Species of the genus Folsomia (Collembola: Isotomidae) of northern Asia

This paper deals with the taxonomy and distribution of Folsomia species from northern Russia with special reference to the Asian regions. Eight new species are described: F. amplissima sp. n., F. ancestor sp. n., F. atropolaris sp. n., F. borealis sp. n., F. brevisensilla sp. n., F. cryptophila sp. n., F. iongidens sp. n., /•'. palaearctica sp. n. Five species, F. taimyrica, F. regularis, F. sp. aff. aitamontana, F. alpha, and F. janstachi (nom. n. for lsotomina gracilis Stach, 1962 nec Folsomia gracilis Latzel, 1922), are re­ described. F. binocidata (Wahlgren) has been recovered and rcdescribed. The positions of F. macrochaetosa, F. magadani, and F. tesari have been defined more exactly on the basis of type material. An identification key of the northern Asiatic species of the ge­ nus is given.


INTRODUCTION
Folsomia is among the largest genera of the Isotomidae.The members of the genus predominate in most collcmbolan communities of the Palaearctic, especially in the northernmost areas.Nevertheless, the taxonomy of the species inhabiting the Russian Arctic remains unclear, since more attention has been paid by Russian taxono mists to Middle Asia and southern regions of Siberia.
The present paper is concerned with the fauna and dis tribution of Folsomia species in the tundra and partly taiga zones of Russia with special reference to the Asian regions.It is based on a large quantity of material col lected during several expeditions to the Arctic by the TAXONOMY AND DISTRIBUTION 31 Folsomia species have been found in the Asian re gion of the northern Palaearctic, including 8 new to sci ence.They can be split into several groups based on the sensilla position on the terga and some traditional charac ters.
Group A: with medial accp-sensilla far in front of prow of dorsal chaetae.

