Klimov, Pavel B. ¹ ; Kolesnikov, Vasiliy B. ² ; Khaustov, Alexander A. ³ and OConnor, Barry M. ⁴

¹✉ X-BIO Institute, Tyumen State University, Tyumen, Russia & Purdue University, Lilly Hall of Life Sciences, West Lafayette, Indiana, USA.
²X-BIO Institute, Tyumen State University, Tyumen, Russia.
³X-BIO Institute, Tyumen State University, Tyumen, Russia.
⁴Department of Ecology & Evolutionary Biology, University of Michigan, 3600 Varsity Drive, Ann Arbor, MI 48109, USA.

2024 - Volume: 64 Issue: 2 pages: 542-553

ZooBank LSID: 65BD239E-46D0-45A5-A7DD-9F4ACB8403DF

Original research

Keywords

Abstract

Introduction

Mites of the genus Fagacarus Fain et Norton, 1979 (Acari: Acaridae: Rhizoglyphinae) feed on white rot fungi growing between the sapwood and heartwood layers in fallen tree trunks at an advanced stage of decay (PBK pers. obs.). As these mites occupy a hidden and very specialized ecological niche, they are only occasionally collected. A single described species, Fagacarus verrucosus Fain et Norton, 1979, is known from the USA (Indiana, Michigan), while undescribed species have been reported from both the Nearctic (Colorado, USA) and Western and Eastern Palearctic realms (Bugrov, 1997; Klimov, 2000; Klimov and Tolstikov, 2011). It is very likely that the genus Fagacarus may be actually more common, diverse, and broadly distributed.

The genus Fagacarus is characterized by a remarkably distinct external morphology, exemplified by long pectinate dorsal setae and a heavily sclerotized and ornamented idiosoma. In contrast, most other mites of the acarid subfamily Rhizoglyphinae typically have a smooth idiosomal cuticle and smooth and short dorsal setae. Based on these distinct (but arguably autapomorphic and homoplastic) character states, the genus Fagacarus was first placed in a new subfamily Fagacarinae in the family Acaridae (Fain and Norton, 1979). However, cladistic analyses based on both adult and deutonymphal morphological characters demonstrated close relationships of Fagacarus with the genera Schwiebea Oudemans, 1916 and Thyreophagus Rondani, 1874 in the subfamily Rhizoglyphinae Oudemans, 1923 (Klimov, 2000). Subsequently, a refined analyses showed sister-group relationships of only two genera, Fagacarus and Schwiebea (Klimov and OConnor, 2003). This placement of Fagacarus among rhizoglyphine lineages was further supported by the discovery of heavily sclerotized and ''hairy'' adults of Schwiebea koerneri Türk et Türk, 1957 (Wurst, 2002), suggesting that these conspicuous character states also occur in other species of the subfamily Rhizoglyphinae and are not restricted only to the genus Fagacarus.

Here, we describe both adults and a heteromorphic deutonymph of a new species, Fagacarus absalom sp. n., from the Russian Far East and provide renewed diagnosis of the genus.

Material and methods

Tree trunks at an advanced stage of decay (almost no bark, wood is brown color, can be easily broken in pieces) were examined in the field. Specimens were collected by hand from colonies of white rot fungi growing between the sapwood and heartwood layers.

The terminology of idiosomal chaetotaxy follows Griffiths et al. (1990) and the terminology of the palp and leg chaetotaxy follows that of Grandjean (1939) and Griffiths (1970) with minor corrections to coxal setation by Norton (1998). The terminology of the spermatheca follows that of Witaliński et al. (1990) and Klimov and OConnor (2008). All measurements are in micrometres (μm).

We sequenced the small subunit ribosomal RNA (18S rRNA) in two species, Fagacarus absalom sp. n. (BMOC 01-0922-004.AD489, see below) and Fagacarus verrucosus (USA: Michigan, Lake Co., 3.5mi WSW Irons, Driftwood Valley Recreation Area, decayed log, 01 Oct 2000, B.M. OConnor, P. B. Klimov, 44°07′54″N 85°59′54″W, BMOC 00-1001-004.AD313). DNA extraction and sequencing were done as described previously (Klimov and OConnor, 2013). Genetic distances were calculated in PAUP v.4a168 (Swofford, 2022) as follows: begin paup; dset distance = p; savedist format = tabtext undefined = asterisk file = 1_p_distances.tab; dset distance = k2p; savedist format = tabtext undefined = asterisk file =2_k2p_distances.tab; end;. 18S rRNA sequences were deposited into the NCBI GenBank database, accession IDs: OR644287-OR644288.

