Scutisotoma acorrelata, 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 : 41-43

Scutisotoma acorrelata sp.n.

Figs. 109–113

Material. Holotype: ♀, Russia, Buryatia, Baikal Lake, mouth of Selenga river , under Trifolium sp. , 28.vii.1982 (52 o 14’N, 106 o 29’E), leg. S. Stebaeva. Paratypes: 9 exx., same sample. 2 exx., Russia, Buryatia, Baikal Lake, village. Maksimikha (53 o 15’N, 108 o 44’E), algae debris on shore, 27.viii.1986, leg. T. Zheltikova ( MSPU) .

Description. Size 0.8–1.4 mm. Colour of live specimens unknown (only cleared animals have been seen). Cuticle with fine but visible granulation. Ocelli 8+8, G and H clearly smaller ( Fig. 113 ). PAO narrow elliptical, not constricted, about 0.8 as long as U 3. Maxillary outer lobe with bifurcate palp and 4 sublobal hairs. Maxillary head with short, unmodified lamellae. Labral formula 4/554. Labium with a full set of papillae and guards. Proximal field with 3 chaetae, basomedian field with 4. Ventral side of a head usually with 4+4 postlabial chaetae. Ant.1 with 2 basal microsensilla (bms, dorsal and ventral) and 2 ventral sensilla (s). Ant.2 with 3 bms and one latero-distal s, Ant.3 with one bms and 5 distal s (AO and one lateral s). Guards of AO almost subequal to inner sensilla. Additional sensilla on Ant. 2–3 in males stick-like, shorter than other chaetae. Sensilla on Ant.4 hardly differentiated, subapical organite small.

Dorsal axial chaetom of Th.II–Abd.III as 8–9,7–8/5,5,5–6. Ventral axial chaetae on thorax absent. Macrochaetae differentiated laterally on Th.II–III and on tip of abdomen, medial ones on Abd.V 0.4–0.5 times as long as tergite. Sensilla on tergites clearly shorter than ordinary chaetae. Sensillary formula 33/22224 (s) and 11/111 (ms). Sensilla on Abd.I–III in mid-tergite position (medial sensillum on Abd.III in posterior quarter of the tergite). On Abd.V the adjacent sensilla of the medial and lateral pairs (accp2 and accp3) set close to each other, sensilla of lateral pair (accp3 and accp4) longer and moved slightly anteriorly ( Fig. 112 ).

Unguis simple, without inner tooth. The number of chaetae on Ti.1–3 (21-21-25) similar to the basic set, Ti.3 sometimes with 1–2 additional proximal chaetae. B-row on Ti.1–2 with 7 chaetae. Chaetae x and B 5 on Ti. 3 in studied males poorly differentiated. Tibiotarsal tenent chaetae (1-2-2) on Ti.1–3 slightly clavate, about as long as unguis (U 3: t.ch. = 0.8–1.1: 1). Ventral tube with 4+4 laterodistal and usually with 5 posterior chaetae. Tenaculum with 3+3 strong teeth and one chaeta. Anterior furcal subcoxa with 22–26, posterior with 7–10 chaetae. Anterior side of manubrium with a pair of distal chaetae, the posterior side with 16–23+16–23 on the main part and with 4+4(3–5) chaetae on the latero-basal lobes ( Fig. 110 ). Dens usually with 6(7) anterior chaetae, basal half (0.5–0.6) without chaetae ( Fig 109, 111 ). Posterior side of dens crenulated, with 10–11(14) chaetae (5–7, usually 6, in basal group, 2(3) outer, 2(3) inner and 1 subapical) ( Figs. 110, 111 ). Mucro with two teeth, subapical one stronger. Ratio of manubrium: dens: mucro = 5.8–8.1: 4.1–5.6: 1. Medial pre-anal chaeta on each anal lobe significantly longer than lateral ones.

Affinity. The species appears to be closely related to S. tenuidentifera sp.n. by identical position of sensilla on Abd.V. Apart from the different number of mucronal teeth ( tenuidentifera usually with 3 teeth), full grown specimens can usually be distinguished by the number of posterior chaetae on dens (6–7 in tenuidentifera , 10–11 in acorrelata ).

S. acorrelata sp.n. is characterised by rather specific chaetotaxy of dens. Comparing with related species, the usual correlation between the number of anterior and posterior chaetae on dens seems not to be operating. In number of anterior chaetae it is closer to ananevae and tenuidentifera , while chaetotaxy of posterior side of dens is almost identical to that of schisti , which has longer dens with more anterior chaetae.

Distribution. Known only from two seashore localities of Lake Baikal.

Name derivation. The name reflects the uncorrelated anterior and posterior chaetotaxy on dens.

Figures

FIGURES 109–113. Scutisotoma acorrelata sp.n. 109, 110, anterior and posterior views of furca; 111, dens, lateral; 112, chaetotaxy of Abd.V (s1–4: accp1–4 sensilla); 113, PAO and ocelli.