The F. taimyrica complex
This is characterized by at most 2+2 ommatidia, 1 + 1 chaetae on the anterior side of the manubrium, and the same arrangement of sensilla as the F. quadrioculata complex, but differs from the latter by having more chae tae on the lateral flaps of the ventral tube (4+4 vs. 3+3) and in the central part of the posterior side of manubrium (4+4-6+6 vs. 2+2).The presence of 2 lateral sensilla on Ant.Ill is also characteristic of all the northern members of the complex.
Even in the proposed restricted diagnosis, the species remains morphologically variable.Our material includes two main forms.As specimens with intermediate charac ters do also exist, both of them are treated as F. taimyrica here: Form 1.The largest macrochaetae on abdominal end are 4-5 times longer than mucro.Unsetaceous band on Abd.V granulated (Fig. 15).Rarely it has a small unpigmented ommatidium.Includes the studied paratypes.
Distribution.Possibly a circumpolar species (Fig. 105), but Nearctic records should be verified.
F. taimyrica f. 1 has a more southern distribution, re stricted to the tundra zone, no records from the true polar desert.Recorded from Yamal Peninsula to the Wrangel Island in Russia, but seems to be absent in the eastern part of the mainland.
F. taimyrica f. 2 is one of the commonest forms on the High Arctic archipelagos (from Spitsbergen to Novosi birsk Islands).Also recorded from the northern belt of the tundra zone, for instance on the Taimyr Peninsula.
The measurements of the holotype are given in Table 2.
Affinities.The new species resembles F. taimyrica in having microsensilla on all terga from Th. II to Abd.Ill, but differs in having only 1 + 1 macrochaetae on Th.Ill (the medial ones are not differentiated) and the presence of a large ommatidium.
Distribution.The species has the most southern distri bution area among the members of F. taimyrica complex (Fig. 105).Widely distributed, inhabiting various commu nities of the northern taiga and tundra of both Asian and European parts of Russia, but no records from the Kola Peninsula or Karelia.
Description.Body length up to 1.8 mm.White, with some grains of black pigment scattered mostly on head and last abdominal segments.2+2 small ommatidia, the posterior cornea smaller and hardly observable (Fig. 19-22) (see also Affinities).Region of ommatidia usually pigmented.PAO narrow, clearly constricted in the middle, with inner "denticles", distinctly longer than width of Ant.I. Maxillary palp bifurcate, outer maxillary lobe with 4 sublobal hairs.
Affinities.The new species can be easily distinguished from F. laimyrica and F. borealis by the absence of microsensilla on Abd.II-III.Besides, F. amplissima has 2+2 ommatidia, no medial unsetaceous band on Abd.V (Fig. 23), and longer macrochaetae.Due to the 2+2 om matidia and a single pair of anterior chaetae on manu brium F. amplissima can be confused with F. quadrioculata, which, however, belongs to another spe cies group characterized by fewer chaetae on the lateral flaps of the ventral tube (3+3 chaetae vs. 4+4) and on the central part of posterior side of manubrium (2+2 vs. 4+4-6+6).Habitually the former differs from pale speci mens of F. quadrioculata by having less diffuse colour pattern and larger pigment grains.
In the material from the westernmost part of the distri bution area of F. ctmplissima we found some populations without pigment in the ocular area and without distinct cornea, but with only irregular primary granulation in their places (open circles on Fig. 105).Their status calls for further study.
Distribution.Widespread in the Arctic and Subarctic from the western slope of Ural Mts range to Chukotka, but seems to be more numerous in the eastern part of the area (Fig. 105).
The F. quadrioculata complex The members of this complex are very similar to the previous group but have fewer chaetae on the lateral flaps of ventral tube (3+3 chaetae vs. 4+4 in F. taimyrica com plex) and in the central part of the posterior side of manu brium (2+2 vs. 4+4-6+6).Four species of this complex are permanent inhabitants of the northern regions of the Palaearctic, namely F. quadrioculata, F. manolachei, F. palaearctica sp. n., and F. sp. n. aff. palaearctica.The two latter species were previously recorded from the northern Palaearctic as F. diplophthalma which is now re defined as a member of the F. sexoculata complex (Pota pov & Dunger, in press).
Folsomia quadrioculata (Tullberg, 1871) Affinities.We separate F. quadrioculata from F. ma nolachei using the characters proposed by Deharveng (1982).Unfortunately after his paper the problem of a broad diagnosis of F. quadrioculata has been replaced by the problem of a broad diagnosis of F. manolachei, which is usually listed as "manolachei complex" in recent publi cations.
It is often considered that F. quadrioculata has a pref erence for colder and wetter habitats than F. manolachei.According to our data, F. quadrioculata is indeed much more frequent and abundant in the northern areas of the Palaearctic, but as a whole, the distribution areas of both species in the Arctic are strongly overlapping.
Distribution.Holarctic.Almost all over the northern part of Russia, excluding some of the High Arctic archi pelagos.More common in the southern tundra subzones.Spitsbergen, Novaya Zernlya, central parts of Taimyr and Wrangel Island are the northernmost areas of its distribu tion.Bagnall, 1939sensu Deharveng, 1982 Affinities.The species often predominates in many southern areas of Russia and is also rather common in the Arctic.This fact supports the opinion that F. manolachei is a complex of several morphologically hardly distin guishable species.

Folsomia manolachei
Distribution.Scattered records all over the northern Palaearctic, much more frequent in the western part.Not found in the High Arctic and, unlike F. quadrioculata, it is not recorded from Spitsbergen (Fjellberg, 1994), No vaya Zernlya and Wrangel Island.
coll.AB; Loc. 63 Habitually it can be confused with pale specimens of F. manolachei, from which F. palaearctica differs by hav ing only 1 + 1 ommatidia, no basal ms on Ant.Ill, and a "corner" sensillum on Th.II always in front of the p-row.
Distribution.Finland, northern regions of European part of Russia and almost all Siberia.In Siberia it is one of the commonest species in both Arctic and boreal com munities.Found also in Arctic Canada (Devon Isl.), but seems to be absent in Spitsbergen and in the easternmost parts of the Russian North (Wrangel Isl., Chukotka).
Affinities.The species is described in separate paper (Potapov & Dunger, in press).It can be defined by: 1 + 1 ommatidia, 1 + 1 anterior chaetae on manubrium, very long postantennal organ (Fig. 26), ms on lateral part of Th.Ill present (Fig. 25), basal ms on Ant.Ill missing.The last character separates it from all the relatives (F.quadrioculata, F. taimyrica, etc.) except F. palaearctica.It can be distinguished from the F. taimyrica complex by the 2+2 chaetae on the central part of posterior side of manubrium and 3+3 lateral chaetae on ventral tube, and from F. quadrioculata and F. manolachei by the 1 + 1 api cal chaetae on posterior side of manubrium and only one ommatidium.Habitually F. sp.n. aff.palaearctica is rather similar to F. borealis, the smallest species of the F. taimyrica complex.
Distribution.Northern parts of the Russian Plain and some scattered records from the central (Putorana Plateau) and southern Siberia.It is known also from Mon golia.

