﻿The genus Oligonychus Berlese (Acari, Prostigmata, Tetranychidae): taxonomic assessment and a key to subgenera, species groups, and subgroups

﻿Abstract A comprehensive taxonomic assessment of the most agriculturally important and highly diverse spider mite genus, Oligonychus Berlese (Acari: Tetranychidae) was performed. The sub-generic division, species groups, doubtful species, species complexes and the interpretation of a key generic character are discussed. Based on the orientation of the male aedeagus, only two subgenera, namely Oligonychus Berlese (aedeagus downturned) and Reckiella Tuttle & Baker (aedeagus upturned), are valid in the genus Oligonychus. The subgenera Homonychus Wainstein, Metatetranychoides Wainstein, and Wainsteiniella Tuttle & Baker are considered to be synonyms of the subgenus Oligonychus, whereas the subgenus Pritchardinychus Wainstein is proposed as a synonym of the subgenus Reckiella. Moreover, based on female morphological characters, four species groups (coffeae, exsiccator, iseilemae, and peruvianus) and 11 species subgroups (aceris, biharensis, coffeae, comptus, exsiccator, gossypii, iseilemae, peruvianus, pritchardi, smithi, and subnudus) are suggested in the subgenera Oligonychus and Reckiella. Fourteen Oligonychus species are proposed as species inquirendae, and potential cryptic species complexes in the genus Oligonychus are briefly highlighted. It is agreed that the clunal seta h1 is always absent, while the para-anal setae h2 and h3 are always present in the genus Oligonychus. A key to subgenera, species groups, and species subgroups of the genus Oligonychus is provided.


Introduction
Oligonychus Berlese (Acari: Prostigmata: Tetranychidae) is the largest genus of the spider mites, comprising > 200 species, and its members have been reported throughout the world (Migeon and Dorkeld 2021). A range of feeding specificity occurs throughout the genus, with polyphagous, oligophagous and monophagous species present on both broad and narrow leaved commercial (fruits, agronomic crops, etc.) and noncommercial (wild trees, shrubs, grasses, etc.) host plants (Pritchard and Baker 1955;Jeppson et al. 1975;Beard et al. 2003;Matsuda et al. 2012;Migeon and Dorkeld 2021). Some economically significant species, e.g., the date palm mite O. afrasiaticus (McGregor), the tea red spider mite O. coffeae (Neitner), the banks grass mite/new world date mite O. pratensis (Banks), and the avocado brown mite O. punicae (Hirst), have been spread across the world and are now widely distributed (Jeppson et al. 1975;Migeon and Dorkeld 2021).
The authenticity of sub-generic division of the genus Oligonychus (Wainstein 1960;Tuttle and Baker 1968) always remains questionable, due to the use of inconsistent characters, e.g., striae pattern on dorsal opisthosoma and number of tactile setae on tibia I (Meyer 1974(Meyer , 1987Bolland et al. 1998;Beard et al. 2003Beard et al. , 2008Khanjani et al. 2018;Li et al. 2018Li et al. , 2019. So, to confirm the current taxonomic status of the six subgenera of Oligonychus suggested by Tuttle and Baker (1968), a comprehensive morphological investigation based on all known Oligonychus species is needed.
The species identity in the genus Oligonychus is usually challenging due to the limited number of potential diagnostic characters, presence of intraspecific variation, minute differences in male aedeagus morphology and interspecific similarities in females (Pritchard and Baker 1955;Meyer 1974Meyer , 1987Jeppson et al. 1975;Li et al. 2018). Mostly, Oligonychus species have been differentiated based only on the aedeagus morphology (Pritchard and Baker 1955;Meyer 1974Meyer , 1987. In addition, specimens of both sexes are frequently required for precise identification of Oligonychus species (Ben-David 2008;Meyer 1987). The aedeagal traits could be unreliable and confusing when male specimens were not mounted in a precisely lateral position, and in some cases the aedeagus was neither described nor illustrated (e.g., O. mangiferus Rahman & Sapra, 1940). Also, intraspecific variations in aedeagus shape or variation in the interpretation of aedeagus shape that can manifest as intraspecific variation, have been observed in species described from various geographical localities (e.g., O. pratensis;McGregor 1939;Pritchard and Baker 1955;Meyer 1959Meyer , 1974Meyer , 1987Baker and Pritchard 1962;Estebanes and Baker 1968;Jeppson et al. 1975;Tuttle et al. 1976;Baker and Tuttle 1994). Furthermore, aedeagus shape may appear variable at different levels of focus under the microscope (e.g., O. ephamnus Beard & Walter, 2003). There are 17 Oligonychus species that have been described based only on females with the males remaining unknown, e.g., O. mactus Tseng and O. nielseni Reeves (Reeves 1963;Tseng 1990), and a few species that were inadequately described, e.g., O. kobachidzei (Reck) and O. stenoperitrematus (Ugarov and Nikolskii), with important morphological information of male/female not included (Ugarov and Nikolskii 1937;Reck 1947). Some species, e.g., O. caucasicus (Reck) and O. daleae Tuttle, Baker & Abbatiello, were reported as new to science, without making any remarks regarding related or similar species (Reck 1956;Tuttle et al. 1976). This lack in clarity has resulted in the suggested existence of several species complexes within the genus Oligonychus e.g., coffeae complex and pratensis complex (Pritchard and Baker 1955;Meyer 1987;Ehara and Gotoh 2007;Lara et al. 2017;Khanjani et al. 2018;Li et al. 2018). Consequently, there is a great need for developing an integrative taxonomic approach to clarify the actual status of many closely related Oligonychus species and for establishing truly diagnostic characters for accurate and consistent species separation.
Keeping in view the importance of taxonomic adversities in the genus Oligoncyhus, the objectives of the present study were to i) assess the current taxonomic status of the sub-generic division of Oligonychus, ii) classify all species of Oligonychus into species groups and subgroups based on consistent morphological characters, iii) construct a diagnostic key to subgenera, groups and subgroups of Oligonychus, and iv) highlight or discuss the doubtful species, species complexes and contradiction/confusion in the identification of para-anal setae in Oligonychus.

