Neelipleona and Symphypleona (Collembola) from a Sampling in the Mesovoid Shallow Substratum of the Sierra de Guadarrama National Park (Madrid and Segovia, Spain): Taxonomy and Biogeography

Simple Summary The material for this study was obtained after intensive sampling in the colluvial mesovoid shallow substratum (MSS) of the Sierra de Guadarrama National Park using 33 subterranean sampling devices (SSD). The data were obtained from the first extraction of the traps between May and October of 2015. This paper presents the results for a small part of the total Collembola captured (4.4% of the total for this sampling), namely, the Neelipleona and Symphypleona. Eleven species belonging to seven families were identified, two of which are new species. Based on the results of this study, and others previously published on Collembola of the MSS in the Sierra de Guadarrama National Park, the presence of epigeal and edaphic species is observed, which, in general, are not as abundant as the newly discovered species. The high abundance and extensive presence in the hypogean environment of most of the new species discovered are indicative that the MSS has a unique and distinct Collembola community. Abstract Megalothorax minimus (Neelidae) and Sphaeridia pumilis (Sminthurididae) had already been identified in surface sampling from Sierra de Guadarrama. In Europe, Sminthurinus gisini (Katiannidae) seems to be associated with environments at specific altitudes, and has little representation in this sampling. Pygmarrhopalites custodum Baquero and Jordana sp. nov. (Arrhopalitidae) coexists with two previously identified surface occurring species of the same family (P. elegans and Arrhopalites caecus). However, P. custodum is more abundant, indicating that it occupies an ecological niche tending to troglophile in the mesovoid shallow substratum (MSS). Moreover, it is also more abundant in the MSS of higher altitude corresponding to the bioclimatic zones cryo-oro-Mediterranean and oro-Mediterranean supra forest. Allacma cryptica Baquero and Jordana sp. nov. (Sminthuridae), is another species that had not been previously detected on the surface in the study area. A. cryptica is an addition to a genus which has eight described species. Gisinurus malatestai (Sminthuridae) appears well represented in the MSS, being a species present very occasionally in the Mediterranean area. Two species of the genera Sminthurides (Sminthurididae) and Fasciosminthurus (Bourletiellidae) have been found, but they could not have been identified to the species level. Finally, a few specimens of Dicyrtomina minuta (Dicyrtomidae), an abundant species on the surface, have been captured.


Introduction
Until recently, intensive studies on the mesovoid shallow substratum (MSS) were lacking. Environmental conditions in MSS are similar to those of caves, but have a much

Site
The sampling was conducted in the Sierra de Guadarrama National Park, located in the eastern half of the Central System (i.e., the Iberian Peninsula). The park covers 33,960 hectares, with a belt of 62,687.26 hectares that functions as a peripheral protection area [5].
The Sierra de Guadarrama mountain range is configured in three axes comprised of the Siete Picos-La Mujer Muerta, Montes Carpetanos, and Cuerda Larga and associated mountainous complex ( Figure 1). The dominant rocks are of metamorphic origin of the orthogneiss type [6]. Glacial and periglacial events fragmented this rock typology forming extensive colluvial deposits [7,8] that make up numerous "scree slopes" that allowed the development of the MSS. The studied area is divided into three bioclimatic zones: supra-Mediterranean, oro-Mediterranean, and cryo-oro-Mediterranean [9,10]. The oro-Mediterranean zone is further sub-divided in forest and supra-forest. The most outstanding characteristics of these bioclimatic zones in the Sierra de Guadarrama, and its most conspicuous vegetation, are summarized in [11]. It should also be noted that there is intense snow precipitation in the cryo-oro-Mediterranean and oro-Mediterranean above the scrub supra-forest line.

