Subisotoma Stach, 1947

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 : 12-15

Subisotoma Stach, 1947

Type species: Isotoma pusilla Schäffer

Diagnosis. Anurophorinae with all abdominal segments clearly separated and a Folsomides-like furca: manubrium without anterior chaetae, dens smooth with 1 anterior and 4 posterior chaetae at most, mucro not clearly set off from dens. Tergal sensilla on abdomen situated in or near p-row of chaetae. B-row of chaetae on Ti.1–2 complete (unpaired B absent).

4/5

Known species characterised by 8+8 ocelli, full set of bms on Ant.1–3, simple maxillary palp with 4 sublobal hairs, reduced number of guards on labial papilla E (at least е 7 absent), only two prelabral chaetae, number of microsensilla on tergites usually reduced (fewer than 11/111), ventral chaetae on Th.II–III present or absent, macrochaetae hardly differentiated, VT with 4+4 laterodistal and 4–5 posterior chaetae, tibiotarsi often with more than 21-21-25 chaetae, tenent hairs (1-2-2) clavate or pointed, unguis with or without tooth.

Discussion. We accept here the strict definition of Subisotoma proposed by Fjellberg (1993). As a result only three species ( pusilla Schäffer , asiatica Martynova , and tenuis Dunger ) out of 10 Palaearctic species assigned to the genus by Potapov (2001) are in agreement with the above diagnosis. Species of the veletensis -group (veletensis Steiner, gisini Gama, meridionalis Dallai , corsica Poinsot & Barra , and navacerradensis Selga) are obviously more related to members of the genus Ballistura (see below), whereas variabilis Gisin and stepposa Martynova belong to the genus Scutisotoma Bagnall which is redefined below.

Nevertheless, the “reduced” Subisotoma seems to be rather diverse in eastern Palaearctic.

There are at least 7 undescribed species in our collection, mainly from the Asian part of Russia.

Two distinct species-groups may be distinguished within Subisotoma s.str., differing in general appearance and some morphological features. The first, pusilla -group, includes rather large and dark species which are characterised by a dense cover of short undifferentiated chaetae, by the presence of ventral chaetae on Th.3 and some additional sensilla on Ant. 3 in both males and females. Tibiotarsi have long, clearly clavate tenent hairs and increased number of proximal chaetae. Furca rather long and slender. Species of the second, asiatica- group, are smaller and lighter with sparser pilosity on the tergites, macrochaetae clearly differentiated. Also the absence of ventral chaetae on thorax and additional sensilla on Ant.3 is characteristic, as well as pointed tenent hairs and a reduced number of chaetae on Ti.3.

Ballistura B ö rner, 1906

Type species: Isotoma schoetti Dalla Torre

Syn.: Clavisotoma Ellis, 1970: 21 (type species: Proisotoma tuberculata Stach, 1947: 218 )

Diagnosis. Anurophorinae with all abdominal segments clearly separated. Furca varies from long with strong mucro and a cylindrical dens which is almost completely covered by chaetae, to a Folsomides-like furca with reduced mucro. Manubrium always without anterior chaetae. Dorsal tergal sensilla are situated near p-row of chaetae. Inner sides of Ti.1–2 with chaetae B 4/5 present.

Ocelli 4–8 on each side of head, PAO usually small and oval. The bms on Ant.1–3 often elongated and hardly distinguishable from ordinary chaetae. Maxillary palp bifurcate with 3 sublobal hairs. Number of guards on labial papilla E reduced (e 7 absent). Prelabral chaetae 2. Number of tergal sensilla often increased, especially on Abd.IV–V. Ventral chaetae on thorax absent. VT with 4+4 or more laterodistal chaetae and, usually with only a single pair of posterior chaetae. Tenent chaetae 1-2-2, usually pointed, rarely clavate. Unguis usually simple without tooth.