Materials and methods
The taxonomic literature of 211 Oligonychus species was critically reviewed to confirm the current status of subgeneric division and doubtful and closely related Oligonychus species, create species groups, and prepare a key for their identification; in addition to discussing the confusion/contradiction associated with the naming of para-anal setae. To verify the consistency in expression of some female morphological characters and their significance in creating species groups and subgroups within the genus Oligonychus, numerous spider mite samples were collected and observed from Egypt, Mexico, Pakistan, Saudi Arabia, USA, and Yemen. In addition, mite samples of some other closely or distantly related Tetranychini genera viz. Tetranychus Dufour, Eotetranychus Oudemans, Mixonychus Meyer & Ryke, Neotetranychus Trägårdh, Sonotetranychus Tuttle et al., and Schizotetranychus Trägårdh, were collected from various localities in different regions of Saudi Arabia, to confirm the absence/presence and shape and position of the clunal (h 1 ) and para-anal setae (h 2 and h 3 ). The nomenclature of Grandjean (1939Grandjean ( , 1944aGrandjean ( , 1944bGrandjean ( , 1947 was followed for body setae, and Lindquist (1985) for leg chaetotaxy and other terminologies.

Taxonomic review of the genus Oligonychus
The genus Oligonychus was erected by Berlese (1886), based on Heteronychus brevipodus Targioni-Tozzetti as the type species (specimen was a nymph), reported from the Holly Oak (Quercus ilex L., Fagaceae) in Italy (Targioni-Tozzetti 1878). Because Berlese (1886) did not clearly describe the presence of proximoventral hairs on the empodial claws in the diagnosis of Oligonychus, Zacher (1913) created a new genus, Paratetranychus, and described the presence of proximoventral hairs in it. McGregor (1950) followed the work of Zacher (1913) by giving priority to Paratetranychus over Oligonychus, and placed all Oligonychus species within Paratetranychus. Five years later, Pritchard and Baker (1955) redefined the genus Oligonychus, and synonymized the genera Paratetranychus and Tacebia (Yokoyama 1929) with Oligonychus.

