A new species of the gamasid mite genus Arctoseius Thor, 1930 (Parasitiformes, Mesostigmata, Ascidae) from Russia with a key to the multidentatus species-group

Abstract A new gamasid mite species belonging to the genus Arctoseius Thor, 1930 is described from Russia. Arctoseius koltschaki sp. n. is distributed in the plain and mountain tundras from Khibiny Mountains to Chukotka on the north and to West Sayan Mountains on the south. A diagnosis and a key for identification of species comprising the multidentatus species-group (Arctoseius multidentatus Evans, 1955; Arctoseius wisniewskii Gwiazdowicz & Kamczyc, 2009; Arctoseius sexsetus Lindquist & Makarova, 2011; Arctoseius haarlovi Lindquist & Makarova, 2011; and Arctoseius koltschaki sp. n.) are given.

introduction Arctic landscapes are populated by at least 26 mite species belonging to the genus Arctoseius Thor, 1930, which constitutes about half of the total diversity of this genus (Makarova in press). Such species richness appears to be unique among terrestrial mite genera within the Arctic. In spite of intensive investigations in the two last decades (Petrova and Makarova 1991;Makarova and Petrova 1992;Makarova 2000a, b;Lindquist and Makarova 2011), several arctic species still remain undescribed, one of which is treated here. It belongs to the multidentatus species-group, for which a diagnosis and a key to species are presented.

Material and methods
In total 59 females, 23 males and 19 nymphs were found in 25 localities. Setal notation for the idiosoma follows Lindquist and Evans 1965, with some modification (Lindquist 1994). The leg and palpal chaetotaxy corresponds to Evans 1963Evans , 1964. The poroidotaxy and adenotaxy are given according to Johnston andMoraza 1991, with small modification (Makarova 2003). The length of all shields was measured on the mid-line, the width at the broadest part, except for the sternal/sternitigenital shield, whose length was measured from the level of setae st1 to the posterior margin, and the width at the broadest part between levels of setae st2 and st3 (Makarova 2000a). The length of the epigynal shield was measured to include the anterior flap, the width -next to the posterior margin. The length of legs and tarsi are given excluding the ambulacrum, and also the pedicel on leg I. The subcapitulum length was measured from its anterior margin without appendages (internal malae, corniculi). The length of the chelicera excludes the basal segment, and the cheliceral digit length is based on the movable digit. Measurements were carried out in 20 females and 10 males and their limits are stated in micrometers (μm). The following ratios were used in the description (Makarova 2000a):

lD/wD
length-to-width ratio of the dorsal shield; J(1-4)min/jmax ratio of the length of dorsocentral setae inserted on the opisthonotal and podonotal regions, notably the shortest among J1-4 and the longest in the series j; (J4-J4)/(J3-J3) ratio of transverse distances between bases of opisthonotal setae J4 and J3 in pairs; Z5/(Z5-Z5) ratio of setae Z5 length to the distance between them; lSt/wSt, lAn/wAn, lVA/wVA length-to-width ratios of the sternal, anal, and ventrianal (in males) shields, respectively; lCo/lD ratio of the length of the corniculus and of the dorsal shield, %; lCh/lD ratio of the length of the chela and of the dorsal shield, %; lCh/lCo ratio of the length of the chela and of the corniculus; lExI/lD, lTIV/lD ratio of the length of leg I or tarsus IV and of the dorsal shield; lTIII/wTIII length-to-width ratio of tarsus III, width taken at basistarsus.
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, Saint Petersburg (ZIRAS). Part of paratypes, 5 females, 1 male, 2 N2, from Suntar-Khayata Range, 1500 m a.s.l. (above sea level), spotty tundra (on one slide) -in Canadian National Collection of Insects, Arachnids and Nematodes, Ottawa (CNC). Description. Adults of middle size, yellowish or brownish, with idiosoma rather narrow and appendages of normal proportions. Idiosomal shields moderately sclerotized, very finely punctate (punctation rarely visible), with clearly reticulate ornamentation only on sternal, genital or sternitigenital shields; dorsal shield smooth with distinct sigillae. Most body setae of moderate size, needle-shaped. Some setae of distal leg segments, especially of leg IV, elongated and finely tipped, subapical setae av-1 and pv-1 on tarsi II-IV strongly formed and blunt.
Differential diagnosis. Adults of Arctoseius koltschaki sp. n. are similar to those of Arctoseius multidentatus Evans, 1955, but the dorsal shield in A. koltschaki sp. n. is often narrower (lD/wD in female 1.96-2.24, in male 2.05-2.17 instead of 1.91-2.05 and 1.85-2.03 respectively), and tarsi II-IV have subapical setae av1 and pv1 more stout and blunt (Figs 11-15). In male of A. koltschaki sp. n., the ventrianal shield is relatively wide (Fig. 17), subtriangular (narrowly oval in A. multidentatus), incorporating the metapodal platelets (free in A. multidentatus); the spermatodactyl is more tightly or sharply sinuate, and more angled apically (not as gently sinuate, and digitiform apically as in A. multidentatus) (cf . Figs 18 and 19). In female of A. koltschaki sp. n., the sternal shield is uniformly reticulated (whereas in A. multidentatus, the reticulation is developed laterally, and separately within an elongate posteromedial "window"). Female of A. koltschaki sp. n. is often smaller (dorsal shield length 528-616 instead of 566-688 in A. multidentatus).
Etymology. The species named in honor of Admiral Alexander Koltschak, the prominent Arctic hydrologist and outstanding Citizen of Russia.
Distribution. At present known from the Russian Arctic and Siberian mountain regions (Fig. 25).

History
Previously and provisionally, there were some indications about the presence of new species close to Arctoseius multidentatus Evans, 1955, namely Arctoseius sp. aff. multidentatus [A. koltschaki sp. n.], «A. haarlovi Lindquist, 1963ms.» (Lindquist 1964) [A. haarlovi Lindquist & Makarova, 2011, together forming a natural group. Their distinguishing features were given in a key to high-arctic Arctoseius species (Makarova 2000b). Recently Lindquist and Makarova (2011) augmented this group by the inclusion of Arctoseius wisniewskii Gwiazdowicz & Kamczyc, 2009 and Arctoseius sexsetus Lindquist & Makarova, 2011. Moreover, they pointed out the similarity between all mentioned species and Arctoseius laterincisus Thor, 1930, the type-species of the genus Arctoseius. In spite of its doubtful validity, this species can be formally considered as a member of the multidentatus-group, in view of its having the following features: shortened peritremes, elongated corniculi, and elongated posterior idiosomatic setae (Lindquist and Makarova 2011). However, subsequent to its original description, this species has not been found either on Svalbard (Ávila-Jiménez et al. 2011) or anywhere in the European Arctic (Makarova 2013). Very probably, its type series included specimens belonging to more than one species of Arctoseius (Lindquist and Makarova 2011). Because of these aspects, A. laterincisus is not included in the key below.
The relatively long paranal setae are considered to be apomorphic for this speciesgroup within the genus Arctoseius. The reduced peritremes and peritrematal shields, somewhat elongated corniculi, and complete lack of dorsal shield ornamentation (retention of deutonymphal condition) are also apomorphic, but are derived independently among some other Arctoseius species.