Scutisotoma baica, 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 : 49-51

Scutisotoma baica sp.n.

Fig. 133–138

Material. Holotype: ♀, Russia, Siberia , Baikal lake, vicinity of Maksimikha village (53 o 15’N, 108 o 44’E), organic debris (algae) on lake shore, 27.viii.1986, leg. T. Zheltikova. Paratypes: 30 exx., same sample ( MSPU) .

Description. Size up to 0.9 mm. Colour spotted greyish, ventral side and extremities almost without pigmentation. Cuticle with visible granulation. Ocelli 8+8, G and H clearly smaller. PAO elliptical, rather wide and slightly constricted, about 1.0–1.3 as long as U 3. Maxillary outer lobe with 4 sublobal hairs and bifurcate maxillary palp. 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 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 1 laterodistal s, Ant.3 with 1 bms and 6 distal s (AO + two lateral s). Additional sensilla on Ant.2–3 present in males, usually stick-like. Sensilla on Ant.4 hardly differentiated, subapical organite small.

Dorsal axial chaetom of Th.II–Abd.III as 6–7,5–6/5–6,5–6,4–6. Thorax without ventral axial chaetae. Macrochaetae poorly differentiated laterally on Th.II–III and on abdominal tip. Medial macrochaetae on Abd.V are 0.2–0.3 as long as tergite ( Fig. 138 ). Sensilla on tergites shorter and thinner than ordinary chaetae. Sensillary formula 33/22224 (s) and 11/111 (ms). Sensilla on Abd.I–III in mid-tergal position (medial sensillum on Abd.III in posterior 1/3 of the tergite). Sensilla on Abd.V set in a line ( Fig. 138 ).

Unguis simple, without inner tooth. All tibiotarsi with additional proximal chaetae (as a whole with 22–23, 24–26, 30–34 chaetae) ( Figs. 136, 137 ). B-row on Ti.1–2 with 7 chaetae. Chaetae x and B 5 on Ti. 3 in males stick-like, thinner and shorter than ordinary chaetae ( Fig. 137 ). Tibiotarsal tenent chaetae (1-2-2) more or less clearly clavate, about as long as unguis length (U 3: t.ch. = 0.8–1.0: 1). Ventral tube with 4+4 latero-distal and 6–7 posterior chaetae. Tenaculum with 3+3 strong teeth (an additional small tooth may sometimes be present) and one chaeta. Anterior furcal subcoxa with 16–19 chaetae, posterior one with 9–12. Furca relatively long, reaching posterior margin of Abd.II. Anterior side of manubrium without chaetae, posterior side with 17–18+17–18 on the main part and usually with 4+4 chaetae on the latero-basal lobes. Anterior side of dens with one distal chaeta only. Posterior side of dens crenulated, with 4 chaetae (3 basal, 1 subapical near mucro) ( Figs. 133–135 ). Mucro with two teeth and more or less clear lateral lamella. Ratio of manubrium: dens: mucro = 5.3–7.4: 3.6–5.0: 1.

Affinity. The new species is similar to S. stepposa (Martynova) , which generally has the same structure of furca. S. baica has more chaetae on posterior side of manubrium (17–18+17–18, in stepposa only 8–10+8–10) as well as on all tibiotarsi. Unfortunately both features are highly size/age dependant and separation of immature specimens of stepposa / baica is not possible.

Despite the Subisotoma -like furca (manubrium without anterior chaetae, dens with 1/4 chaetae only), other characters of baica / stepposa clearly indicate the genus Scutisotoma .

Distribution. Known only from the type-locality (Lake Baikal). Probably a neustonic

form.

Name derivation. The name associates the new species with Lake Baikal, and is also a local Siberian name for a squirrel.

Figures

FIGURES 133–138. Scutisotoma baica sp.n. 133, posterior view of furca and female genital field; 134, dens, anterior; 135, dens, lateral; 136, tibiotarsus 1; 137, tibiotarsus 3; 138, chaetotaxy of Abd.V (s: accp-sensillum).