Taxonomy

Family Acaridae Latreille, 1802

Subfamily Rhizoglyphinae Oudemans, 1923

Genus Fagacarus Fain et Norton, 1979

Type species: Fagacarus verrucosus Fain et Norton, 1979, by original designation.

Diagnosis — Adults. Chelicera with one setae; subcapitulum with paired hemispherical filter apparatus at sides of oral opening well developed, with oblique rows of spines. Grandjean's organ reduced; dorsal setae long, rounded in cross-section, heavily barbed; setae ve, f₂ and c₃ absent, other idiosomal (vi, se, si, c₁, c₂, c_p, d₁, e₁, d₂, e₂, h₁, h₂, h₃) setae present. Cuticle dorsally and in part ventrally with numerous raised protuberances forming a reticulate pattern. Tarsal setae ba I-II spiniform, situated near ω₁; setae ~~aa absent. Genua I with two solenidia. Genual setae nG III absent. Female. copulatory opening separated from anus by distance greater than half of anus length. Male. genital apparatus located between tarsi IV, with widened posterior part. Anal suckers vestigial, ad₁ and ad₂ minute or reduced to alveoli; ps₃ situated at anterior level of anus. Tarsus IV with a dorsal crista.

Heteromorphic deutonymph. Gnathosoma with one pair basal setae on dorsal sides, palpal remnants without setae. Setae ve absent. Setae si, c₁, d₁, e₁ and h₁ longer than other dorsal setae. Setae 1a, 3a, 4a conoidal. Coxal fields II and III closed. Sternum long, free. Ventrum present. Attachment organ with conoidal setae ps₂ situated at posterior level of central suckers ad_{1 +2}, posterior to midline of ad₁₊₂ and anterior to ps₁. Bases of conoidal setae ps₁ and ps₂ with long subcuticular sclerites, distinctly extending beyond the posterior level of attachment organ. On tarsus I, solenidion ω₃ situated in common field with solenidion ω₁; setae ba I and aa absent. Tarsal seta e I with very large, triangle-shaped tip; e II needle-like, shorter than claw; e III-IV spiniform. Tarsal setae p, q III and IV with attenuate tips. On genu III, setae nG III and solenidion σ III absent.

Remarks — The genus Fagacarus is similar to the genus Schwiebea sharing the following character states. Adults: Grandjean's organ is reduced; setae ve, c₃ and f₂ are absent; tarsal setae r IV, gT I-II and hT I-II are spiniform; tarsal setae aa are absent; tarsal setae ba I-II spiniform, situated at base of tarsus, adjacent to ω₁; tarsal setae e I-II are spiniform, massive. Female: adanal setae ad are absent; tarsal setae la I-II are spiniform; tarsal setae ra and la I-II are subterminal. Male: the genital apparatus is posterior to anterior apodemes IV and close to the anus; tarsal setae f I-II are simple; solenidion φ IV is spiniform. Heteromorphic deutonymph: setae ve are absent; tarsal setae aa absent; solenidion σ III is absent.

The genus Fagacarus differs from Schwiebea by the following character states. Adults: the subcapitulum has paired, hemispherical structures representing a filter apparatus (absent in Schwiebea); the dorsal setae are heavily barbed (smooth or very finely barbed in Schwiebea koerneri); the dorsal body is covered with raised, sclerotized protuberances (smooth in most Schwiebea, pitted in S. koerneri, or with small network of protuberances in nymphal stages of S. koerneri). Male: the opisthosomal shield is absent (present in Schwiebea); the anal suckers are vestigial (well developed in Schwiebea); adanal seate ad₁ and ad₂ are minute or alveolar (ad₁ and ad₂ or their alveoli absent in Schwiebea). Heteromorphic deutonymph: dorsal setae si, c₁, d₁, e₁ and h₁ are longer that other dorsal setae (subequal in Schwiebea); palps of gnathosoma without setae (with a pair of setae in Schwiebea); tarsal setae e I with a very large, triangle-shaped tips (setae e I rounded, not triangle-shaped in Schwiebea).