Methodology
Thirty-three sampling points were established ( Figure 1). The sampling was performed mainly using subterranean sampling devices (SSD) that consisted of a PVC cylinder 11 cm in diameter and 1 m in length, with perforations of 8 mm in diameter (with a separation of 15 mm between them) in its lower half, placed in a suitable substrate. A pitfall trap (10 cm in diameter), that fit within the PVC tube, was filled with 1,2-propanediol, and a vial containing cheese, was slid to the bottom of the tube, the top was closed and the entire unit was placed in the soil. Other details that describe the placement of traps and other methodology for capturing the animals have been described in [1]. The authors who performed the sampling included a team that consisted of V. M. Ortuño, E. Ledesma, J. D. Gilgado, A. Jiménez-Valverde, G. Pérez-Suárez, and E. Baquero. Permits to collect samples were obtained from the appropriate authorities (General Directorate of Environment of the Community of Madrid and Territorial Service of the Environment of the Junta de Castilla y León). Traps (Table 1) were placed between 20 May 2015 and 9 July 2015, and the first series of samples were obtained between 17 September 2015 and 6 November 2015.
inder 11 cm in diameter and 1 m in length, with perforations of 8 mm in diameter (with a separation of 15 mm between them) in its lower half, placed in a suitable substrate. A pitfall trap (10 cm in diameter), that fit within the PVC tube, was filled with 1,2-propanediol, and a vial containing cheese, was slid to the bottom of the tube, the top was closed and the entire unit was placed in the soil. Other details that describe the placement of traps and other methodology for capturing the animals have been described in [1]. The authors who performed the sampling included a team that consisted of V. M. Ortuño, E. Ledesma, J. D. Gilgado, A. Jiménez-Valverde, G. Pérez-Suárez, and E. Baquero. Permits to collect samples were obtained from the appropriate authorities (General Directorate of Environment of the Community of Madrid and Territorial Service of the Environment of the Junta de Castilla y León). Traps (Table 1)    After the preliminary sorting to separate the Neelipleona and Symphypleona from other Collembola, some representative specimens of each species were selected and mounted in Hoyer's medium for observation under a compound microscope (phase contrast and Differential Interference Contrast-DIC microscopy). A portion of the specimens were cleared in Nesbitt's fluid. The remaining samples were stored in 70% ethyl alcohol.
The abbreviations used are: Abd-abdomen or abdominal segment; Ant-antennal segment or antenna/ae; a.s.l.-above sea level; MSS-mesovoid shallow substratum; SSDsubterranean sampling devices. The chaetae are marked in bold in the text.

Summary
Neelipleona and Symphypleona accounted for 4.4% of the Collembola captured in the traps (1860 specimens) in the total number of samples used to obtain data for this study (42,745 specimens). Specimens of nine genera and eleven species, belonging to seven families (Neelidae, Sminthurididae, Katiannidae, Arrhopalitidae, Sminthuridae, Bourletiellidae, and Dicyrtomidae) were captured, but with unequal representation. One of the species, belonging to the genus Pygmarrhopalites and new to science, was overwhelmingly abundant (1532 specimens; 83%) and had an extensive distribution; a second new species belonging to the genus Allacma Börner, 1906 [18], was poorly represented (45 specimens; 2%), and had a more restricted distribution. The other species, with the exception of Gisinurus malatestai Dallai, 1970 [19], were almost anecdotal in terms of abundance and distribution.

Remarks
Present in the Holarctic Region and tropics, and previously referred for Guadarrama by Selga (1971) [24].

Ecology
Very poorly represented in the MSS of the study area ( Figure 2). Observed only in the oro-Mediterranean bioclimatic zone, both in the forest and supra-forest belts. However, it appears to have a wide spatial distribution in the Sierra de Guadarrama National Park, having been detected in the underground of all three mountainous axes ( Figure 3A). This species is syntopic with some Symphypleona cited in this work: Sphaeridia pumilis, Allacma cryptica Baquero and Jordana sp. nov., Pygmarrhopalites elegans, Arrhopalites caecus, and Pygmarrhopalites custodum Baquero and Jordana sp. nov. Exclusively observed in one of the sampled locations near the Montón de Trigo mountain, part of the Mujer Muerta-Siete Picos mountainous axis ( Figure 3A). The MSS is found in a colluvial deposit covered by pine forest (Pinus sylvestris) and located at an altitude slightly higher than 1800 m a.s.l., and therefore located in the oro-Mediterranean forest bioclimatic zone. In this MSS, it has been observed that S. pumilis is syntopic with Megalothorax minimus (Neelipleona), and the Symphypleona, Pygmarrhopalites custodum sp. nov. and Allacma cryptica Baquero and Jordana sp. nov.   Spain, Sierra de Guadarrama, Segovia, SSD-2, four specimens on a slide 03 and one on a slide 11. Ortuño et al. leg [27].