Subdivision of the genus Oligonychus
The genus Oligonychus has a history of subdivision into species groups (Pritchard and Baker 1955;Ehara 1999) and subgenera (Wainstein 1960;Tuttle and Baker 1968). Initially, Pritchard and Baker (1955) divided Oligonychus species into five species groups viz. ununguis, peruvianus, pritchardi, pratensis, and mcgregori. The ununguis group was further divided into five species subgroups viz. aceris, bicolor, boudreauxi, subnudus, and ununguis. This grouping was based on both male and female morphological characters, including the shape or orientation of the male aedeagus, number of tactile setae on tibia I, number of tactile setae on tarsus I proximal to the proximal duplex setae and ventrally near or beyond the duplex setae, number of proximoventral hairs on the empodium, the pattern of striation on the female dorsal opisthosoma, shape of dorsal body setae, and the shape of the peritreme (Pritchard and Baker 1955). Another species group, the clavatus species group, was subsequently created by Ehara (1999) based on the number of tactile setae on tibia I, aedeagal morphology and female body color, and included O. clavatus (Ehara) and O. pustulosus Ehara. Wainstein (1960) proposed five sub-genera of Oligonychus, namely Oligonychus Berlese, Homonychus Wainstein, Metatetranychoides Wainstein, Pritchardinychus Wainstein, and Paratetranychus Zacher. This sub-generic division was based on both male and female morphological characters, including the length of dorsal body setae, dorsal setae set on tubercles or not, the total number of setae on each of female tibia I, tibia II and tarsus I, and the morphology of the male aedeagus. Three of these sub-genera were further categorized into species groups based on the chaetotaxy of legs I and II. The subgenus Oligonychus was divided into three species groups: boudreauxellus, bakerellus, and berlesellus; the subgenus Paratetranychus was divided into two groups: ununguellus and zacherellus; and the subgenus Pritchardinychus was divided into three groups: pritchardellus, pratensellus, and mcgregorellus. Later, Tuttle and Baker (1968) synonymized the subgenus Paratetranychus with the subgenus Oligonychus, and retained four of the subgenera created by Wainstein (1960). Tuttle and Baker (1968) then created two more sub-genera, namely Wainsteiniella Tuttle & Baker and Reckiella Tuttle & Baker. This sub-generic division was again based on both male and female characters, including the pattern of the striation of the female dorsal opisthosoma, shape and orientation of male aedeagus, length of dorsal body setae, and total number of tactile setae on tibia I. Tuttle and Baker (1968) relied heavily on one of the female morphological characters to differentiate these six subgenera, the pattern of the striae on the dorsal opisthosoma. Mitrofanov (1977) also used this character to raise the sub-genera Metatetranychoides, Pritchardinychus, and Homonychus to genus level, and to erect a new genus, Neonychus, with O. licinus Baker & Pritchard as type species. Meyer (1987) found that the pattern of dorsal striae on the female opisthosoma to be a variable character, and disagreed with the subgeneric divisions of Oligonychus made by Wainstein (1960) and Tuttle and Baker (1968). The sub-genera are not always well distinguished and several morphological characters, such as the pattern of striae between dorsal setal pairs e 1 and f 1 on the female and the number of tactile setae on tibia I, have been found to be variable (Meyer 1987). Such variable characters cannot be used to reliably separate the subgenera of Oligonychus (Meyer 1987;Beard et al. 2003Beard et al. , 2008Li et al. 2018). These sub-divisions were made for practical rather than phylogenetic reasons, and have resulted in a somewhat artificial classification (Helle et al. 1981;Meyer 1987). Bolland et al. (1998) supported homogeneity within only the two sub-genera viz. Reckiella and Oligonychus, based on biological, morphological, and molecular data, but felt that the homogeneity of the remaining four sub-genera (Homonychus, Metatetranychoides, Pritchardinychus, and Wainsteiniella) requires further investigation to confirm their validity. Additionally, molecular investigations have revealed the presence of "polyphyly" in the genus Oligonychus (Navajas et al. 1996;Ben-David et al. 2007;Matsuda et al. 2014).

Species complexes in the genus Oligonychus
Several species complexes within the genus Oligonychus have been suggested by various authors in the past, for example a coffeae complex, pratensis complex, perseae complex, subnudus complex, sacchari complex, and ununguis complex (Pritchard and Baker 1955;Cromroy 1958;Meyer 1987;Ehara and Gotoh 2007;Lara et al. 2017;Li et al. 2018), and these are difficult to resolve based on morphology (Pritchard and Baker 1955  . Originally, the Japanese population of O. gotohi was considered to represent a single species, however, genetic crossing studies revealed that this population is a complex of three reproductively isolated species (O. gotohi, O. castaneae, and O. amiensis)  . Moreover, some variation in morphological characters, e.g., aedeagus shape and number of tactile setae on tarsus II, have been observed in various taxa identified as O. coffeae reported from different geographical localities (Pritchard and Baker 1955;Baker and Pritchard 1960;Ehara 1969Ehara , 1999Meyer 1974Meyer , 1987Wang 1981), highlighting the possibility that further cryptic species could be separated within this complex .
The pratensis complex sensu Pritchard & Baker has been recognized by various authors based on observed variations or differences in some morphological characters among different populations identified as O. pratensis, e.g., aedeagus shape and striae pattern on dorsal hysterosoma (Pritchard and Baker 1955;Meyer 1974;Li et al. 2018). A subnudus complex was suggested by Pritchard and Baker (1955), when morphological variations in shape/length of some hysterosomal setae and the stylophore were observed between two populations of O. subnudus (McGregor) from two different localities in the United States.
Based on the variations in descriptions and illustrations of two morphologically similar Oligonychus species, O. sacchari (McGregor) and O. saccharinus Baker & Pritchard (McGregor 1950;Pritchard and Baker 1955;Baker and Pritchard 1960;Meyer 1974), the sacchari complex was proposed by Khanjani et al. (2018). Although, Meyer (1974) comprehensively discussed the morphological differences between these two closely related species, their taxonomic identities remain doubtful, and require further investigations through the combined use of morphological and molecular data (Khanjani et al. 2018).
The possibility of an ununguis complex was suggested by Pritchard and Baker (1955) Pritchard & Baker, O. punicae, and O. ununguis (Jacobi). The taxonomic identities of most species in the ununguis complex remain questionable, and the females are indistinguishable. The minute differences in the shape of the aedeagus and the size of female palp spinneret are often used for differentiating these closely related species (Pritchard and Baker 1955;Meyer 1987;Khanjani et al. 2018). However, some of these species e.g., O. mangiferus, O. punicae, and O. vitis, are very close morphologically, can be exceedingly difficult to differentiate as separate species, and are part of the "greatest taxonomic problem" in the genus Oligonychus (Meyer 1987;Khanjani et al. 2018).

