently used in their keys the shape of the female squama genitalis, which made identification of males and young stages impossible except by association. The present paper contains an identification key to European Eosentomon species based primarily on chaetotaxy, shape, and position of sensilla on the foretarsus and the shape of sensilla on the maxillary palpi.

Materials and methods
Type materials were examined of 31 Eosentomon species deposited in the collection of Prof. Szeptycki in the Institute of Systematics and Evolution of Animals PAS, eight Eosentomon species in the collection of J. Rusek in the Institute of Soil Biology BCCAS and one species deposited in the collection the State Museum of Natural History NASU. Information about the taxonomy of other Eosentomon species was taken from original descriptions or redescriptions of type materials in Tuxen (1964), Nosek (1973) and various other papers. Head chaetotaxy is labelled as in Szeptycki (1984), and body chaetotaxy is labeled according to Bernard (1990).

Results and discussion
Taxonomic characters used in the key are present in juvenile stages as well as the adults. The shapes of the parts of the adult female squama genitalis may have great phylogenetic value and can serve as additional characters for identification of species. The characters used in the key, such as shape of maxillary palpi, chaetotaxy of the head, shape and position of sensilla on the foretarsus and position of seta P1a on tergite VII are stable from the second juvenile stage (larva I) (Nosek 1973, Szeptycki 1965b. All setae on the notal tergites and on the abdominal segments are present from the maturus junior stage (Imadaté 1965).
Analysis of the geographical distribution of European Eosentomon shows that nearly all species have been collected only in Europe, except for two (E. delicatum and E. mixtum) that have also been recorded from northern Africa. The majority of the species have been recorded only from Central Europe, probably due to many years of active work in this region by Josef Nosek, Josef Rusek and Andrzej Szeptycki. Sixteen species are known from Western Europe and only two species have been reported from Eastern Europe (Table 1). Nineteen species occur in Southern Europe and only two species have been noted from Northern Europe. If intensity of collection and study is correlated with number of recognized species then there are more species yet to be discovered in Europe. Thirteen of the 61 species are known only from their type localities: therefore, it is difficult to assert endemicity within such a poorly studied group of microarthropods. Nevertheless, two species could be endemics of the East Carpathians. The occurrence of E. carpaticum has been confirmed in the Carpathian Mountains of Ukraine, Romania, Hungary (Shrubovych andSterzyńska 2015, Shrubovych et al. 2015) and in the Slovakian Carpathians (unpublished data). Therefore, it can be considered an Eastern-Carpathian endemic ( Table 1). The collection of E. carpaticum in the lowlands of the Transcarpathian region is consistent with an earlier report that this species has a wide ecological plasticity and environmental distribution pattern, and can predominate in Protura communities outside of mountain habitats in azonal habitats, such as floodplain forests (Sterzyńska et al. 2012). A similar situation exists with E. enigmaticum, which was collected in the Ukrainian and Slovakian Carpathians (Shrubovych and Sterzynska 2015, unpublished data) and has been considered an Eastern-Carpathian endemic (Shrubovych and Sterzynska 2017); however, in Poland this species was found outside of mountainous habitats.
The existence of at least 61 European Eosentomon species, with only two of its species outside the continent (Mediterranean Africa), strongly suggests that the remaining continents must have many more species than are currently known from them. All of the world's Eosentomon species have ranges restricted to a single continent, and most are apparently specialized to a particular biome or specialized habitat. Much more collecting needs to be done, even in Europe, for us to understand the diversity of these enigmatic hexapods. Sternites IX -X with 6 setae, female squama genitalis of "wheeleri" type (Nosek 1973: p. 95;Tuxen 1964: fig. 105)   Rostral seta thinner than subrostral seta (see Szeptycki 1986a: fig. 6), lateral sensillum on maxillary palpus longer than dorsal sensillum (see Nosek 1973: fig. 28C';Szeptycki 1986a: fig. 7), foretarsal sensillum t1 nearer to α3 than to α3'(see Nosek 1973: fig. 28A;Szeptycki 1986a: fig. 20) or midway between α3 and α3' (see Szeptycki 1986a: fig. 21