Remarks
Type locality: Germany. Present in the Holarctic Region and Australia. Previously cited for the Iberian Peninsula and in Guadarrama [15,28,29].

Ecology
Exclusively observed in one of the sampled locations near the Montón de Trigo mountain, part of the Mujer Muerta-Siete Picos mountainous axis ( Figure 3A). The MSS is found in a colluvial deposit covered by pine forest (Pinus sylvestris) and located at an altitude slightly higher than 1800 m a.s.l., and therefore located in the oro-Mediterranean forest bioclimatic zone. In this MSS, it has been observed that S. pumilis is syntopic with Megalothorax minimus (Neelipleona), and the Symphypleona, Pygmarrhopalites custodum sp. nov. and Allacma cryptica Baquero and Jordana sp. nov. Katiannidae Börner, 1913 [30], sensu Bretfeld 1999 [15] Gama, 1965 Material Studied Spain, Sierra de Guadarrama, Segovia, SSD-16, one female and one male on a slide (05); Madrid, SSD-27, two juveniles on a slide (03). Ortuño et al. leg [31].

Ecology
Found in a very low number of specimens of a single locality (oro-Mediterranean forest bioclimatic zone of the Cuerda Larga mountain axis). In this MSS, it has been observed that P. caecus is syntopic with Megalothorax minimus (Neelipleona), and Pygmarrhopalites custodum sp. nov. (Symphypleona).

Remarks
Described originally from the south of the Iberian Peninsula [37], previously cited from Guadarrama [24].