Para-anal setae in the genus Oligonychus
In many genera of the tribe Tetranychini Reck, three pairs of h setae (h 1 , h 2 , and h 3 ) are consistently present on the fifth segment (H) of opisthosoma (Pritchard and Baker 1955;Lindquist 1985;Bolland et al. 1998). However, one of these setae (h 1 or the clunals) is absent in some Tetranychini genera, e.g., Oligonychus and Tetranychus (Pritchard and Baker 1955;Lindquist 1985;Bolland et al. 1998;Seeman and Beard 2011;Alatawi and Kamran 2018;Khanjani et al. 2018). Earlier, Pritchard and Baker (1955) believed that seta h 1 (clunal seta) is consistently present and h 2 (one of a pair of para-anal setae displaced terminally to become a post anal seta) is absent in Oligonychus. Later, Lindquist (1985) analyzed and discussed the relative position and shape of the para-anal setae (h 2 and h 3 ) with respect to setal homologies and concluded that the clunal seta h 1 is apparently absent in Oligonychus, whereas seta h 2 is consistently present (Lindquist 1985), as previously explained by many authors (Oudemans 1930;Pritchard and Baker 1952;Attiah 1970). According to Lindquist (1985) and Seeman and Beard (2011), para-anal setae h 2 and h 3 , are consistently present in Oligonychus. Confusion is generated when simple positions are used to name setae rather than homologies. Lindquist (1985) and Seeman and Beard (2011) use homologies and state that "two pairs of para-anal setae h 2 and h 3 , are consistently present in Oligonychus and related genera". Many authors have followed the work of Lindquist (1985), and also consider setae h 1 to be absent, and h 2 and h 3 to be present in Oligonychus (Beard et al. 2003(Beard et al. , 2008Kamayev 2017;Li et al. 2017Li et al. , 2018Li et al. , 2019Arabuli and Gotoh 2018;Khanjani et al. 2018). Pritchard and Baker (1952) originally assumed that seta h 1 (clunal) was present or absent, with two pairs of para-anal setae present; however, Prtichard and Baker (1955) altered this view and assumed the clunal setae to always be present. They stated that there are two pairs of para-anal setae, and that the posterior pair get displaced terminally to become a post-anal seta, and that it is this seta that is absent in two genera (Oligonychus and Tetranychus). Meyer (1987) and Bolland et al. (1998) also use a positional approach to naming setae and assume the clunals to be consistently present, but interpret setae h 2 or h 3 as absent in Oligonychus and Tetranychus. Thus the setae are named as one pair of clunals and two pair of para-anals, and as a consequence, the statement "one pair of para-anal setae is present in Oligonychus and related genera" appears to be contradictory to what other authors believe. Many authors also mention the presence of h 1 and only one pair of para-anal setae (either h 2 or h 3 ) in the genus diagnosis and descriptions/illustrations of different Oligonychus species (Rimando 1962;Tuttle and Baker 1968;Chaudhri et al. 1974;Meyer 1974;Jeppson et al. 1975;Tuttle et al. 1976;Zaher et al. 1982;Tseng 1990;Gupta and Gupta 1994;Smiley and Baker 1995;Ehara 1999;Knihinicki and Flechtmann 1999;Flechtmann and Etienne 2006;Ehara and Gotoh 2007;Zeity 2015Zeity , 2016Alatawi and Kamran 2018).
Diagnosis (based on male). Male aedeagus with shaft bending ventrad, downturned part mostly tapering distally, forming an acute or blunt tip.
Only two subgenera are hereby recognized: Oligonychus Berlese and Reckiella Tuttle & Baker, instead of five and six subgenera as proposed by Wainstein (1960) and Tuttle and Baker (1968), respectively. The subgenera Homonychus, Metatetranychoides and Wainsteiniella are considered to be synonyms of the subgenus Oligonychus, and subgenus Pritchardinychus is recommended as a synonym of the subgenus Reckiella. In total, 76 species are placed in the subgenus Oligonychus, whereas 118 Oligonychus species are designated to the subgenus Reckiella. However, approximately 17 Oligonychus species could not be assigned to any of the two subgenera, because their descriptions were based only on females, with males remaining unknown in the original and subsequent descriptions.