The F. regularis complex
The morphology of the members of this complex is within the limits of the traditional understanding of F. regularis (1 + 1 ommatidia, from 2+2 to 7+7 (usually 3-5) chaetae on the anterior side of manubrium).They are also characterized by a quadrioculata-like arrangement of sensilla, by the chaetotaxy of the posterior side of manu brium resembling that of the F. taimyrica complex (Fig. 39), and by the presence of a single lateral sensillum on Ant.Ill (F.regularis from Devon Isl.exceptionally show a variation in the latter character).
Below we fix the positions of F. regularis and F. ozeana, recover F. binoculata and describe a new species of the complex.All these species have previously been listed as F. regularis from different parts of the Holarctic.Hammer, 1953 Material.AR: WRANG Loc. 20, 4 ex., coll.AB; NWT Loc. 22, 22 ex., coll.AB; Loc. 23,14 ex.,coll. AB.Affinities.The species was described from Ellesmere Island (Arctic Canada) and subsequently recorded all over the Arctic and in more southerly regions (in East Asia as a senior synonym of F. ozeana).Our material from Ellesmere Island and Devon Island indicates the presence of 2+2 macrochaetae on Th.Ill in Canadian specimens, whereas Palaearctic F. regularis always has only 1 + 1 macrochaetae on Th.Ill (medial macrochaetae are not differentiated).Our new definition of the species considerably restricts its area of distribution.F. regularis s. str.can be characterised by two macrochaetae on each side of Th.Ill (Fig. 37 and 38) and 4+4 latero-distal chae-tae on ventral tube, white colour, and by the presence of a basal microsensillum on Ant.III.
F. regularis is very similar to the northern European species -F.agrelli Gisin, 1944.The latter differs only in the lower number (2+2) of anterior chaetae on the manu brium.Hammer (1954), however, indicated considerable variation of this character in F. regularis (from 2+2 to 4+4), which is in agreement with our material (Fig. 40).We have studied several specimens of F. agrelli from Norway (Oppland) and found a few specimens with an abnormal number (3+2 or 3+4) of anterior chaetae on the manubrium.Possibly, F. agrelli and F. regularis belong to the same variable species.
Variability.The above redescription is based mainly on the largest form of this species (form 1).It frequently occurs together with another form, which is even more numerous and common for the Russian North.These specimens (form 2) are distinctly smaller (0.8-1.0 mm), with darker and diffuse pigmentation (Fig. 54), and their macrochaetae are shorter, 2.7-3.9 times as long as mucro (cf.Figs 51 and 52).As a rule, manubrium bears more an terior chaetae, usually 4+4-5+5, the posterior side of the ventral tube usually has a smaller number of chaetae (5-7).However, the variability of the characters overlaps.
The two forms can easily be distinguished in mixed populations by coloration and body size.Their status and relationships call for further investigation.Both of them are referred as F. binoculata here.
Distribution.Widely distributed all over the Arctic.F. binoculata f. 1 has been recorded in various polar desert and Arctic tundra communities on the High Arctic archipelagos, found also in the northern Taimyr.
F. binoculata f. 2 is more common and widespread in the Russian Arctic (from Franz Josef Land to Chukotka).Inhabits all the tundra subzones penetrating into the polar deserts as well.The most inland record: alpine zone of Verkhoyansky Mt.Range.No records from Spitsbergen and Wrangel Island.
Affinities.As a member of F. regularis complex, F. atropolaris is characterized by 1+1 ommatidia and the usual number of anterior chaetae on manubrium and dens, but can easily be distinguished by its dark colour, shorter dorsal sensilla on the terga and by the absence of a basal microsensillum on Ant.III.Weak differentiation of cor nea is also characteristic.It also differs from F. regularis and F. binoculata by having only 3+3 latero-distal chae tae on the ventral tube.
Variability.Specimens from the northern areas differ from the main form by smaller size and shorter macrochaetae.The ratio of the largest abdominal macrochaetae : rnucro is 3.8-4.9 in the main form of F. ozeana (Kurile Islands), whereas it is only 2.1-3.4 in the northern form (Wrangel Island).As no other differences are observed, the status of this north ern form remains unclear pending further investigation.As a whole, it is very similar to F. binoculata f. 2 but has only 3+3 latero-distal chaetae on the ventral tube.In a mixed population they can be separated by body colour: grey in F. binoculata f. 2, and yellowish-white in Fol somia sp.aff.ozeana.
Distribution.Widespread in the eastern part of the Russian Arctic (from Taimyr to Wrangel Isl.).Most re cords are from the tundra zone, but found also in taiga.