Fagacarus absalom Klimov, Kolesnikov et OConnor, sp. n.

ZOOBANK: D81AB2F8-CCE7-4511-9AEA-A248C4FF94FF

Type material

Holotype female — RUSSIA: Primorskiy Kray, Ussuriysky District, Kaymanovka village, banks of Barsukovka stream, inside rotten stump of a deciduous tree (between sapwood and heartwood layers), on edge of colony of white rot fungi, 7 Oct 1998, coll. P.B. Klimov (slide #1.1).

Paratypes: 5 females – same slide as holotype; Paratypes: 1 female, 2 males – same data, slide#1.2; 3 females, 7 males – same data, slide#1.3; 3 females, 5 males – same data, slide#1.4; 8 females, 5 males – same data, slide#1.5; 12 females, 3 males – same data, slide#1.6. 1 heteromorphic deutonymph – RUSSIA: Primorskiy Kray, Khasanskiy rayon, Kedrovaya pad' reserve, fallen log with Lasius niger (Hymenoptera, Formicidae), 29 Apr 1999, coll. P.B. Klimov (mounted 21 Mar 2000) (slide #2.2); 2f – same data, slide #2.2.

Non-type material — Immatures (not counted) – slides 1.3, 1.6; 1 TN – slide 1.4, 1 TN – slide #2.2; 1 TN – slide #2.3.

Type deposition — Holotype and paratypes (slides #1.6, 2.3) are deposited in the Zoological Institute, Russian Academy of Sciences, Saint Petersburg, Russia (ZISP). The remaining paratypes (slides #1.1-1.5, 2.1, 2.2) are deposited in the University of Michigan, Museum of Zoology, Ann Arbor, Michigan, USA (UMMZ).

Description

Female — (Figures 1A, B, 2A–H, 3B, C, 6A). Subcapitulum (57 × 38, holotype) elongated, trapezoid, lateral sides straight, strongly converging towards apex, at base, with two pairs of weakly sclerotized fields (muscle attachment sites). Paired filter apparatus at sides of oral opening well developed, forming dense hemispherical structures with oblique rows of spines. Subcapitulum with 1 pair of filiform setae h and short setae a. Palps with setae sup and cm; supracoxal seta (elcp) absent. Palp solenidion ω rod-shaped; ul′ and ul″ present, minute, button-shaped. Chelicerae (55) elongated, twice as long as height, with one distal sharp tooth and seta cha.

Idiosoma, length × width, 287 × 175 (holotype), 276–347 × 160 –187 (paratypes, n=21), length / wide ratio 1.53–1.90 (n=12). Propodosoma 88, hysterosoma 199, ratio of hysterosoma/propodosoma 2.26 (n=1). Cuticle pigmented, not forming distinct shields or shield. Dorsolateral idiosoma with raised protuberances (sometimes conical and spiniform in shape); on dorsal hysterosoma, these protuberances form a network consisting of 4-6-sided polygons; central part of dorsum has two pairs of curved and heavily sclerotized ridges at level of scapular setae; on middorsal propodosoma, pattern is weakly developed; lateral propodosoma and hysterosoma with irregular protuberances, not forming clear pattern. Idiosoma smooth ventrally, except for posterolateral hysterosoma and transverse area at level of posterior coxal apodemes II, which covered by small, conical protuberances. Dorsal sejugal furrow well developed.

