Review of the mite subfamily Arctoseiinae Evans with a key to its genera and description of a new genus and species from Siberia (Parasitiformes, Mesostigmata, Ascidae)

Abstract We redefine the subfamily Arctoseiinae of the family Ascidae, and describe a new genus, Maxinia gen. n., based on a new species, Maxinia arctomontana sp. n., whose adults display a combination of attri-butes uniquely different from other genera of the subfamily. The geographical range of Maxinia arctomontana is limited by arctic and mountain landscapes of Siberia. This description provides furtherdata on the arctic distribution and morphological diversity of the subfamily Arctoseiinae, which is unusually well represented in that region (26–83% in local gamasid mite faunas). Conceptual problems with the genus Iphidonopsis Gwiazdowicz, 2004 are reviewed, and a new combination, Iphidonopsis magnanalis (Ma & Yin, 1999) comb. n., is presented for Iphidozercon magnanalis Ma & Yin, 1999 from China. The genus Diseius Lindquist & Evans, 1965 is provisionally moved from the family Ascidae to the Digamasellidae. A new key to the genera of Arctoseiinae is presented.


introduction
The subfamily Arctoseiinae at present numbers ca 90 species (including 19 undescribed species at hand of the genus Arctoseius Thor, 1930), among which approximately one-third (30 species, 26 -Arctoseius) inhabit Arctic landscapes (Makarova 1999, in press). The separate local faunas within the Arctic include 5-16 arctoseiine species which constitute 26-83 % of the local gamasid mite diversity (Makarova 2012, in press). In this communication we account for a recently discovered, undescribed species with a unique combination of attributes that defies its placement in any of the five, currently recognized genera of Arctoseiinae Evans, 1963. We present a diagnosis of the subfamily, which argues for placement of the new genus in it, a description of the new genus, and detailed descriptions and illustrations of adult female and male of the new species. Previous confusion and disparity concerning the genus Iphidonopsis Gwiazdowicz, 2004 is considered. The monobasic genus Diseius Lindquist & Evans, 1965, previously allocated by various authors to either the Arctoseiinae or the Ascinae of the family Ascidae, is tentatively moved to the Digamasellidae.

Material and methods
Setal notation for the idiosoma follows Lindquist and Evans (1965), with some modification (Lindquist 1994). The leg and palpal chaetotaxy complies with Evans (1963Evans ( , 1964. The poroidotaxy and adenotaxy are given according to Johnston and Moraza 1991, with small modification (Makarova 2003). The length of all shields was measured along the mid-line, and the width at the broadest part (in case of sternal/sternitigenital shield, the width was accepted as the distance between gland openings gvb; see Fig. 15). The length of the epigynal shield was measured to include the anterior flap, and the greatest width at the posterolateral corners. The length of all legs and tarsi are given excluding the ambulacrum, and also its pedicel on leg I. The subcapitulum length was measured from its anterior margin excluding projections (internal malae, corniculi). The measurements were carried out on all available specimens and their limits are stated in micrometers (μm). The following ratios were used in the description (Makarova 2000):

lD/wD
the length-to-width ratio of the dorsal shield; lSt/wSt, lVA/wVA the length-to-width ratios of the sternal (in females) and ventrianal (in females and males) shields respectively; lCh/lD the ratio of the lengths of the movable cheliceral digit and the dorsal shield, %; lCh/lCo the ratio of the lengths of the movable cheliceral digit and the corniculus.
In the male description, the features common with the female are omitted. Holotype and most of paratypes are deposited in Zoological Institute, Russian Academy of Sciences (ZIRAS), St.-Petersburg; part of paratype series (6 females and 1 male from Suntar-Khayata Range) in Canadian National Collection of Insects and Arachnids, Ottawa (CNC).