The F. sexoculata complex
Five species which can be considered as representatives of this group have been found in our northern material.Very short dorsal chaetae and a particular arrangement of sensilla [Ant.I always with 3 sensilla (Fig. 65), medial sensillum on Abd.II-III is between Mac2 and Mac3, that on Abd.IV situated in anterior part of the tergum] is char acteristic of all of them.
Distribution.For the time the west of Taimyr Penin sula is the easternmost record of this species.It lives along the sea coast or banks of lakes and streams met with river deltas.(Axelson, 1902) Material.EUR: Loc. 27
Distribution.Probably widespread in the northern re gions of the European part of Russia and Siberia: re corded from Karelia to Yana Delta.In the Arctic it prefers sites enriched with organic material (debris in the supralittoral, mammal nests), southwards it was also recorded in litter from damp woods.All previous records of F. di plophthalma should be verified.Folsomia cf.diplophthalma (Axelson, 1902) Affinities.This form is very similar to the previous species but differs in having more anterior chaetae on the manubrium (from 7+7 to 9+9) and less chaetae (9-12) on the anterior furcal subcoxa.Its species status is doubtful.

Affinities.
Well distinguished species because of the two contrasting large black eye spots on each side of head (Fig. 73), but specimens with reduced or absent eye pig ment are rather common in some populations.The ante rior chaetotaxy of the manubrium (adults) varies from 2+2 to 3+3.The variant with 2+2 chaetae is not very common, but specimens with 3+2 chaetae occur with the same frequency as 3+3.F. microchaeta can easily be dis tinguished from F. diplophthalma and associated species by the 2+2 equally large ommatidia and few chaetae on the manubrium.They can also be separated by the length of the sensilla on Th.II: contrary to F. diplophthalma and F. similis, the sensillum near the hind corner of the seg ment is always slightly shorter than the common chaetae in F. microchaeta (cf.Figs 77 and 78).
Distribution.Probably widespread in the Arctic and mountainous regions of Russia, recorded from Kolguev Isl. to Yakutia.Rather rare species, indicated only from nival slopes and tundra sites.
On the other hand, the only distinct difference between F. sp.aff.altamontana and F. microchaeta is their col oration.Other subtle differences are as follows: sensillum near hind corner of Th.II is within (F.microchaeta) or in front of (F.aff.altamontana) the p-row of chaetae (Figs 77,76), the second lateral chaeta on the ventral lube is out of (F.microchaeta) (Fig. 74) or in a line with the other chaetae (F.aff.altamontana) (Fig. 68).
Distribution.Arctic and nival communities of central Siberia.

The F. funetaria complex
This group includes species without broad sensilla on abdomen, the sensilla on the abdominal terga are within the p-row, and ventral chaetae on Th.Ill are usually miss ing.Five species of the complex have been recorded in the northern regions of Russia.

Affinities.
A well defined species due to the few chaetae (2+2) on anterior side of the manubrium which is rare in the F. fimetaria complex.The number of anterior chaetae on the dens is rather variable, ranging from 12 to 19, usually 13-15.Specimens with asymmetric arrangement, e.g.12+15, 13+18, are also common.
Distribution.The species was described from Jan Mayen and has been recorded from all over the Arctic (circumpolar).In real tundra it usually prefers the warm est and richest biotopes, southwards it can be found only in bogs and other damp sites.
Name derivation.Named after type habitat (crypta -covered passage in Latin).