In the present study, we suggest using the male aedeagus shape and its orientation as a consistent and strong morphological character to redefine the two valid Oligonychus subgenera, instead of using inconsistent or variable characters, e.g., striation pattern on dorsal hysterosoma and number of tactile setae on tibia I (Wainstein 1960;Tuttle and Baker 1968;Meyer 1987). The proposed suggestion also agreed well with the molecular separation of various Oligonychus species into two groups, which successfully coincided with their morphological grouping based on male aedeagus i.e., aedeagus upturned vs. aedeagus downturned (Ben-David et al. 2007;Matsuda et al. 2012;Unpublished results). This study also supported the findings of Meyer (1974Meyer ( , 1987, who was the first to disagree with the subgeneric divisions of Oligonychus (Wainstein 1960;Tuttle and Baker 1968), due to the inconsistency of diagnostic characters, e.g., striation pattern on dorsal opisthosoma and number of tactile setae on tibia I. Meyer (1987) found that some of the African Oligonychus species, e.g., O. andrei Gutierrez and O. pennisetum, would not fit in any of the six Oligonychus subgenera (Tuttle and Baker 1968). Subsequently, more authors disagreed with the six-subgeneric Oligonychus system (Flechtmann and Alves 1976;Helle et al. 1981;Bolland et al. 1998;Beard et al. 2003Beard et al. , 2008Khanjani et al. 2018;Li et al. 2018). However, there are still authors who continue using the Oligonychus six-subgeneric system (Kamayev 2017;Li et al. 2019), without confirming the validity of characters that were initially devised to erect these subgenera (Tuttle and Baker 1968).
Based on the upturned aedeagus and tibia I with nine tactile setae (Tuttle and Baker 1968;Jeppson et al. 1975), we here synonymize the subgenus Pritchardinychus with the subgenus Reckiella. Both of these subgenera were previously separated using an inconsistent character of regarding the dorsal hysterosomal striae on the female (Meyer 1987) -longitudinal striae only between setae f 1 -f 1 in Reckiella or transverse striae on entire hysterosoma in Pritchardinychus (Tuttle and Baker 1968). In the present study, we observed longitudinal, irregular longitudinal or oblique striae present between both e 1 -e 1 and f 1 -f 1 setae in 10 Oligonychus species of the valid subgenus Reckiella, and longitudinal striae between only e 1 -e 1 setae in a species (O. andrei Gutierrez) of Reckiella (Zacher 1921;Baker and Pritchard 1960;Rimando 1962;Meyer 1964Meyer , 1965Meyer , 1974Meyer , 1987Ehara 1966;Gutierrez 1966Gutierrez , 1967Lo and Ho 1989;Tseng 1990). Furthermore, 13 Oligonychus species of the valid subgenus Reckiella with upturned aedeagus have seven or less than seven tactile setae on tibia I rather than nine setae. Also, the pattern of opisthosomal/ hysterosomal striae of some of these 13 species varies, e.g., reticulated pattern of irregular elongate elements in the case of O. comptus Meyer & Bolland, (Hirst), transverse except V-shaped/ irregular pattern between e 1 -e 1 setae in O. megandrosoma Flechtmann & Alves, and transverse except slightly U-shaped pattern between e 1 -f 1 area in O. poutericola Feres & Flechtmann (Hirst 1924;Baker and Pritchard 1962;Estebanes and Baker 1968;Livshits 1968;Gutierrez 1969;Paschoal 1970;Meyer 1974Meyer , 1987Flechtmann and Alves 1976;Meyer and Bolland 1984;Feres and Flechtmann 1986;Mendonca et al. 2010).