taxonomy / Systematics
Subfamily Arctoseiinae Evans, 1963 The Arctoseiinae was first proposed by Evans (1963) in a rather informal manner, based on the results of his observations on the chaetotaxy of the legs among a wide variety of free-living Gamasina. It was based on a reduced chaetotaxy of the genu and tibia of legs II to IV, and "the nature of the chaetotaxy of the opisthonotal region of the dorsal shield in the female." A formal and more detailed description of the subfamily, including a key to its genera was presented by Lindquist and Evans (1965). From that description, the subfamily may be based on the following apomorphic characteristics within the family Ascidae: Dorsal shield of adults entire, with or without lateral incisions; opisthonotal region usually with maximum of four pairs of lateral S setae, S2 usually not added in change from larva to protonymph. Marginal (r-R) series of setae lacking r6 in podonotal region and often R6 posteriorly; marginal R setae on lateral soft cuticle on female, but variably on dorsal shield margin on male. Submarginal (UR) setae absent. Female sternal shield with third pair of lyrifissures on its posterior margin; fourth pair of sternal setae on soft cuticle. Sternal setae st5 usually on soft cuticle flanking female epigynal shield. Male sternitigenital shield free from, but sometimes abutting ventrianal shield, and often not fully integrated with endopodal strips alongside coxae III-IV. Maximum number of setae on genua I-II-III-IV, respectively, 12-10-8-7; on tibiae, 12-9-7-7; in change from protonymph to deutonymph, seta pd-3 not added to genu and tibia I, pl-2 not added to genu and tibia II, and al-2 not added to genua and tibiae III-IV.
Plesiomorphic attributes of the Arctoseiinae include the following: Gnathotectum basically bi-or tri-ramous, each process simple or denticulate; cheliceral movable digit bidentate in female, unidentate in male, without pointed process on mid-ventral surface, and with fringed arthrodial envelope basally; cheliceral fixed digit with small, setiform pilus dentilis, with usually few teeth restricted to apical half of masticatory surface, and without hyaline serrate rim near base on paraxial surface. Tarsus I usually with a conspicuously lanceolate-tipped seta dorsodistally. In change from protonymph to deutonymph, seta ad-3 added to genua I-II and tibia I, ad-2 added to tibia II, and al-2 added to tibia II.
Diagnosis. Adults of Maxinia are immediately distinguished from those of other arctoseiinae genera by having a broad, robust dorsal shield; by the female having an apomorphically expansive ventrianal shield that encompasses the metapodal plates anterolaterally and includes setae JV1 anteriorly; and by the male having an expansive sternitigenital shield that is fully consolidated with the endopodal strips beside coxae III-IV and abuts an expansive ventrianal shield. In addition, apomorphically, the subcapitulum has a narrow deutosternal groove, and the opisthogaster lacks a setal pair ZV1. None of the setae on tarsi II to IV are conspicuously differentiated by thinness, elongation, or curved shape. As in Arctoseius Thor, 1930 andIphidozercon Berlese, 1903, genu III carries only seven setae (lacking pv-1) on adults; however, adults of those genera have less robust, parallel-sided dorsal shields, retain opisthogastric setae ZV1, and their females have free metapodal plates and an anal shield. As in Zerconopsis Hull, 1918, adults have robustly sclerotized dorsal and ventral shields. However, adults of Zerconopsis have two to several pairs of dorsal shield setae (at least s4 and Z5) uniquely paddle-like in form, retain opisthogastric setae ZV1, eight setae (including pv-1) on genu III, and the female ventrianal shield is never expansive enough to include the metapodal plates and setae JV1; also, their tarsi II to IV have dorsal proximal setae ad-2, pd-2 conspicuously elongated and curved.
Idiosomal venter.Tritosternum with laciniae free for most of length (Fig. 17). Ventral shields well sclerotized and ornamented (Figs 1,2,12,22). Female sternal shield entire, continuous with well developed endopodal extensions between coxae I-II, but free from those between coxae II-III (Figs 13-16); endopodal extension between I-II with gland pore apically where approaching or abutting exopodal strip; sternal shield with three pairs of sternal setae and three pairs of poroids; setae st4 isolated on soft cuticle. Female with well developed endopodal strips alongside coxae III-IV. Female epigynal shield widened behind level of setae st5, its posterior margin broadly convex and nearly abutting ventrianal shield; setae st5 and paragenital poroids iv5 on soft cuticle; postgenital furrow with two pairs of small platelets. Female with expansive ventrianal shield encompassing metapodal plates anterolaterally and including setae JV1 anteriorly, along with other opisthogastric and circumanal setae, except JV5, ZV4 on soft cuticle; ventrianal shield with paranal setae inserted at level of anterior margin of anus, with gland pores gv3 on posterolateral margins, and with cribrum formed as partitioned strip behind level of postanal seta. Male with expansive sternitigenital shield consolidated with presternal platelets and endopodal strips alongside coxae I-IV and abutting expansive ventrianal shield. Male ventrianal shield similar in expansiveness, setation, and other structure to that of female (Fig. 22); setae JV5, ZV4 on soft cuticle, ZV5 absent. Peritrematal shield consolidated with exopodal strips behind coxae IV, with two poroids and one gland pore in area behind stigma, and a gland pore and poroid at level between coxae II-III; exopodal strip alongside coxae II-III usually fragmented.
Etymology. The name of the genus is tribute to the first author's spouse, Maxine Lindquist. Together with him for 55 years, she supported his acarological endeavors, accompanied him in field work throughout North America, and hosted many visiting colleagues at home. The name is feminine in gender.
Remarks. Among some of the characteristics that distinguish them from other taxa of Arctoseiinae, adults of Maxinia resemble those of the genus Neojordensia Evans, 1957, Ascinae, e.g., the relatively broad, well sclerotized dorsal shield, expansive ven-trianal shield encompassing the metapodal plates and insertions of setae JV1, absence of setae ZV1, expansive male sternitigenital shield, and the narrow rows of deutosternal denticles. However, adults of Neojordensia present dorsal shield setation and leg chaetotactic attributes typical of the subfamily Ascinae, and are characterized by different distinctive attributes, including absence of paravertical setae z1, absences of setae av-2 on genu I and ad-2 on tibia II, gnathotectum with convex, smooth anterior margin, female with most or all of r-R marginal series of setae on margins of dorsal shield, female sternal region with setae st1 on separate presternal plates, and female epigynal shield apomorphically widened to include the paragenital poroids as well as setae st5 Evans 1965, Athias-Henriot 1973 Description. Middle-sized dark-yellow or brownish mites with rather broad, somewhat pyri-form idiosoma (Figs 1, 2). Idiosomal shields well sclerotized, very finely punctate, with clearly reticulate ornamentation on nearly all surfaces except peritrematal plates. Many setae of body and appendages with fine, hair-like tips, which often are broken off. Dorsal shield rather broad, covering entire dorsal idiosoma, without lateral incisions.
Gnathosoma. Hypostome, gnathotectum (Fig. 33), chelicera similar to adults, setae hp-3 and pc absent. Legs. Leg chaetotaxy typical for larvae of Ascidae (Lindquist and Evans 1965). Etymology. The species name indicates its geographical range. Distribution. At present known from zonal and mountainous tundra landscapes of West, Middle and East Siberian sectors (Fig. 35).
Ecology. Recorded from dry and humid tundra sites, meadows including zoogenic ones (e.g. bird of prey outlooks) and shrub (Pinus pumila, dwarf Betula, Rhododendron) communities.
Variability. The species is rather stable morphologically. The variability concerns mainly the form of gnathotectum (26)(27)(28), the degree of coalescence of endocoxal and exocoxal elements around coxa II (Figs 13-16), sometimes the symmetrical or asymmetrical absence of setae s1, s2, z1, and the numbers of denticles in the rows on the deutosternum (usually 2-5 or 3-7).