Ecology
This species was not very abundant (25 specimens) ( Figure 2) in the sample. However, while it has been observed very scarcely in the study area, it is widely distributed across the three mountainous axes of the Sierra de Guadarrama National Park ( Figure 3B). This species is present in the subsoil of the three bioclimatic zones, but it appears better represented in the oro-Mediterranean (seven locations out of nine collected), both in the forest and supraforest belt. In this MSS it has been observed that P. elegans is syntopic with Megalothorax minimus (Neelipleona), and the Symphypleona Pygmarrhopalites custodum sp. nov., and Allacma cryptica Baquero and Jordana sp. nov.  Table 2.
Insects 2021, 12, x FOR PEER REVIEW 10 five distinct sub-segments, with five evident whorls: one at the distal part of b sub-segment; one on each of the three intermediate sub-segments; and another one a basal part of the terminal sub-segment. Apical sub-segment with knobbed subapica ganite and short chaeta-like A1p; one of the chaetae on this area has a narrowing terminal half. Foreleg ( Figure 5A and 5D): precoxae 1, 2, and coxa with 1, 0, 1 chaetae, respecti Trochanter with three anterior and 1 posterior chaetae. Femur with 12 chaetae, a4 tu perpendicularly to the longitudinal axis of the segment, p1 and p3 thin and short. T tarsus with 43 chaetae: whorl I with 9 chaetae among which Ja curved and somew thickened, whorls II-V with 8, 8, 8, and 7 chaetae respectively; region F with 3 primar  chaetae (e, ae, pe) and secondary chaeta FSa. Pretarsus with 1 anterior and 1 posterior chaetae. Foot complex: claw thin, with reduced tunica, with inner tooth and two pairs of indistinct lateral teeth; empodium thin, with corner tooth, and long apical filament surpassing the tip of the claw. Mid leg (Figures 5B and 5E): precoxae 1 and 2 with 1, 1 chaetae respectively, precoxal process present, coxa with three chaetae and a microsensillum. Trochanter with three chaetae and a trochanteral organ. Femur with 15 chaetae, p1 and p3 very small. Tibiotarsus with 44 chaetae: whorl I with 9 chaetae, whorls II-IV with 8, 8, 8, and 7 chaetae respectively; region F with three FP chaetae and FSa chaeta. Foot complex: claw wider than foreleg claw, with reduced tunica and inner tooth, and two pairs of small lateral teeth; empodium with corner tooth and long apical filament surpassing the tip of the claw.
Hind leg ( Figures 5C and 5F): precoxae 1 and 2 with 1, 1 chaetae respectively, process on precoxa 1 present, coxa with three chaetae and a microsensillum. Trochanter with three chaetae and a trochanteral organ. Femur with 13 chaetae, p1 and p3 as micro chaetae. Tibiotarsus with 44: whorl I with 9 chaetae, whorls II-IV with 8, 8, 8, and 7 chaetae respectively; region F with three FP chaetae and FSa chaeta. Foot complex: claw wider than foreleg claw, with reduced tunica, inner tooth, and two pairs of small lateral teeth;  (10) and 23 specimens in ethyl alcohol; SSD-8, one female and one juvenile on a slide (11) and 41 specimens in ethyl alcohol; SSD-20, five specimens in ethyl alcohol; SSD-22, one female and one juvenile on a slide (09); Madrid: SSD-9, two females on a slide (03) and 146 specimens in ethyl alcohol; SSD-10, two females, one male and one juvenile on a slide (01) and approximately 300 specimens in ethyl alcohol; SSD-23, two females and one juvenile on a slide (05) and 136 specimens in ethyl alcohol; and SSD-24, one female and approximately 725 specimens in ethyl alcohol. Additional material, Segovia: SSD-1, one female on a slide (04) and nine specimens in ethyl alcohol; SSD-2, three females on three slides (01, 11, and 12) and 53 specimens in ethyl alcohol; SSD-3, one female on a slide (08); Madrid: SSD-26, one female on a slide (03); SSD-28, two females on a slide (01) and 66 specimens in ethyl alcohol; and SSD-32, one female on a slide (02).