Affinities.
The new species differs from F. bisetosa by the presence of only 2 prelabral chaetae and the absence of the microsensillum on Abd. I (cf. Figs 79 and 80).The former character was never found in Folsomia species be fore.Specimens of F. bisetosa, including those from the type locality (Jan Mayen) always have 4 prelabral chaetae and the microsensillum on Abd. I present (A. Fjellberg, pers. comm.).
Distribution.Known only from the type locality.
Folsomia sp.aff.Stella Grow & Christiansen, 1976 Material.NESIB: Loc. 57,3 ex.,coll. MSPU.Affinities.This species, with 3+3 anterior chaetae on the manubrium and the presence of ventral chaetae on Th.Ill, resembles species of the F. macrochaetosa complex, differing from both F. macrochaetosa and F. brevisensilla by the absence of medial macrochaetae on Th.II-III.As a whole it fits the description of F. Stella, but has only 2 sensilla on Ant.I (vs. 3 sensilla in the original descrip tion).This form has been already recorded from northern regions (Norway, Spitsbergen, Alaska) (Christiansen & Bellinger, 1980;Fjellberg, 1988).Nevertheless, its status remains unclear to us as it is hardly possible from the eco logical point of view to find this southern cave species (the types originate from Iowa) in the Arctic.
Distribution.Only one record from the East Palaearctic.

The F. macrochaetosa complex
The species of this group differ from the F. fimetaria complex by having 2+2 macrochaetae on both Th.II and Th.Ill, and a quadriociilata-Yike PAO.In addition, the ventral chaetae on Th.Ill is present, which is uncommon for the F. fimetaria complex.The group includes two northern species: F. macrochaetosa and F. brevisensilla sp.n.F. inocúlala Stach, 1947 also belongs to this group.It is widespread in Russia (the Caucasus, Siberia, and Far East), but not in the Arctic regions (middle stretch of Yenisei River is its northernmost record).Martynova, 1977 Material.EAST: Loc. 64,3 ex. (paratypes), coll.ZMAS.

Folsomia macrochaetosa
Distribution.East Palaearctic.Rather common in Sub arctic forests (Kamchatka, unpubl.)but has not been found in the tundra zone yet.
Affinities.The new species differs from F. macrochae tosa by the short sensilla, the absence of the nricrosensillunr on Abd. I (cf. Figs 85 and 86), and fewer (14-17) anterior chaetae on dens.The most characteristic feature of the new species is the absence of chaetae in the middle part of posterior side of dens.Such condition was known only in some Folsomici species with shortened furca, e.g.F. brevicauda Agrell, 1931or F. dovrensis Fjellberg, 1976.This feature makes it possible to separate the new species from F. Stella, which, according to Fig. 6, F of "paratype 1" in Christiansen & Tucker (1977) bears two chaetae at the middle part of posterior side of dens.
Distribution.Only two records: Lower (southern tun dra) and upper (mountainous forest) Kolyma River.

The F. sensibilis complex
Several species having 2+2 strongly broadened sensilla at the end of abdomen have been described from different parts of the Holarctic, viz.: F. sensibilis Kseneman, 1934(Carpathians), F. tesari Dunger, 1970 (the Sudeten), F. gracilis (Stach, 1962) (Spitsbergen), F. alpha Christian sen &Tucker, 1977 (Alaska), andF. magadani Marty nova in Martynova, Berman &Chelnokov, 1977 (Russia: Magadan Region), lsotoma coeruleogrisea Hammer, 1938 (Canada: NWT) probably also belongs here.All of them have a long furca, many chaetae on anterior side of manubrium (excluding F. sensibilis), and no ommatidia.We have found that each side of Abd.V bears 6 sensilla, three slender medial, two leaf-or stick-like in more lateral positions, and one chaeta-like ventrolateral (Fig. 98).The dorsal side of Abd.VI also has some sensillum-like chae tae.These species form a natural group which should probably be removed from the genus Folsomia in future.Our material from the northern regions includes 4 forms of the complex, two of them are described below as new to science.
The main differences of the species of this complex are summarized in Table 1.Only three of them (F.ancestor, F. sensibilis, F. longidens) are well distinguished from the relatives.The other members of this group having two outer chaetae in the middle part of dens and an incom plete sensorial set, combine poorly separable taxa which require further study.Kseneman, 1934 Material.EUR: Loc. 25,3 ex.,coll. MSPU; SOUTH: Loc. 76, 1 ex., coll.MSPU; Loc. 78,1 ex.,coll. MSPU.Affinities.F. sensibilis differs from all the other spe cies of the group by having few chaetae on anterior side of dens (2+2, rarely 3+3).Chaetotaxy of lateral parts of the furca (only one lateral chaeta on each side of manu brium and dens) is also specific and shared only with F. longidens sp.n. (see Affinities of the latter).