Genus Iphidonopsis
In the following key, Iphidonopsis Gwiazdowicz, 2004 is treated in the original sense (Gwiazdowicz 2004) to include I. sculptus Gwiazdowicz, 2004 and Iphidonopsis mag- nanalis (Ma & Yin, 1999) comb. n. Subsequently, Gwiazdowicz (2007) broadened his concept of Iphidonopsis to accommodate other species (e.g., Lasioseius pulvisculus Berlese, 1921, Seiulus minutus Halbert, 1915 and thereby distinguished his genus from Arctoseius Thor, 1930, primarily on the dorsal shield of adults having lateral incisions in Arctoseius, and these lacking in Iphidonopsis. However, not only is the lack of such incisions a plesiomorphic character state, as presented in some other genera of Arctoseiinae, but the presence of lateral incisions varies greatly among species, and is even subject to geographical variability and sexual dimorphism among some species, in Arctoseius. So, Athias-Henriot (1959, 1961 reported the lack of lateral incisions on females of Arctoseius pannonicus Willmann, 1949 from Corsica and Spain, although shallow ones are present in other parts of Europe. Observations by Lindquist (1964) also indicated these differences between some populations. Illustrations of A. venustulus (Berlese, 1917) by Bernhard (1963) show just the slightest of lateral incisions on females, and entire lateral margins on males. Also, since Seiulus minutus has been previously designated as type-species of the genus Arctoseiopsis by Evans (1954), Gwiazdowicz's placement of this species in Iphidonopsis would render this genus a junior synonym of Arctoseiopsis. With added phylogenetic perspective, we continue to follow the previous concept of Lindquist and Evans (1965), such that the genus Arctoseius embraces Arctoseiopsis and includes the species A. minutus, A. pulvisculus, and A. venustulus. Adult females of Iphidonopsis are somewhat similar to those of Arctoseius and Iphidozercon in retaining what may be considered to be an apomorphically reduced, deutonymphal form of anal shield. However, as indicated in the following key, the genus Iphidonopsis shares with Zerconopsis and Xenoseius Lindquist & Evans, 1965 the apomorphic attribute of having dorsoproximal setae ad-2 and pd-2 of tarsi II to IV elongated, curved, somewhat whip-like, while retaining the plesiomorphic attribute of genu III having eight setae, including pv-1. In contrast, Arctoseius, Iphidozercon and Maxinia are plesiomorphic in having setae ad-2 and pd-2 of tarsi II to IV unmodified, but sharing the apomorphic attribute of genu III having seven setae, in absence of pv-1.