Discussion. The absence of anterior chaetae on manubrium and the presence of medial inner chaeta on Ti.1–2, as well as the number of sublobal hairs on maxillary outer lobe and posterior chaetae on VT, clearly point to a relationship between Ballistura and Folsomides . Also the furca of the representatives of these two genera may be quite similar. The only significant differences between these genera are the position of the sensilla on the abdominal tergites (mid-tergal in Folsomides and near p-row in Ballistura ) and the increased number of laterodistal chaetae on VT in Ballistura .

Known species of the genus. Apart from the Palaearctic species of Ballistura listed by Potapov (2001), some traditional Subisotoma (veletensis, gisini, meridionalis , corsica , navacerradensis) also fit the above diagnosis.

Proisotoma B ö rner, 1901

Type species: Isotoma minuta Tullberg

Syn.: Hemisomia Bagnall, 1949: 92. (type species: Proisotoma guthriei Linnaniemi, 1912: 132 ) Folsomidiella Bagnall, 1949: 59 (type species: Folsomidiella inaequalis Bagnall, 1949: 59 )

Diagnosis. Anurophorinae with all abdominal segments clearly separated and a complete but relatively short furca. Manubrium with a pair of distal chaetae on anterior side. Sensilla on abdominal tergites are within the p-row of chaetae. The number of microsensilla reduced (10/000 or 00/000). B-row on Ti.1–2 complete.

Colour usually light, integument without visible granulation. Ocelli from 0 to 8 on each side. Ant.3 without bms. Maxillary palp simple, with 4 sublobal hairs. Labial papilla E with only 5 guards (e 4 and e 7 absent). Labral chaetotaxy 3/554. Sensilla (s) of Th.II–Abd.V 33/22224 or 44/33334 (sometimes 5 on Abd.V). Thorax often with ventral chaetae. Ventral tube with 4+4 laterodistal and more than two posterior chaetae. Chaetae x and B 5 on Ti. 3 in males hardly differentiated. Tenent chaetae 1-2-2 (A 1 on Ti.1, A 1 and A 7 on Ti.2–3) present, clavate or pointed. Unguis simple, without tooth. Dens with relatively low number of chaetae (4–6 anterior and 3–7 posterior), anterior chaetae extending to base. Mucro with 2–3 teeth, no lateral lamella.

Discussion. The main diagnostic character of the genus is the strictly posterior position of sensilla on abdomen and the partial reduction of the microsensilla. Under such a definition the genus comprises only those species which are morphologically — and ecologically — similar to the type species P. minuta . Virtually all included species are typical inhabitants of temporary organic debris.

The majority of other Palaearctic species which are traditionally assigned to Proisotoma ( armeriae Fjellberg , ananevae Babenko & Bulavintsev , beckeri Stebaeva , fjellbergi Dunger , oirota Vilkamaa , subarctica Gisin , tianshanica Martynova , and a number of Asian and Nearctic forms described below) have the abdominal sensilla clearly in front of the p-row and a complete set of microsensilla (11/111). These taxa are allocated to various other genera (see below).

Known species of the genus. P. clavipila (Axelson) , P. minima (Absolon) , P. subminuta Denis , P. coeca Gama , P. juaniae Luciáñez & Simón , P. brevidens Stach. Apart from these species, the following are probably also to be included (specimens have not been checked): P. dottrensi Gisin , P. fraterna Rusek , P. fungi Selga , P. najtae Selga , and P. oliveira Deharveng. In addition several new species in our material will be described in future papers.

There are at least two candidate genera for the “residual” species which fall outside our new strict definition of the genus Proisotoma : Scutisotoma proposed by Bagnall (1949) for Proisotoma titusi Folsom and Dimorphotoma Grinbergs 1975 with Dimorphiella muriphila Grinbergs as the type species. The main diagnostic character of the former is a strongly rugose integument on the tip of abdomen. Dimorphotoma was originally distinguished on the basis of clear sexual dimorphisms (development of spinelike macrochaetae in mature males). Our own study revealed that the only difference between S. titusi and our “residual” species is an increased number of tergal sensilla, a feature which probably developed independently in several lines of Anurophorinae . Apart from sexual dimorphism, D. muriphila is even more similar. We thus conclude that Dimorphotoma is a junior synonym of Scutisotoma , which is defined by the following diagnosis.