The subgenus Oligonychus (Oligonychus) Berlese (Acari, Prostigmata, Tetranychidae), diagnostic keys to world species, and taxonomic notes

Abstract The present study provides the morphotaxonomic-based identification of the world species of the subgenus Oligonychus (Oligonychus) Berlese. In this lieu, five diagnostic keys were erected to identify Oligonychus (O.) species belonging to five subgroups; smithi, peruvianus, subnudus, aceris, and coffeae. In addition, taxonomic notes on some Oligonychus (O.) species, either briefly described or closely related, were provided, and intraspecific variations were highlighted.

Accurately identifying Oligonychus species compulsorily requires carefully observing both male and female morphological characteristics (Meyer 1987;Ben-David 2008;Mushtaq et al. 2021).This raises complications in species identification due to morphological similarities in females of several species, intraspecific variations, brief descriptions of some species, presence of doubtful species, the involvement of sibling/cryptic species complexes, minor differences in key traits of male aedeagus and above all, the unavailability of male specimens (Pritchard & Baker 1955;Jeppson et al. 1975;Meyer 1987;Khanjani et al. 2018;Li et al. 2018;Mushtaq et al. 2021Mushtaq et al. ,2023b)).
In the subgenus Oligonychus (O.), some closely related species are practically indistinguishable due to almost similar aedeagal morphology, minor interspecific differences, or intraspecific variations (Pritchard & Baker 1955;Beard et al. 2003;Khanjani et al. 2018;Li et al. 2018;Mushtaq et al. 2021).On the other hand, some species can only be differentiated based on the inhabited host plant (Pritchard & Baker 1955).Despite the taxonomic division of the subgenus Oligonychus (O.) into two species groups and five subgroups (Mushtaq et al. 2021), no comprehensive diagnostic key, other than some regional keys, has been developed yet, to identify world species of the subgenus Oligonychus (O.), e.g.(Pritchard & Baker 1955;Meyer 1974Meyer , 1987)).However, diagnostic keys have been developed to identify the two subgenera, four *Correspondence: F. J. Alatawi, Department of Plant Protection, College of Food and Agriculture Sciences, King Saud University, Riyadh 11451, Saudi Arabia.Email: falatawi@ksu.edu.sa The European Zoological Journal, 2024, 559-567 https://doi.org/10.1080/24750263.2024.2350197species groups, and 11 subgroups of the genus Oligonychus (Mushtaq et al. 2021) and for the identification of world species of the subgenus Reckeilla (Mushtaq et al. 2023a).
The present study aimed to develop the diagnostic key to each of five species subgroups (viz. aceris, coffeae, peruvianus, smithi, and subnudus) of the subgenus Oligonychus (O.).Brief taxonomic notes on some morphologically closely related species and intraspecific variations in some species are also discussed.

Materials and methods
The published morphotaxonomic literature of the 74 species of Oligonychus (O.) was collected using the websites of different research journals, various search engines, and personal contacts with acarologists around the globe.All Oligonychus (O.) literature, i.e., descriptions, redescriptions, illustrations of all 74 species, and regionally prepared identification keys, were critically investigated to develop five dichotomous keys for identifying all Oligonychus (O.) species.Moreover, following Mushtaq et al. (2023b), different aedeagal traits were measured for distinguishing among closely related Oligonychus (O.) species.O. tshimkenticus Wainstein could not be included in the key to species of the coffeae subgroup due to missing morphological information about the comparative length of members of duplex setae and the number of tactile setae proximal to the proximal duplex on tarsus I in female.

Notes on species of the subgroup subnudus
In the species subgroup subnudus, most Oligonychus (O.) species possess more or less similarly shaped aedeagus.In contrast, a few species (O.karamatus, O. laricis, O. milleri, etc.) generally have different aedeagal morphology.In the literature, variations are noticed in key characters of some species described from various geographical localities.For example, O. boudreauxi was described as having seven tactile setae on tibia I in the original description (Pritchard & Baker 1955), whereas Baker and Tuttle (1994) illustrated only five tactile setae.For the key preparation, the original description was followed by considering seven tactile setae on tibia I in O. boudreauxi (Pritchard & Baker 1955).