Genus Diseius
Provisionally, we exclude the monotypic genus Diseius Lindquist & Evans, 1965 from the Arctoseiinae and from the following key to its genera. Based exclusively on gnathosomatic and idiosomatic attributes, the type-species of this genus, Iphidozercon ulmi Hirschmann, 1962, has been assigned to various genera as conceived by different authors' concepts of the family Ascidae, i.e., to the genus Iphidozercon (including Arctoseius, Leioseius Berlese, 1916 andGamasellodes Athias-Henriot, 1961) sensu Hirschmann (1962), to the subgenus Leioseius (Leioseius) (including Gamasellodes but excluding Arctoseius and Iphidozercon) by Bernhard (1963), and to the genus Leioseius (including Gamasellodes but excluding Arctoseius and Iphidozercon) by Karg (1971Karg ( , 1993. Accounting for those attributes and additional ones of leg chaetotaxy, the species was assigned to a separate, new genus and placed in the tribe Ascini of the subfamily Ascinae (separate from the subfamily Arctoseiinae), by Lindquist and Evans (1965). The chaetotaxy of the genu and tibia of legs III and IV of Diseius ulmi resembles that of genera of Arctoseiinae, but Diseius was excluded from the Arctoseiinae based primarily on the adult female having a fully divided dorsal shield, and having sternal setae st5 inserted on the edges of the epigynal shield, rather than on soft cuticle. However, new observations of female Diseius by one of us (EEL) revealed the unanticipated absence of setae al-3 and pl-3 on basitarsi III-IV, and also of pd-3 on basitarsus IV. The chaetotaxy of tarsi II to IV is extremely constant throughout the families of free-living Gamasina (Evans 1963). Losses of basitarsal setae on legs II to IV are not known to occur among species of Ascidae, but they have been noted sporadically in the Digamasellidae and Rhodacaridae (Lindquist 1975). Also, in the absence of j2, setae j3 are inserted relatively closely behind j1 in Diseius, much as in the digamasellid subgenus Longoseius (Longoseius) Chant, sensu Lindquist (1975). The subcortical habitat association of D. ulmi with scolytid beetles is in common with that of a variety of digamasellids. As adult females of D. ulmi are highly neotenous in both leg and dorsal idiosomal setation, the similarity of leg chaetotaxy on genua and tibiae II-IV with Arctoseiinae is thought to be convergent, and our new observations persuade us to place Diseius tentatively in the Digamasellidae instead of the Ascidae, until such time as additional attributes become available from description of the female spermathecal system and of attributes of the adult male.
Academy of Sciences supported the second author during certain aspects of this work. Tables of pictures and maps have been prepared for publication with the invaluable participation of K.V. Makarov. Three anonymous reviewers supplied us with very useful comments and corrections. We are very grateful to all. Open access to this paper was supported by the Encyclopedia of Life (EOL) Open Access Support Project (EOASP).