Subdivision of Oligonychus species into groups and subgroups
In the present study, four species groups and 11 species subgroups are recognized under the valid subgenera of Oligonychus and Reckiella, based on the combination of three morphological characters of the adult female: the number of tactile setae on tibiae I and II, the length of dorsal hysterosomal setae c 1 , and the pattern of striae on the dorsal hysterosoma. These characters were previously used to erect species groups by Pritchard and Baker (1955) and subgenera by Tuttle and Baker (1968) in Oligonychus.

Species groups and subgroups in the subgenus Oligonychus
The Oligonychus (Oligonychus) is subdivided into two species groups, the peruvianus species group (Pritchard and Baker 1955) and the newly proposed coffeae species group. The peruvianus species group is further categorized into two newly proposed species subgroups, the smithi species subgroup and the peruvianus species subgroup; whereas the coffeae species group is categorized into three species subgroups, subnudus species subgroup (Pritchard and Baker 1955), aceris species subgroup (Pritchard and Baker 1955), and the newly proposed coffeae species subgroup.
The following two species of the subgenus Reckiella could not be assigned to any species group/subgroup, because they were briefly described and/or certain key characters of the female were not included: O. annonicus (McGregor, 1955) O. stenoperitrematus (Ugarov & Nikolskii, 1937) iseilemae new species group Exemplar species. Paratetranychus iseilemae Hirst, 1924: 524. Diagnosis (based on female). Seven or less than seven (five or six) tactile setae on tibia I. Diagnosis (based on female). More than seven (eight, nine or rarely ten) tactile setae on tibia I.
** The available molecular data for O. amiensis (Matsuda et al. 2012) is proved association with the subgenus Oligonychus. Furthermore, based on female morphology, it belongs to the species group/subgroup coffeae.
Host and distribution. Picea sp. (Pinaceae); Italy. Remarks. Oligonychus picei (Canestrini) was described briefly based only on female, male remains unknown in original (Canestrini 1889) and subsequent descriptions (Pritchard and Baker 1955). Although Pritchard and Baker (1955) examined female paratypes, they did not provide a detailed re-description. They mention that it resembles O. subnudus (described from USA on Pinus sp., Pinaceae), differing by having comparatively longer dorsal setae. The identity of O. picei is doubtful until the male and female are comprehensively described from the type host and locality. (Oudemans, 1931) Paratetranychus primulae Oudemans, 1931: 291. Host and distribution. Primula obconica (Primulaceae); Netherlands.

Oligonychus primulae
Remarks. Oligonychus primulae (Oudemans) was very poorly described using only the female, without illustrations, and the male was unknown in both the original (Oudemans 1931) and subsequent descriptions (Geijsks 1939;Pritchard and Baker 1955). Furthermore, Oudemans (1931) did not compare it specifically with any closely related species, except mentioning the resemblance of its empodium with O. ununguis (Jacobi, 1905) (described from Germany on Picea abies, Pinaceae). Pritchard and Baker (1955) confirmed it to be an Oligonychus, but highlighted its taxonomic position as doubtful due to the absence of the male. The identity of O. primulae is uncertain until the male and female are comprehensively described from the type host and locality. (Reck, 1947) Paratetranychus kobachidzei Reck, 1947: 472. Host and distribution. Corylus avellana (Betulaceae), Juglans regia (Juglandaceae), Platanus occidentalis, P. orientalis (Platanaceae) and Ulmus sp. (Ulmaceae); Armenia, Azerbaijan and, Georgia.

Oligonychus kobachidzei
Remarks. Oligonychus kobachidzei (Reck) was described from male and female specimens from type host Platanus occidentalis and type locality Georgia; however, the description lacked the key characters necessary for species confirmation (Reck 1947). Moreover, Reck (1947) did not specifically compare it with any other closely related Oligonychus species. Although Bagdasarian (1957) re-described the species from other hosts (Juglans regia and Ulmus sp.) and locality (Armenia), the description still lacked details of the important morphological characters of both sexes. Further, it seems to us, based on the published literature that Bagdasarian (1957) did not observe the types of O. kobachidzei, as it necessary for confirming the taxonomic identity of Armenian specimens, when original description of O. kobachidzei was poor. Also, the illustration of aedeagus (Bagdasarian 1957) was not clear and did not appear to be in a completely lateral position. The identity of O. kobachidzei and its redescription is doubtful until the male and female are comprehensively described from the type host and locality. (Reck, 1956) Paratetranychus caucasicus Reck, 1956: 17. Host and distribution. Carpinus betulus, Corylus avellana (Betulaceae); Georgia. (Reck) was briefly described from only the female, and the male was unknown (Reck 1956). Although the description lacked illustrations, it indicated that the species did belong to the genus Oligonychus (Reck 1956). However, its species identity remains questionable, because the author neither described the male nor compared it specifically with any closely related species. The taxonomic status of O. caucasicus will be resolved after collecting and describing the male and female from the type host and locality.
Remarks. Oligonychus proteae Meyer & Ryke was described from only females, and details of the male were absent in both the original (Meyer and Ryke 1959) and subsequent descriptions (Meyer 1974). The taxonomic status of this species has remained doubtful since it was proposed (Meyer and Ryke 1959;Meyer 1974), and it was excluded from the list of Oligonychus species reported from Africa (Meyer 1987). The species identity of O. proteae will be resolved after collecting and describing the male from the type host and locality. Reeves, 1963 Oligonychus nielseni Reeves, 1963: 57. Host and distribution. Pinus strobus (Pinaceae); United States.