Grandjean's organ not observed. Supracoxal seta of leg I (scx) 28, smooth, long, nearly spiniform. Dorsal setae heavily barbed, thick, rounded in cross section, situated on protuberances (except for vi, scx, h₃). External scapular setae se anterior to posterior propodosomal shield ridges; si posterior to se, 1.5 times shorter than se. Dorsal setae c₁ and c₂ at anterior margin of hysterosoma. Setae c₃, f₂ and ve absent. Length of setae vi 41, se 73, si 48, c₁ 63, c₂ 44, c_p 67, d₁ 104, d₂ 61 e₁ 79, h₁ 70, h₂ 56, h₃ 34 (n=1). Distance vi-vi 13, se-se 63, si-si 21, c₁-c₁ 44, d₁-d₁ 20, e₁-e₁ 27, h₁-h₁ 41. Cupules ip situated ventrally, approximately between insertion points of legs III-IV, ih situated on sides of anterior end of anus, cupules ia and im not observed. Opisthonotal gland openings gla situated on cuticular protuberances, slightly anterior to e₂. Anterior coxal apodemes I fused to form sternum, other coxal apodemes free, posterior coxal apodeme IV absent. Coxal fields I-IV with large sclerotized areas. Distance between posterior end of posterior coxal apodemes II and free tip of anterior coxal apodemes III shorter than length of the latter. Ovipore wide, in form of an inverted Y; situated between coxal fields III-IV. Coxisternal setae 1a, 3a, 4a, 4b and genital setae g subequal in length, filiform; setae g anteriad of setae 4a. Genital papillae well sclerotized and pigmented, rounded; distal tip rounded, with pigmented cap; proximal part slightly narrowed forming a neck. Anus shifted from posterior edge of body by more than 3/4 of its length. Setae ps and ad absent. Copulatory opening ventral, near posterior margin of body (separated from anus by a distance greater than half of anus length); inseminatory canal of spermatheca long, and widened near spermatheca (diameter 1.6); spermatheca spherical (18.9), pigmented and sclerotized; efferent ducts in form of elongated V-like funnels (5.6 × 1.9).

Legs I-IV short, legs length (without empodium) 99, 90, 90 and 120 (n=1). Pretarsi with claws attached to condylophores. Leg setation: trochanters 1–1–1–0, femora 1–1–0–1, genua 2(2)–2(1)–0(1)–0, tibiae 2(1)–2(1)–1(1)–1(1), tarsi 12(3+ε)–12(1)–10–10. Trochanters I–III with filiform setae (pR I–II, sR III). Femoral setae vF I–II and wF IV filiform. Genual setae cG I–II short, spiniform, mG I-II robust, sword-shaped, nG III absent. Tibial setae gT I–II spiniform, hT I–II and kT III robust, sword-shaped, kT IV slightly sword-shaped. Tarsi I-II without foliate setae, setae ba and e robust, spiniform (at least 1/2 of claw length), ba situated near ω₁; wa and la spiniform; d, f, ra filiform, d and f shorter than tarsus, ra slightly widened basally; s, v, u, p and q spiniform, p I and q II shorter than other spiniform setae. Tarsi III-IV without foliate setae; setae w, e, r, s, u, v, p and q spiniform; d and f filiform, shorter than tarsus, p and u larger than q and v. Solenidion ω₁ I bacilliform, thin, slightly curved, slightly widened apically, reaching level of seta e; ω₂ I anterolaterad ω₁ I, bacilliform, about 5 times shorter than ω₁ I; ω₃ I curved, with rounded tip, not reaching half of claw; famulus ε short. Solenidion ω II well expanded apically in comparison with ω₁ I. Solenidion φ I long, tapering; φ II shorter than φ I, projecting slightly beyond apex of tarsus; φ III and IV short, tapering, shorter than tibia. Solenidia σ′ and σ″ I distinctly longer than tibia, σ′ slightly longer than σ″; σ II distinctly longer than tibia, with rounded tip; σ III curved, with rounded tip, shorter than genu III.

Male — (Figures 1C, D, 2I, J, 3A, D, E, 6B). Gnathosoma as in a female. Subcapitulum 51 × 35 (n=1). Chelicerae 44 (n=1).

Idiosoma, length × wide, 287– 237 × 140 – 176 (n=13), length / wide 1.54-1.92 (n=12).