Diagnosis
Eyes 1 + 1. Bothriotricha ABC almost aligned. Ant IV with five distinctly separated sub-segments. Head dorsum with 4 + 4 spine-like chaetae; three unpaired clypeal chaetae. All claws with tunica and inner tooth; empodia I-II with corner tooth, III with or without subterminal tooth; all empodia with filament, surpassing tips of corresponding claws. Manubrium with 7 + 7 posterior chaetae; dens without ventral spines, two outer and one inner spines present; ventral dens formula: 3,2,1,1,0. Abd VI with winged and serrated circumanal chaetae, and anal appendage gutter-like with lateral and terminal fringes (last third approximately). Ecology. This is the Symphypleona species dominant in the MSS of the study area. This species was observed in 14 representative localities of the three mountainous axes, and therefore indicative of being widely distributed in the MSS of the Sierra de Guadarrama National Park ( Figure 3C). Its abundance was truly remarkable, as they comprised 83% (1532 specimens) of the specimens analyzed in this study ( Figure 2). Mostly present  Table 2. Comparison among the species that share with P. custodum Baquero and Jordana sp. nov. the presence of the reduced distal formula 021 for the dens (number of ventralexternal-internal spine-like chaetae); or one eye + eight spine-like chaetae on head vertex; or a 0 chaeta not bifurcated + at least a 1 winged and serrated at the base on female anal valves. 1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33 34 D
Antenna ( Figure 4C-F) shorter than the body (ratio 0.80) and ratio Ant/head as 1.67 (n = 18); basal sub-segment of Ant IV 1.23 times longer than Ant III. Ant I with 7 chaetae, p as micro chaeta. Ant II with 15 chaetae, two interior ones longer than others. Ant III with 15 chaetae, the two usual sensilla, and three guard sensilla; Ant IV ( Figure 4F) with five distinct sub-segments, with five evident whorls: one at the distal part of basal sub-segment; one on each of the three intermediate sub-segments; and another one at the basal part of the terminal sub-segment. Apical sub-segment with knobbed subapical organite and short chaeta-like A1p; one of the chaetae on this area has a narrowing since terminal half.
Mid leg ( Figure 5B,E): precoxae 1 and 2 with 1, 1 chaetae respectively, precoxal process present, coxa with three chaetae and a microsensillum. Trochanter with three chaetae and a trochanteral organ. Femur with 15 chaetae, p1 and p3 very small. Tibiotarsus with 44 chaetae: whorl I with 9 chaetae, whorls II-IV with 8, 8, 8, and 7 chaetae respectively; region F with three FP chaetae and FSa chaeta. Foot complex: claw wider than foreleg claw, with reduced tunica and inner tooth, and two pairs of small lateral teeth; empodium with corner tooth and long apical filament surpassing the tip of the claw.
Hind leg ( Figure 5C,F): precoxae 1 and 2 with 1, 1 chaetae respectively, process on precoxa 1 present, coxa with three chaetae and a microsensillum. Trochanter with three chaetae and a trochanteral organ. Femur with 13 chaetae, p1 and p3 as micro chaetae. Tibiotarsus with 44: whorl I with 9 chaetae, whorls II-IV with 8, 8, 8, and 7 chaetae respectively; region F with three FP chaetae and FSa chaeta. Foot complex: claw wider than foreleg claw, with reduced tunica, inner tooth, and two pairs of small lateral teeth; empodium with or without a subapical small tooth, and a long apical filament surpassing the tip of the claw.
Large abdomen ( Figure 6A): Th II with a single sensillum in row a and three chaetae in row m with m 1 bigger than other. Th III with a sensillum in row a and three chaetae in row m. Abd row a with five chaetae, row m with four, and three p short chaetae, anterior to bothriotrichal complex. Bothriotrichal complex: ABC almost in a linear pattern; bothriotrichum A with one posterior accessory short chaeta (a 1 ); bothriotrichum B with one posterior accessory short chaeta (b 1 ); bothriotrichum C with two anterior accessory short chaetae (c 1 and c 2 ). Posterior lateral complex with seven, and furca base complex with six chaetae. Posterior dorsal complex with three rows with 9, 10, and 8 long chaetae each. Ventral complex with three chaetae. Fifth abdominal segment: with two chaetae and bothriotrichum D in row a, and two chaetae in row p. Sixth abdominal segment ( Figure  6B,C): Abd VI with broadened, winged, and serrated circumanal chaetae (a 0 , a 1-3 , av 1 and AV 1 ; sometimes such chaetae are only winged; in some specimens a 0 has its tip simple, doble or four-branched); anal appendage gutter-like with lateral and terminal fringes (last third approximately) ( Figure 6D).
Etymology. The name is derived from the Latin term custōs (guard, protector), (masculine name in genitive plural) in tribute and recognition of the people who work in the management and protection of the Sierra de Guadarrama National Park.