Oligonychus nielseni
Remarks. Oligonychus nielseni Reeves was described from only females, and details of the male were absent in both the original (Reeves 1963) and subsequent description (Baker and Tuttle 1994). The species was not specifically compared with any other closely related Oligonychus species (Reeves 1963). Moreover, variations have been reported in length of dorsocentral setae of the females from different populations, e.g., c 1 reaching to bases of e 1 (Reeves 1963) or c 1 shorter than the interval between c 1 -e 1 (Baker and Tuttle 1994). The species identity of O. nielseni will be resolved after collecting and describing the male from the type host and locality.

Remarks.
Oligonychus longus Chaudhri, Akbar & Rasool was briefly described from female specimens only, and the male was unknown (Chaudhri et al. 1974). The species was placed in the subgenus Reckiella, and although the authors did not specifically compare it with any other closely related species of Oligonychus, they did mention that based on the pattern of dorsal striae and length of the female body, this species differed from all species in that subgenus. However, the morphological information available for the female clearly indicates that this species does not actually match the subgenus Reckiella diagnosis of that time (e.g., Tuttle and Baker 1968;Chaudhri et al. 1974;Jeppson et al. 1975). The taxonomic identity of O. longus is doubtful until the male is collected and described from the type host and locality.
Remarks. Oligonychus conostegiae Tuttle, Baker & Abbatiello was briefly described from only females, and details of the male were absent in both the original (Tuttle et al. 1974) and subsequent description (Tuttle et al. 1976). The authors compared it with the female of O. gambelli (described from USA on Quercus gambelii, Fagaceae), and both species differed based on the often variable number of tactile setae proximal to the proximal duplex on tarsus I (Tuttle et al. 1974). Later, O. conostegiae was separated from females of O. platani (described from USA on Platanus occidentalis, Platanaceae) in a diagnostic key, using differences in the comparative lengths of the members of the duplex setae (McGregor 1950;Tuttle et al. 1976). These three species are distributed in similar geographical localities (Migeon and Dorkeld 2021), and their separation requires further taxonomic scrutiny. The species identity of O. conostegiae will be clear after collecting and describing the male from the type host and locality.
Remarks. Oligonychus daleae Tuttle, Baker & Abbatiello was described from females only, and details of the male were unknown (Tuttle et al. 1976). The authors did not specifically compare it with any other closely related species. However, O. daleae females were differentiated from the females of O. propetes (described from USA on Quercus alba) and O. quercus (described from Mexico on Quercus sp.) in a diagnostic key, using the striae pattern and comparative length of setae on the dorsal hysterosoma (Tuttle et al. 1976). These three species share similar geographical distributions (Migeon and Dorkeld 2021). The species identity of O. daleae will be clear after collecting and describing the male from the type host and locality.

Oligonychus changi Tseng 1980
Oligonychus changi Tseng, 1980: 152. Host and distribution. Pinus sp. (Pinaceae); Taiwan Remarks. Oligonychus changi Tseng was poorly described from females only, and details of the male were absent in both the original (Tseng 1980) and subsequent descriptions (Tseng 1990;Lo and Ho 1989). The author did not specifically compare it with any other closely related species. However, O. changi females were distinguished from the females of both O. subtropicus (another questionable species reported in same paper; described from Taiwan on Juniperus chinensis, Cupressaceae) and O. perditus (described from Japan on Juniperus communis) in a diagnostic key, using the pattern of dorsal hysterosomal striae (Tseng 1980(Tseng , 1990. The species identity of O. changi will be clear after collecting and describing the male from the type host and locality.
Remarks. Oligonychus jiangxiensis Ma & Yuan was briefly described from females only without detailed morphological characterization, and details of the male were absent. The authors did not compare it with any other closely distributed or closely related Oligonychus species, but instead compared it with O. endytus described from the United States on Quercus sp. (Fagaceae) (Migeon and Dorkeld 2021). Recently, Li et al. (2019) observed the type material of O. jiangxiensis, but did not re-describe or confirm its actual taxonomic status. The species identity of O. jiangxiensis will be clear after collecting and describing the male from the type host and locality. Tseng, 1980 Oligonychus subtropicus Tseng, 1980: 147. Host and distribution. Juniperus chinensis (Cupressaceae); Taiwan.

Oligonychus subtropicus
Remarks. Oligonychus subtropicus Tseng was described from only females, and details of the male were absent in both the original (Tseng 1980) and subsequent descriptions (Tseng 1990;Lo and Ho 1989). Although the author did not specifi-cally compare it with any closely related species, it was differentiated from O. perditus (from Japan on J. communis) in a diagnostic key, using slight differences in the pattern of dorsal hysterosomal striae (Tseng 1980(Tseng , 1990. Moreover, the author did not even compare it with another closely related species, O. chamaecyparisae Ma & Yuan (1976) reported from China on Chamaecyparis pisifera and other Cupressaceae hosts. Bolland et al. (1998) synonymized O. chamaecyparisae with O. perditus andMigeon andDorkeld (2021) follow this classification, despite it recently being reinstated as valid species by Li et al. (2019). The species identity of O. subtropicus will be clear after collecting and describing the male from the type host and locality. Tseng, 1990 Oligonychus mactus Tseng, 1990: 146. Host and distribution. Pinus sp. (Pinaceae); Taiwan.

