Megalothorax sanctistephani Christian, 1998

Figs 5–12, Table 1–2

Type material. Holotype: female on slide, Austria, Vienna, catacombs of St. Stephan’s cathedral [48.2084° N, 16.3732° E]; gravelly substrate mixed with human bone fragments (see Christian 1998); 30.09.1996 – 12.01.1997; leg. E. Christian; NMW (COLLEMBOLA Inv. Nr. 130 to 134). Four paratypes (sex undetermined), same data as for the holotype.

Other material. Three females, four males and one juvenile on slides, France, Paris, underground tunnels of the Jardin des Plantes (botanical garden) [48.8450° N, 2.3605° E]; silty substrate without visible organic debris; 17.03.2011; leg. C. Schneider; MNHN (EA040126–131, 133, 135). Five females, one juvenile and two sex undetermined on slides; Kaliningrad; Botanic garden Immanuel Kant Baltic Federal University [54.7365° N, 20.5156° E]; soil in an old greenhouse; leg. D. Levochko; 09.08.2023 and 04.02.2024. One female on slides, Russia, Saint-Petersburg, Peter the Great Botanical Garden [59.9731° N, 30.3272° E]; fern curtain; leg. N. Kuznetsova and M. Potapov; 25.04.2024.

Obtained molecular data. France. One individual sequenced for the partial 28S rDNA (d1, d1), COI mtDNA and 16S mtDNA [PQ900094, PQ900415, PQ900095], same collection data as above RUSSIA. One individual sequenced for near-complete n-rDNA operon sequence [PQ900093], same collection data as above.

Redescription. General aspect. Habitus and segmentation typical of the genus (Fig. 10A, B). Body length around 0.2–0.34 mm (biggest Russian individual found: 0.26 mm). Specimens whitish in alcohol. Body chaetotaxy sparse including chaetae, s-chaetae, trichobothria (Fig. 12C), neosminthuroid chaetae, wax-rods and inner sensilla within sensory fields 2–6. Body chaetae ordinary, without any remarkable development (Figs 5A, 6, 10A, B).

Integument. Secondary granulation made of the usual dorsal rough granules (e.g. Fig. 11C–F). Integumentary channels extending laterally and dorsally in anterior and posterior parts of head, anterior canal branching (detailed topology on Fig. 5A, also see Fig. 10A–E). Channels connection with linea ventralis could be compared to a roundabout.

Sensory fields and wax rods (Figs 5A, 6, 8B, C, 10A–E, 11 C–F). Same configuration as in the genus, but with a novel sensory field (sf 7) associated with wrc 7, being a depression devoid of secondary grain, without inner sensilla (s) (Fig. 11D, E). All s are flame-shaped (Figs 10A, 11C, F). The size of the sensilla increases from anterior to posterior sf: sf 2 has the smallest s, sf 6 has the largest s.

Mouthparts. Labrum as typical for the genus (Fig. 5B). Chaetae a1 and a2 forked, with one tooth. Labium with 4 + 4 proximal chaetae (Fig. 5C). Basomedian fields with 3+3 chaetae, basolateral fields with 1+1 chaetae on tubercle (Fig. 5A). Labial palp (Fig 5C), as common for the genus (A, B, C, D, E, b1, b2, d1, d 2, 2e, H, h1, h2). Oral fold with two chaetae (Fig. 5A). Maxillary outer lobe with two sublobal hairs (Fig. 5D). Maxillary head without strong modification (Fig. 5E, F). Mandibula ordinary (Fig. 5G).

Head chaetotaxy (Fig. 5A). Dorsal anterior area with 9 + 9 chaetae and only one unpaired chaeta, a0 missing and replaced with a “nose”: an oval papilla with an axial groove, like a coffee bean (Figs 5A, 10C, D). Clypeal-labral formula: 2, 2, 5, 4, 5/ 5, 4. Lateral anterior area with 1 + 1 chaetae (one missing in pr. a row). Dorsal posterior area with 12 + 12 chaetae. Ventral side with three pairs of postlabial chaetae. Trend for posterior chaetae to be longer and stronger than anterior chaetae.