Ecology. This is the Symphypleona species dominant in the MSS of the study area. This species was observed in 14 representative localities of the three mountainous axes, and therefore indicative of being widely distributed in the MSS of the Sierra de Guadarrama National Park ( Figure 3C). Its abundance was truly remarkable, as they comprised 83% (1532 specimens) of the specimens analyzed in this study ( Figure 2). Mostly present in MSS of enclaves at altitudes close to 2000 m a.s.l., or higher, in the oro-Mediterranean supraforest and cryo-oro-Mediterranean bioclimatic zones; and less abundant at altitudes below 2000 m a.s.l., in the oro-Mediterranean forest and supra-Mediterranean zones ( Figure 3C)  Body. Bothriotricha insertion linear or with a slightly obtuse open forward angle; bothriotrichum D inserted on small abdomen. Trochanter with a posterior spine-like chaeta. Claws broad, with tunica and serrated pseudonychia; inner tooth present on all legs; empodium of all legs without an internal tooth, pointed and acuminated, and with a filament not reaching the tip of claw on legs I-II, surpassing the claw on leg III; pretarsus with two chaetae (Figures 8A-B). Body with nearly smooth but broad chaetae. Tenaculum with two chaetae on the corpus. Manubrium with six dorsal chaetae, in three rows: three distal, two medial, and one proximal. Dens anterior with 3, 2, 2, 2, 1…1 chaetae (the Holotype, asymmetrically, has 3, 2, 2, 2, 2…1); posterior chaetae of normal length, with
Remarks. The species with which the new species should be compared are A. fusca (Linnaeus, 1758) [54], A. gallica (Carl, 1899) [55], and A. koreana Yosii and Lee, 1963 [56]. A   Body. Bothriotricha insertion linear or with a slightly obtuse open forward angle; bothriotrichum D inserted on small abdomen. Trochanter with a posterior spine-like chaeta. Claws broad, with tunica and serrated pseudonychia; inner tooth present on all legs; empodium of all legs without an internal tooth, pointed and acuminated, and with a filament not reaching the tip of claw on legs I-II, surpassing the claw on leg III; pretarsus with two chaetae (Figure 8A,B). Body with nearly smooth but broad chaetae. Tenaculum with two chaetae on the corpus. Manubrium with six dorsal chaetae, in three rows: three distal, two medial, and one proximal. Dens anterior with 3, 2, 2, 2, 1 . . . 1 chaetae (the Holotype, asymmetrically, has 3, 2, 2, 2, 2 . . . 1); posterior chaetae of normal length, with six internal, five external, eight medial and five proximal ( Figure 8C). Mucro with smooth external and lobulated internal edge; mucronal chaeta present ( Figure 8D). Small abdomen with normal chaetae, smooth and not broadened at its base; anal appendage long, cylindrical, blunt, and without fringes ( Figure 8E).
Etymology. The new species is named cryptica, from the Latin crypticus, alluding to "hidden", referring to its subterranean life.
Remarks. The species with which the new species should be compared are A. fusca (Linnaeus, 1758) [54], A. gallica (Carl, 1899) [55], and A. koreana Yosii and Lee, 1963 [56]. A comparison among the above three and the new species, demonstrates differences in the head (post-antennal chaetae shape, vertex macrochaetae sculpture, macrochaetae number and number of sub-segments on the antenna), claws, empodium, dens, and anal appendage. The presence of a tooth on the internal claw in all legs allows the differentiation of the new species from A. fusca and A. gallica (A. koreana has it sometimes following Bretfeld 1999 [15]). Another definitive character, to differentiate the new species from A. fusca and A. gallica is the form of the anal appendage of the female, which in the case of the new species is acuminate while for the other three species it is broad and with a short ciliation. The principal differences among the new species and those previously described are schematized in Table 3.
Ecology. This species was observed in eight localities in the study area; present in the MSS of the three mountainous axes, although not very abundant (45 specimens, with a maximum collection/SSD of 16 specimens) ( Figure 3D). However, while these data must be taken with caution given the small number of specimens collected, a trend is observed that it occurs at higher frequency in the MSS at lower altitudes, corresponding to the supra-Mediterranean and oro-Mediterranean forest bioclimatic zones ( Figure 3D). Allacma cryptica Baquero and Jordana sp. nov. is syntopic with Megalothorax minimus (Neelipleona), and the Symphypleona Sphaeridia pumilis, Pygmarrhopalites elegans, and Pygmarrhopalites custodum sp. nov. Dallai, 1970 Material Studied Spain, Sierra de Guadarrama, Madrid, SSD-14, three females on two slides (04 and 05), and approximately 216 in ethyl alcohol; SSD-15, one female on a slide (06) and eight in ethyl alcohol. All Ortuño et al. leg [19].