Oligonychus mactus
Remarks. Oligonychus mactus Tseng was described from females only, and the male is unknown. Tseng differentiated the female from the females of O. clavatus (Ehara 1959) (described from Japan on Pinus spp.) and O. subnudus (McGregor 1950) (from USA on Pinus sp.), using the patterns of dorsal hysterosomal striae, the comparative lengths of prodorsal setae and the number of setae on tibia II (Tseng 1990). These three species inhabit Pinus spp., but were described from geographically well separated localities (Migeon and Dorkeld 2021). The male of O. mactus still needs to be collected and described from the type host and locality.
Remarks. The description of Oligonychus pongami Sivakumar & Kunchithapatham was based on just one morphological character, that the female differs from O. biharensis by having longitudinal striations between e 1 -e 1 vs. transverse in the later. There are numerous species in the genus Oligonychus which have longitudinal striation between setae e 1 -e 1 . No details of the male were provided, and the taxonomic identity of O. pongami is doubtful until detailed descriptions of the male and female type specimens are provided.

Species complexes in the genus Oligonychus
The term species complex, also referred to as sibling or cryptic species complex, is an informal taxonomic term or "open nomenclature qualifier" that is used when two/more morphologically indistinguishable but biologically separate species are present or sever-al distinct species are suspected to exist under one name, which results in the taxonomic uncertainty of a taxon (Sigovini et al. 2016). Species complexes are notoriously difficult to resolve when based on morphology alone (Pritchard and Baker 1955). However, such complex taxonomic issues have been efficiently and effectively addressed in different tetranychid genera, e.g., Mononychellus Wainstein, Oligonychus, and Tetranychus through the combination of morphological, molecular, and biological data (Navajas et al. 1994(Navajas et al. , 2001Gotoh et al. 1998Gotoh et al. , 2007Gotoh et al. , 2009Matsuda et al. 2013;Zeity et al. 2017). Therefore, integrative taxonomic approaches are needed to clarify the actual status of all closely related species, and species complexes, in the genus Oligonychus.
Within the genus Oligonychus, we recognized five new species complexes, viz. the afrasiaticus species complex, the litchi species complex, the punicae species complex, the plegas species complex and the tylus species complex, along with two previously highlighted complexes, the sacchari complex (Khanjani et al. 2018) and the pratensis complex (Pritchard and Baker 1955;Meyer 1974;Li et al. 2018 Presence and absence of para-anal setae in the genus Oligonychus Lindquist (1985) used the form and position of the H setae when determining setal homologies and recognized that both of the para-anal setae h 2 and h 3 are always present, and that the clunal seta h 1 is always absent in the genera Oligonychus and Tetranychus. This nomenclature was first interpreted by Oudemans (1930) and then later by Pritchard and Baker (1952), and as is followed here.
The setal shapes can be helpful when determining the presence or absence (and hence names) of para-anal setae in Oligonychus and its closely related genera, as previously highlighted by Lindquist (1985) and Seeman and Beard (2011). Moreover, the form of setae h 1 in different genera, for example Mixonychus and Schizotetranychus, are similar to other dorsal setae (Figs 1A, B), whereas setae h 2 and h 3 are usually similar in form to the anals, genitals, and other ventral setae, in all Tetranychini genera (Figs 2, 3).   The term "para-anal setae" was introduced by Pritchard and Baker (1955) and can be confusing when interpreting setae in the Tetranychini. So, for practical purposes, the first couplet (page 9, section 2.3) of the diagnostic key to genera of the tribe Tetranychini provided in the world catalogue of spider mites by Bolland et al. (1998) may be interpreted by users as, "3 pairs of h setae present, h 1-3 " versus "2 pairs of h setae present, h 2-3 and h 1 absent".
Key to subgenera, species groups, and subgroups of the genus Oligonychus Berlese Female with medial dorsal hysterosomal striae longitudinal, irregular longitudinal, oblique with/without forming a V/U-shaped pattern between setae e 1 and e 1 and between/posterior to f 1 and f 1 , and with/without forming a diamond pattern between setal rows E and F (Fig. 4A) (Fig. 4B), irregular longitudinal (Fig. 4C), oblique with/without forming a V/U-shaped pattern (Fig. 4D) between either setae e 1 and e 1 or between/posterior to f 1 and f 1 , and not forming a diamond pattern between setal rows E and F ..