Antennal chaetotaxy (Figs 7A–C, 10E, F, 11A). Ant. I and II with one and four chaetae, respectively. Ant. III with six chaetae and two long S-chaetae (S1 and S4). Striations of Ant III sensory organ short sensilla (S2 and S3) distinguishable in light microscopy. Ant. IV with five chaetae (X-chaeta missing) and 10 long S-chaetae. Sensory organ with s-chaetae Sx, Sy, Or, a, sa. Organite (Or) short, seems apically flared. Diagram of the chaetotaxy of the antenna in Fig.7C. Summary on antennal chaetotaxy provided in Table 1.

Th. II—Abd. VI chaetotaxy (Figs 6, 8B, C, 10A–B, 12 A–C). Th. II with 12 + 12 chaetae, 1 + 1 tubular and curved s1-sensilla (Figs 6A, 8B). Th. III with 10 + 10 chaetae, 6 + 6 free wax-rods (wrc 1–6). Chaetae p4 far from wrc2. Chaeta a5 smaller than a6. Abd I–V terga with 19 +19 ordinary chaetae. Chaeta ζ4 absent. Globular s-chaeta s3 present (Fig. 12B), smaller than s2 (Figs 11F, 12A), equidistant from chaetae γ1 and δ1 and in lateral position to γ1 and δ1. Chaetae of body subequal, slightly thickened.

Abd VI chaetotaxy: nine dorsal chaetae as usual; one chaeta on each anal valve (av); mature specimens have 7 + 7 ventral chaetae (Fig. 9A). Males have additionally two axial pairs of swollen chaetae with blunt apex, the outer pair of chaetae being larger than the most axial ones (Fig. 9B).

Genital plate. Female with 2 + 2 chaetae as usual (Fig. 9A), male with 8+9 chaetae (observed on the only preparation, Fig. 9B).

Abd. IV sternum and furca (Fig. 9A). Abd. IV sternum with 2 + 2 neosminthuroid chaetae, 2 + 2 chaetae and 1 +1 tegumentary lobe. Manubrium with 2 + 2 posterior chaetae. Proximal subsegment of dens with a posterior chaeta; distal subsegment posteriorly with two basal spines and one median chaeta. Anterior side of dens with five apical spines, spines without elongated apex. Mucro narrowing in the distal ⅖, with all three lamellae entirely smooth.

Legs chaetotaxy typical of the genus (Table 2), consisting of ordinary chaetae of variable size (Figs 8A–C).

Claws. Claw III bulkier than claw I and II. Claws subequal in unguis length (with a trend as unguis I>unguis II> unguis III). Unguis basal and posterior auxiliary lamellae (la, lp and Bp) well developed (Figs 8A–C). Unguiculus 0.5–0.6 as long as unguis.

Tenaculum and ventral tube. Tenaculum with 3 + 3 hook-like teeth (Fig. 9C). Ventral tube bulky with two apical pairs of chaetae.

Ecology of Megalothorax sanctistephani

Megalothorax sanctistephani has been found from three special localities. First, the catacombs of the cathedral St. Stephan in Vienna (Christian 1998). Second, the underground galleries under the Jardin des Plantes (botanical garden) in Paris; and third, near an old greenhouse in the botanical garden of Kaliningrad, under furniture debris. Whether the botanical garden or subterranean habitat is the key factor is unclear. In Vienna and Paris, it seems clear that the soil was poor in organic material. In Paris, M. sanctistephani had no direct competition from other springtails. In Kaliningrad it was found twice: once alone, and in a second sample coexisting with M. minimus.

Megalothorax sanctistephani as never been found in natural caves, and its morphology is typical of soil species, without any of the usual troglobiontic modification (such as increased body size, elongated claws and enlarged sensorial organs of the antenna). Since European troglophilous Neelidae have received some attention (Deharveng 1978, Deharveng & Beruete 1993, Kováč & Papáč 2010, Papáč & Kováč 2013, Papáč et. al 2016, Papáč et. al 2019) we estimate that M. sanctistephani is at least uncommon there, if not really absent.

The known ecological niche of M. sanctistephani can thus be described as follows: poor soil with low to no competition in frost-dampened urban habitat. Its natural habitat is likely comparable. It may have been overlooked in Europe due to a cryptic habitat (such as Mesovoid Shallow Substratum, e.g. Pipan & Culver 2019) or perhaps the species was imported with plants from tropical regions.