Propodosomal length 85, hysterosomal length 158, ratio of hysterosoma/propodosoma 1.86 (n=1). Iddiosomal cuticle as in female; distinct shields or sclerites absent. Sejugal furrow present. Grandjean's organ absent. Dorsal setae as in female; length of setae scx 25, vi 34, se 56, si 38, c₁ 66, c₂ 36, c_p 50, d₁ 82, d₂ 53 e₁ 61, h₁ 56, h₂ 41, h₃ 27 (n=1). Distance vi-vi 13, se-se 60, si-si 20, c₁-c₁ 27, d₁-d₁ 21, e₁-e₁ 25, h₁-h₁ 30. Opisthonotal gland opening gla, cupules and apodemes as in female. Setae g slightly anteriad of setae 4a. Genital apparatus between apodemes IV and close to anus. Aedeagus rod-shaped, slender, not curved, not protruding beyond the edge of genital capsule. Genital papillae as in a female. Anus shifted from posterior margin of body by more than 3/4 of its length. Anal suckers vestigial. Cuticle posteriad of anus with irregular small papillae. Setae (ps) present: ps₁ (4) and ps₂ (3) situated posteriad of anus, short, slightly spiniform, distance ps₂-ps₂ longer than distance ps₁-ps₁; ps₃ situated near anterior sides of anus, filiform, slightly longer than ps₁ and ps₂. Setae ad_1-2 adjacent to each other, situated on anal suckers, ad₁ very short, ad₂ represented by alveoli.

Legs I–III as in a female, except solenidion ω₃ I thickened, almost reaching tip of claw. Trochanter and femur IV as in female. Tibia IV with slightly sword-shaped seta kT IV. Tarsus IV with dorsal crista, seta f filiform, situated anteriad of crista; setae d and e represented by suckers (shifted to distal part of tarsus); other setae (w, r, s, u, v, p and q) spiniform. Solenidion φ of tibia IV shortened and thickened.

Heteromorphic deutonymph — (Figures 4, 5, 6C–E). Body rounded, 160 × 140, length/width ratio 1.1. Gnathosoma with elongated subcapitular remnant (slightly longer than wide, 12 × 7) and short palps (slightly longer than wide) with filiform apical palpal solenidia ω (25), and one pair of dorsobasal setae.

Dorsal idiosoma smooth, except regions of sejugal furrow and lateral sides of hysterosomal shield, which have lineate pattern. Length of hysterosoma (130), length of propodosoma (32) 4.0. Anterior propodosoma with rounded rostrum and convave sides. Ocelli absent. Setae vi short (4), filiform, smooth. Setae ve absent. Setae se (10) longer that vi, finely serrate at tips; setae si (18) longer that se, finely serrate; setae si posterior to se. Setae scx (9) with bifurcate tip. Hysterosomal setae c₁ (22), d₁ (25), e₁ (20) and h₁ (15) finely serrate; length of d₁ and e₁ shorter than distance between bases of d₁-e₁ and e₁-h₁, respectively. Setae c₂ (10), c_p (7) and d₂ (7) shorter than c₁, d₁, e₁ and h₁, all somewhat finely serrate. Setae e₂ (6), f₂ (4), h₂ (6) and h₃ (10) smooth, shorter than c₁, d₁, e₁ and h₁ (setae h₃ longer than e₂, f₂ and h₂). Opisthonotal gland openings gla situated between d₂ and e₂. Cupules ia present, situated posterior to c₂.

Surface of coxal fields without any well-developed pattern. Anterior apodemes of coxal fields I fused, forming a sternum. Posterior end of sternum acute, not touching anterior apodemes II, ending near ends of anterior apodemes II; distance between its end and ventrogenial shield much shorter than its length. Coxal fields II and III closed, posterior apodemes II and anterior apodemes III adjacent. Anterior apodemes of coxal fields III not fused with each other, with a short gap between them. Anterior apodemes of coxal fields IV fused to each other and connected to apodemes III. Ventrum present, not reaching anus. Setae 1a, 3a, 4a conoidal, well-developed; 4a situated posterior to g; 4b (3) and g (4) filiform. Setae c₃ (3) filiform, ventral, situated between transverse levels of trochanters II–III. Attachment organ 40 long, 48 wide (length/width ratio 0.8), situated near posterior edge of the body. Cuticular suckers present. Setae ps₁ and ps₂ conoidal; ps₂ situated posterior to midline of ad₁₊₂ and anterior to ps₁; sclerites of ps₁ and ps₂ large, elongated. A pair of small refractile spots (ps₃) anterolaterad of median suckers (ad₁₊₂). Suckers ad₃ round; ad₁₊₂ much larger, consisting of paired vestigial alveoli surrounded by wide sclerotized border.

