Subisotoma pusilla ( Schäffer, 1900 ), Schaffer, 1900

Potapov, Mikhail, Babenko, Anatoly, Fjellberg, Arne & Greenslade, Penelope, 2009, Taxonomy of the Proisotoma complex. II. A revision of the genus Subisotoma and a description of Isotopenola gen. nov. (Collembola: Isotomidae), Zootaxa 2314, pp. 1-40 : 8-10

Subisotoma pusilla ( Schäffer, 1900)

Figs 6 , 15–16 , Tab. 2

Isotoma pusilla Schäffer, 1900: 254

Proisotoma pusilla ( Schäffer, 1900) : Gisin 1942, Folsomides pusillus ( Schäffer, 1900) : Gisin 1960; Clavisotoma pusilla ( Schäffer, 1900) : Palissa 1964

Material. Female and 2 males, France, Harroque, Gresigae Forêt, Quercus ilex , 25.xi.1973, leg. L. Deharveng (AF); 15 specimens, France, Lot, Labastide–Marinhac, oak forest, litter, 26.x.1979, leg. L. Deharveng ( MNHN); 10 specimens, Poland, Sudetes, Wapniarka Mt., mosses on rocks, 23.ii.2003, leg. D. SkarŻyński (WU); 11 specimens, Ukraine, L’viv Province, Kolodrubi village, flood-plain oak-forest, mosses on bark, 04.x.2002, leg. I. Kaprus’ (LPM).

Redescription. Size 0.9–1.0 mm. Colour dark, bluish-black with numerous light spots, ventral side and extremities lighter. Abd.VI small, partly hidden under Abd.V. Cuticle always with unequally sized hexagons, largest ones (pits) vary in size from small to large depending on population (as on Figs 21 , 27 ).

Ocelli 8+8, G and H clearly smaller. PAO elliptical, wide, without constriction, about 1.5–2.0 as long as ocellus diameter and 0.7–0.9 as long as U3. Maxillary outer lobe with simple maxillary palp and with 4 sublobal chaetae (abnormal variability was detected in a single specimen). Labral formula as 2/554. Labium with all five papillae (А–Е) present, papilla E without guard chaeta e7. Proximal part of labium with 3 chaetae, basomedian field with 4 chaetae. Ventral side of head with 4+4(5) postlabial chaetae. Ant.1 with 2 bms, dorsal and ventral, and 2 ventral sensilla (s). Ant.2 with 3 bms and 1 laterodistal s. Ant.3 with 1 bms, 5 distal s ( AO and one lateral s) and 3(4) additional sensilla on its dorsal side. Sensilla on Ant.4 hardly differentiated, subapical organite small, microsensillum present.

Tergal sensilla only slightly shorter but clearly finer than ordinary chaetae. Sensillar formulas 44/33334 (s) and 10/101 (ms) ( Figs 6 , 15 ). Macrosensilla on Abd.I–III set between p-row and the next row of chaetae. All terga with dense cover of short, uniform chaetae, macrochaetae not developed. Largest chaetae on Abd.V about 0.2 times as long as this tergite length. Dorsal axial chaetom of Th.II–Abd.III as 8–9,8–9/7,6–7,7. Sternum of Th.III with 2+2(1) chaetae.

Unguis usually simple, without inner tooth. Unguiculus with narrow basal lamella. Ti.1–3 with 21, 23–24, 25–26 chaetae respectively. According to Fjellberg (1993) reproductive males with modified chaeta x on Ti.3 (we have not seen mature males in our material). Tibiotarsal tenent chaetae (1–2–2) long and clearly clavate (U3: t.ch. = 0.6–0.7). Ventral tube with 4+4 laterodistal and 5–7 caudal chaetae. Tenaculum with 3+3(2) teeth and one chaeta. Anterior furcal subcoxae with 10–11 chaetae, posterior ones with 7–10. Anterior side of manubrium without chaetae, posterior side with about 20–22 chaetae on the main part and usually with 4+4 chaetae on laterobasal lobes. Dens with one subapical chaeta anteriorly and (3)4 subequal posterior chaetae. Mucro rather long, with two teeth, the subapical one usually strongest (but only one tooth was mentioned by Palissa (1964). Ratio of manubrium: dens: mucro = 5.2–6.5: 3.0–3.7: 1 ( Fig. 16 ). Each anal lobe with 3 minute equal setulae.

TABLE 2. Morphological characteristics and distribution of species of genera Subisotoma and Isotopenola .

Affinity. Although we have not seen type material of S. pusilla or specimens from the type locality (Urach, Germany), it is highly probable that our samples of this widespread European species are true S. pusilla . It is easily distinguished from all known species and the new species described below by the presence of four macrosensilla on the terga of Th.II–III, two of which are associated with the p-row (accp-sensilla) and the other forms a lateral group (al-sensilla). Our understanding of this species is in a full agreement with morphological redescription given by Potapov (2001). The sensillar formula 54/32434 (s + ms) given for the species by Deharveng (1979) and Luciáñez & Simón (1994) indicates that more than one species might be included under this name in Western Europe. The specimens studied by us all have formula 54/43434 [44/ 33334(s) + 10/101(ms)].

Distribution. Known from most of Europe. According to Potapov (2001) it has been recorded from Germany, France, Italy, Poland, Czechia, Slovakia, and Hungary. Some records need verification as at least one closely related species ( S. pomorskii sp.n., see below) occurs in eastern parts of Europe.

Figures

FIGURES 1 – 6. Sensillar chaetotaxy of Subisotoma: 1, Subisotoma tenuis, Subisotoma bisensillata sp. nov. (3,3 / 2,2,2,2,4); 2, Subisotoma guzeriplica sp. nov. (3,3 / 2,2,2,3,4); 3, Subisotoma pomorskii sp. nov. (3,3 / 2,2 – 3,3,3,4); 4, Subisotoma homonomica sp. nov. (3,3 / 3,3,3,3,4); 5, Subisotoma quadrisensillata (3,3 / 3,3,3,4,4); 6, Subisotoma pusilla (4,4 / 3,3,3,3,4).

FIGURES 15 – 16. Subisotoma pusilla: 15, chaetotaxy of Th. III and Abd. I; 16, ventral chaetotaxy of Abd. II – V (tenaculum not shown), sf – II – setaceous field on Abd. II.

FIGURES 17 – 23. Subisotoma pomorskii sp. nov.: 17, chaetotaxy of Th. III, Abd. I and II; 18, dens and mucro, lateral view; 19 – 20, ocelli and PAO, two views; 21, fine structure of cuticle near ocelli D and H; 22 – 23, Ant. 3, dorsal view (variation).

FIGURES 24 – 27. Subisotoma homonomica sp. n.: 24, chaetotaxy of Th. III and Abd. I; 25, Ant. 3; 26, dens and mucro; 27, ocelli and PAO (area of ocelli D and H shown in more details).