Folsomia sensibilis
Distribution.Northern records of this species are re stricted to the western Palaearctic: Kola Peninsula, Fin land (Vilkamaa, 1989), Norway (Fjellberg, 1988).Not yet found in the true tundra.

Affinities.
Two main features distinguish this species from the relatives: Body pigmentation and the posterior position of sensilla on the terga.Unfortunately the latter feature is unstable both in our material and in the type population (Grow & Christiansen, 1976).F. tesari is also pigmented but differs by having only one chaeta on each of the lateral sides of the manubrium and considerably larger pigment grains on the body.See also Affinities of F. janstachi and F. magadani.
Affinities.Large collection of this species from Hornsund, southern Spitsbergen (the type locality of Isotomina gracilis) allowed us to ascertain the precise taxonomic position of this species.Fjellberg (1984) sharply defined this form under the name Folsomia gracilis (Stach, 1962) and considered F. alpha as its junior synonym. Later Fjellberg (1986) began to use the name alpha instead of gracilis which was preoccupied.We prefer to keep both forms as separate species and thus we propose the new re placement name janstachi, in memory of Jan Stach who originally described it.F. janstachi and F. alpha can be distinguished by different body pigmentation (absent vs. present) and the position of sensilla on Abd.I-III (ante rior to p-row vs. inside p-row), although the latter charac ter is not stable in F. alpha.
We studied a paratype of Folsomia magadani from "Magadan Region, Snezhnaya Valley" (coll.ZMAS).The specimen differs from F. janstachi in having no ventral chaetae on Th.Ill and more slender sensilla on Abd.V. Obviously the position of F. magadani calls for further study but so far it is known only from the type locality and is absent in our materials.
Chaetotaxy of the lateral side of the manubrium and dens of the new species mostly resembles that of F. sensi bilis which differs from F. longidens by having ventral chaetae on Th.Ill present, and a lower number of anterior chaetae on the manubrium (4-6 vs. 10-12 in F. longi dens).
Distribution.Unclear: It was recorded in two separate regions -western (from Kolguev Island to Taimyr Penin sula) and eastern (Chukotka) but seems to be absent in Novosibirsk Islands (Fig. 105), where F. alpha is common.
T y p e m a te ria l.Holotype: 9 (slide), labelled "Magadanskaya region, upper Kolyma, ca 60 km W from Sinegorie vi 1., Ken- 1 -body length; 2-5 -length of 12dens;     The measurements of the holotypes of the new species are given in Table 2.

Distribution .
Described from floodlands of the south ern Siberia.The most northern records are from the north ern taiga.

F
Fig. 105.Distribution of species of F. sensibilis and F. taimyrica complexes (data for Spitsbergen mainly based on Fjellberg, 1994).

Affinities .
The new species is characterized by having the most complete set of sensilla among the members of the F. sensibilis complex (Figs 99, 103).All the others have a reduced number of sensilla on Th.II and Abd.IV.Three sensilla on Ant.I and stick-like (instead of leaf-like) sensilla on Abd.V are also unique characters.Distribution.Known only from the vicinity of the type locality.

Table 1 .
Diagnostic features of the species of F. sensibilis complex.
* The morphological peculiarities of F. tesari and F. magadani were confirmed by our examination of ten syntypes of the former species from the Sudeten Mts (Heuluder, Braunberg, Maly Staw) and a paratype of the latter from Magadan Region (settlement of Magadan, Snezhnaya Valley).

Table 2 .
The lengths of extremities, chaetae and sensilla in the holotypes of the new species.