Notes on species of the subgroup coffeae
The species subgroup coffeae is very complicated, with many similar species, which are very difficult to distinguish among each other, including some species complexes, e.g., coffeae complex, punicae complex, and ununguis complex (Pritchard & Baker 1955;Meyer 1987;Ehara & Gotoh 2007;Khanjani et al. 2018;Mushtaq et al. 2021Mushtaq et al. , 2023b)).Among them, the punicae species complex has been discussed recently using both morphological and molecular data by suggesting O. mangiferus (Rahman & Sapra) and O. vitis Zaher & Shehata as junior synonyms of O. punicae, as well as revealing a cryptic Oligonychus (O.) species (Mushtaq et al. 2023b).Furthermore, intraspecific variations were observed in some important morphological traits of a few species (O.perditus, O. castaneae, O. coffeae, O. ununguis, O. bicolor and O. coniferarum), either within a population or among different populations that reported from various geographical localities (McGregor 1950;Pritchard & Baker 1955;Meyer 1974;Jeppson et al. 1975;Tseng 1990;Baker & Tuttle 1994;Gupta & Gupta 1994;Smiley & Baker 1995;Ehara 1999;Uysal et al. 2001;Ehara & Gotoh 2007;Arabuli & Gotoh 2018).For example, the number of tactile setae behind to proximal duplex in females was found to be variable (usually three, rarely four) among and within a population of O. castaneae (Ehara & Gotoh 2007) and O. coffeae (Meyer 1974), respectively.Whereas the aforementioned tactile setae in O. ununguis are usually four, three were also observed persistently among females of a population (Pritchard & Baker 1955).Additionally, various populations of O. coffeae had been described with straight peritreme (not hooked or U-shaped distally) in both males and females from different localities (Meyer 1974;Baker & Tuttle 1994;Gupta & Gupta 1994;Ehara 1999).However, one population of O. coffeae reported from Taiwan showed a strongly hooked peritreme in females (Tseng 1990).Also, Arabuli and Gotoh (2018) claimed that peritreme is hooked in the Japanese population of O. coffeae.
The shape of the aedeagus was also found to be variable among different populations of some Oligonychus (O.) species, viz.O. bicolor, O. coniferarum, Morphotaxonomic identification of Oligonychus (O.) spp.
O. perditus, etc.For example, as compared to all other descriptions of O. perditus (Pritchard & Baker 1955;Ehara 1962;Lo & Ho 1989;Li et al. 2019), only one population/illustration did not show a notch near the base of the shaft dorsal margin (Tseng 1990).Likewise, the aedeagus shape of O. bicolor is different (aedeagus not sausage-like, shaft axis forming an acute angle with the axis of the bent part, bent part gradually narrowing distally) (McGregor 1950), as compared to all other described populations (aedeagus sausage like, shaft axis forming an obtuse angle with the axis of the bent part, bent part abruptly narrowing distally) (Pritchard & Baker 1955;Meyer & Ryke 1959;Reeves 1963;Jeppson et al. 1975;Lo & Ho 1989;Tseng 1990;Baker & Tuttle 1994).The aedeagal shape of O. coniferarum that was reported/illustrated from Yemen (Smiley & Baker 1995) and Turkey (Uysal et al. 2001) seems somewhat different (bent part comparatively longer, directed ventrally and shaft axis forming right angle with the axis of bent part) as compared to all other reported populations (bent part short, directed caudoventrally and shaft axis forming obtuse angle with the axis of bent part) (McGregor 1950;Pritchard & Baker 1955;Jeppson et al. 1975;Baker & Tuttle 1994;Khanjani et al. 2018).
The relative length/width of palp spinneret has been used as an essential key character to distinguish among various Oligonychus species of the subgroup coffeae, e.g (Meyer 1987;Khanjani et al. 2018).Previously, O. quercifolius Wainstein (spinneret about two times longer than its width) (Wainstein 1956) was synonymized with O. longiclavatus (spinneret about four times longer than its width) (Bagdasarian 1957) by Reck (1959).In the original description, O. quercifolius was compared and separated from O. longiclavatus based only on the shape of palp spinneret (Wainstein 1956).Therefore, Wainstein (1960) did not accept this synonymy and separated O. quercifolius from O. longiclavatus in the key as valid and separate Oligonychus species.However, later authors (Bolland et al. 1998;Migeon & Dorkeld 2022;Akimov & Zhovnerchuk 2010) agreed with the synonymy of O. quercifolius with O. longiclavatus (Reck 1959).We suggest following an integrative taxonomic approach using both morphological and molecular data to reconfirm the taxonomic status of O. quercifolius.

Conclusions
The present study comprehensively investigated all published morphotaxonomic literature of the subgenus Oligonychus.Five diagnostic keys to identify the world species of the subgenus Oligonychus are developed.It was noticed that some Oligonychus (O.) species are morphologically very similar and hardly separated based on available morphological information.
Moreover, taxonomic notes highlighted some closely related species and intraspecific variations depicted in some Oligonychus (O.) species.Therefore, observing multiple specimens of a species from a population or different populations is strongly suggested to authenticate the persistency of a key morphological character.An integrative taxonomic approach is crucial to distinguish among closely related Oligonychus (O.) species or of resolving species complexes in the subgenus Oligonychus.