Scutisotoma ladaki ( Denis, 1936 ) Potapov & Babenko & Fjellberg, 2006

Potapov, Mikhail, Babenko, Anatoly & Fjellberg, Arne, 2006, Taxonomy of the Proisotoma complex. Redefinition of genera and description of new species of Scutisotoma and Weberacantha (Collembola, Isotomidae), Zootaxa 1382 (1), pp. 1-74 : 21-23

Scutisotoma ladaki ( Denis, 1936) , comb.n.

Figs. 40–47

Proisotoma ladaki Denis, 1936: 262

Material studied. Numerous specimens, Russia, Siberia, Taimyr Peninsula, northern

shore of Pjasina Lake [70 o 05’N 87 o 27’E], stony beach, leg. A. Babenko. 25 exx., Russia, Siberia, Tuva Republic, Alashskoye Plateau, shore of Kara-Khol Lake , near water edge, leg. S. Stebaeva; 1 ex., Mongolia, Ar-Khangayski Aimak, Tuvshrulekh, leg. A. Druk; 2 exx., from another unknown locality in Mongolia ( MSPU) .

Redescription. Size 1.20–1.35 mm. Usually pale green, dark specimens can be almost violet. Extremities paler. Cuticle with visible primary granulation, unmodified on Abd.V. 8+8 ocelli, G and H smaller. PAO elliptical, not constricted and almost twice as long as the ocellus or 0.6–0.8 as long as inner unguis. Maxillary outer lobe with 4 sublobal hairs, maxillary palp bifurcate. Labral formula 4/554. Labium with a full set of papillae and guards, 3 proximal and usually 5 basomedian chaetae. Ventral side of a head usually with 5+5 postlabial chaetae. Ant.1 with 2 basal microsensilla (bms), dorsal and ventral, and 2 ventral sensilla (s). Ant.2 with 3 bms and one laterodistal s. Ant.3 with one bms and 5 distal s (4 in AO and 1 lateral). In most mature males additional sex sensilla (antennal “spurs”) absent, only some specimens have few in the middle part of Ant.3. Sensilla on Ant.4 hardly differentiated, subapical organite minute.

Tergal sensilla well differentiated, shorter than ordinary chaetae. Sensillar formula 33/ 22224 (s), 11/111 (ms) ( Fig. 46 ). The most lateral sensillum on Th.II is on the same level as other lateral sensilla. Medial sensillum on Abd.III in midtergal position. Abd.V sensilla situated in a line ( Fig. 45 ). Macrochaetae poorly differentiated, those on Abd.V 0.2–0.3 as long as tergite. Axial chaetom of Th.II–Abd.III as 9–10,7–8/5,5–7,7–8. Th.III with 30–35 chaetae in p-row. Thorax without ventral chaetae.

Unguis without inner tooth, slightly elongate ( Fig. 43 ). Tibiotarsi 1–3 with more than 21-21-25 chaetae. B-row complete, T-chaetae absent. Tibiotarsal tenent chaetae (1-2-2) clearly clavate, shorter than unguis (U 3: t.ch. = 1,0–1,5: 1). Chaetae x and B 5 on Ti. 3 in males short and chaeta-like ( Fig. 43 ). Ventral tube with 4–5 laterodistal and 6 posterior chaetae ( Fig. 47 ). Tenaculum with 3+3 teeth and one chaeta ( Fig. 44 ). Sometimes an additional reduced tooth present on one of the rami. Anterior furcal subcoxa with 27–33 chaetae, posterior one with 12–14 chaetae. Anterior side of manubrium with a pair of distal chaetae, posterior side with more than 30+30 chaetae. Laterobasal lobes with 4–5 chaetae. Dens with 28–39 anterior chaetae, basal 1/5 without chaetae. Posterior side of dens tuberculated with 9–12 chaetae in basal group, 7–10(12) outer, 7–9 inner, and one subapical chaeta near mucro ( Fig. 40–42 ). Mucro strong and lamellate, with 3 subequal teeth. Ratio of manubrium: dens: mucro = 5,5–7,4: 5,6–6,8: 1. Chaetotaxy of anal lobes as in S. karadagi sp.n.

Taxonomic remarks. The above redescription is in full agreement with Denis’ (1936) original description. We only add some additional characters not mentioned by Denis. The association with fresh water habitats probably explains the wide distribution of the species, from high mountains of northern India to tundra landscapes of northern Siberia. Dunger’s (1982) Ballistura sp. from Mongolia is probably the same species.

S. ladaki is the most similar to S. muriphila . Apart from absence of sexual dimorphism in mature males, S. ladaki differs from S. muriphila in having 5 basomedian chaetae on labium and differing position of sensilla on Abd.V (cf. Fig. 45 and Fig. 39 ). Moreover, the dens of S. ladaki is cylindrical while more tapering in muriphila , and its dorsal side usually tuberculated (crenulated in muriphila ).

Denis’ original type material from northern India is probably lost (Prof. J.-M. Thibaud, pers. comm.) .

Distribution. Widely distributed in Asia. A neustonic form inhabiting microsites near fresh cold water.

Figures

FIGURES 40–44. Scutisotoma ladaki. 40, 41, anterior and posterior sides of furca; 42, dens, lateral view; 43, tip of leg 3 in male; 44, tenaculum (B5, x: male modified chaetae).

FIGURES 45–47. Scutisotoma ladaki. 45, sensillary chaetotaxy of Abd.V; 46, distribution of tergal sensilla and microsensilla (ms); 47, ventral tube.

FIGURES 36–39. Scutisotoma muriphila. 36, 37, anterior (36) and posterior (37) sides of furca; 38, tip of leg 3 in male; 39, sensillary chaetotaxy of Abd.V (s1–4: accp1–4 sensilla).