Remarks
It was not possible to determine the identity of the species because a single juvenile was available.

Ecology
According to the abundance ( Figure 2) and distribution data ( Figure 3A), the presence of this species in the MSS of the Sierra de Guadarrama National Park seems anecdotal. This species was found in a locality on the Cuerda Larga mountainous axis, and in the oro-Mediterranean forest bioclimatic zone.
Two other Symphypleona species, P. elegans, and P. custodum sp. nov., were found in the same locality.

Remarks
It was not possible to determine the identity of the species because a single juvenile was available.

Ecology
This species was found in a locality in the confluence of the three mountain axes, and in the oro-Mediterranean forest bioclimatic zone. According to the abundance ( Figure 2) and distribution data ( Figure 3B), the presence of this species in the MSS of the Sierra de Guadarrama National Park appears anecdotal.

Remarks
A species with wide distribution, both in the Holartic region and in many localities in the southern hemisphere [15]. Until now it had only been identified in the north zone of the Iberian Peninsula [24,62,63] and Portugal S [64]; Selga (1971) [24] cited the subspecies D. minuta flavosignata in Madrid.

Ecology
The presence of this species in the MSS of the study area is almost anecdotal (Figure 2). It was only observed in the underground of a locality of the Cuerda Larga mountainous axis ( Figure 3B), and in the oro-Mediterranean forest bioclimatic zone, being syntopic with another Symphypleona, S. gisini.

General Discussion
Previous intensive Collembola collection activities throughout the 20th century in the Sierra de Guadarrama were conducted in edaphic and epi-edaphic environments, and did not reveal the presence of these species except for S. pumilis [29], A. caecus [24] and P. elegans [24]. Of these species, S. pumilis has a surface habit, and the other two have been characterized as troglophiles [65]. Therefore, it came as no surprise that they might have been found in the MSS, which they could use as a refuge. Although cited here for the first time from the Sierra de Guadarrama, S. gisini and D. minuta are widely-distributed species and thus also likely seeking refuge in the MSS. However, as Figure 2 shows, it does not seem fortuitous that the most abundant species (P. custodum sp. nov., G. malatestai, A. crypticae sp. nov., P. elegans, and A. caecus) are all either troglobite or troglophilous species. Accounting for 83% of the combined abundance of Symphypleona, P. custodum sp. nov. is the dominant species of the group in the MSS of the Sierra de Guadarrama. It is seconded by G. malatestai, a very rare species in Europe that has been described as either superficial or troglophilous and found only in very sparse populations, but that in the MSS is rather abundant (223 ex.). We thus suspect that this species belongs to the MSS or caves proper. Allacma crypticae sp. nov. is the next most abundant species (45 ex.). The genus had never been cited in the Sierra de Guadarrama, and it also seems a characteristic species of the MSS.
The MSS has more moderate maximum and minimum temperatures than those found in the epigean environment, but this does not mean that the different MSS localities sampled in the study area are comparable. This circumstance is evidenced by the association of P. custodum sp. nov. with underground spaces. The number of specimens obtained for P. custodum sp. nov. (83% of the total of Symphypleona and Neelipleona collected) allows us to safely conclude that it has a preference in the MSS at higher altitudes ( Figure 3C): cryo-oro-Mediterranean and oro-Mediterranean supra-forest (≈95% of specimens) vs. oro-Mediterranean forest and supra-Mediterranean (≈5% of specimens) ( Figure 2).
Our data indicate that the MSS has a unique and distinct Collembola fauna, constituting a new biotope for these Hexapoda. Epigean and edaphic species have also been collected in the MSS but, in general, they are less abundant and are less widespread in the underground of the studied area.
This study, comprises a new step toward a more thorough knowledge base and understanding of the MSS biocenosis. It demonstrates that the MSS region has enormous potential to serve as a refuge for surface and subterranean faunal species. Furthermore, it indicates that the management objectives for the MSS should include research of the biodiversity of its unique and distinct natural spaces.