Legs I long (96), longer than half the body length; legs II-IV (71, 50 and 63, respectively) shorter than half the body length; all segments free, with dorsal and ventral longitudinal ridges near the setae. Leg setation: trochanters 1–1–1–0, femora 1–1–0–1, genua 2(1)–2(1)–0–0, tibiae 2(1)–2(1)–1(1)–1(1), tarsi 8(3+ε)–9(1)–8–8. Trochanters I–III each with filiform seta (pR I, II, sR III). Femoral setae vF I, II spiniform with attenuate tips, wF IV setiform. Genual setae mG I, II spiniform, sword-shaped; cG I, II spiniform, short; nG III absent. Tibial setae gT I–II and kT III, IV spiniform; setae hT I, II spiniform, sword-shaped, larger than other sword-shaped leg setae. All pretarsi with hooked empodial claws arising from tarsal apices and connected to short, paired condylophores. Tarsus I with 4 setiform setae (d, f, p and q), 3 spiniform setae (ra, la and wa) and 1 modified setae e (wide triangle-shaped tips). Tarsus II with 3 filiform setae (d, p and q), 5 spiniform setae (e, ba, ra, la and wa) and foliate seta f. Tarsi III, IV with 2 filiform setae (d and f) and 6 spiniform setae (e, p, q, w, e and s), with attenuate tips. Solenidia ω₁ I-II on tarsi I cylindrical, with rounded apices, situated in basal part of tarsus; solenidion ω₃ situated close to ω₁, longer than ω₁, cylindrical; solenidion ω₂ cylindrical, with rounded apex, shorter than ω₁. Famulus ε short, spiniform, adjacent to solenidion ω₁. Solenidia φ I, II tapering, shorter than tarsi; φ II, IV short, bacilliform. Genua I, II with single solenidion (σ), σ I, II elongate, cylindrical; σ I shorter than ω₂; σ II shorter than σ I; σ of genu III absent.

Diagnosis

Fagacarus absalom differs from F. verrucosus by the following character states: in adults, solenidion σ II reaches tarsus II (vs. not reaching in F. verrucosus), posterior coxal apodemes II are curved (vs. nearly straight in F. verrucosus); in males, pseudanal setae ps₂ are smooth, situated at the posterior edge of the anus, not protruding beyond the posterior margin of the body (vs. barbed, posterior to this level, protruding in F. verrucosus), pseudanal setae ps₁ and ps₂ are slightly spiniform, about 5 times shorter than h₃ (vs. filiform, less than 2 times shorter in F. verrucosus), the dorsal crista of tarsus IV is about 3/4 of the height of tarsus IV (vs. about 1/2 in F. verrucosus); in females, setae d IV are shorter than tarsus IV (vs. slightly longer in F. verrucosus). Heteromorphic deutonymphs are known only for Fagacarus absalom and therefore cannot be compared with those of any other species of the genus Fagacarus.

Molecular identification

A NCBI BLAST search of both 18S rRNA sequences (Fagacarus absalom sp. n. and F. verrucosus) returned close matches with Schwiebea pseudotsugae (97.40–96.73%) and Rhizoglyphus sp. AP-2010 (97.26–96.78%), thus confirming our morphology-based placement of Fagacarus in the subfamily Rhizoglyphinae and as a sister lineage to Schwiebea (Klimov and OConnor, 2003). Uncorrected 18S rDNA genetic distance between Fagacarus absalom sp. n. and F. verrucosus was 1.52%. This distance is sufficient to delimit the two species, since 18 rDNA (a nuclear gene) evolves much slower than the COX1 gene (a mitochondrial gene) (Klimov at al. 2019).

Etymology

Absalom is a biblical figure known for his luxuriant, long hair, which is reminiscent of the long and barbed dorsal setae of Fagacarus absalom. This specific name is treated here as a nomen in apposition.

Distribution

Russia: Primorsky Krai.

Biology

These mites live inside rotten trunks of a deciduous tree (between sapwood and heartwood layers), on the edge of colony of white rot fungi. Associated astigmatan species included Histiostoma sp. aff ruehmi (Histiostomatidae), Schwiebea sp. aff koerneri (Acaridae).

Acknowledgements

This research was supported by the cooperative agreement No. FEWZ-2021-0004 from the Russian Ministry of Science and Higher Education.

References

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