Schizotetranychus Trägårdh, 1915

Kamran, Muhammad, Hafeez, Abdul, Alatawi, Fahad Jaber & Flechtmann, Carlos Holger Wenzel, 2024, Species groups, subgroups, and key to world species of the genus Schizotetranychus Trägårdh, 1915 (Acari, Prostigmata, Tetranychidae), ZooKeys 1211, pp. 131-150 : 131-150

Genus Schizotetranychus Trägårdh

Type species.

Tetranychus schizopus Zacher, 1913 .

Diagnosis.

Dorsal hysterosoma with ten pairs of setae (c 1-3, d 1-2, e 1-2, f 1-2, and h 1), setae h 2 and h 3 present on ventral opisthosoma, empodial claw divided deeply into two claw-like structures, palp tarsus with seven structures / setae (one spinneret, two eupathidia, one solenidion, three setiform setae); dorsal hysterosoma medially usually with transverse striations, but may be longitudinal or irregular between d 1 and e 1; two sets of duplex setae on tarsus I present distally, nearly adjacent to each other.

Background and taxonomic review of the genus Schizotetranychus

The genus Schizotetranychus was erected by Trägårdh (1915) based on Tetranychus schizopus Zacher, 1913 and distinguished from the genus Tetranychus by having the leg empodia divided deeply into two claw-like structures. Two years later, Banks (1917) erected the genus Stigmaeopsis and designated St. celarius Banks its type species. Banks (1917) described the genus Stigmaeopsis very briefly and did not provide any diagnostic characters which could separate it from the closely related genus Schizotetranychus . Later, McGregor (1950) synonymized the monospecific genus Stigmaeopsis with Schizotetranychus . This synonymy was accepted and followed by Pritchard and Baker (1955), Baker and Pritchard (1960), Tuttle and Baker (1968), Gutierrez (1968), Meyer (1974, 1987), Tuttle et al. (1976), Bolland et al. (1998), and Ehara (1999).

For the first time, Ehara (1999) introduced species groups in the genus Schizotetranychus by dividing the nine Schizotetranychus species reported from Japan into two species groups: schizopus (six species) with transverse striations in the anterior portion of dorsocentral area on dorsal opisthosoma and celarius (three species) with longitudinal striations in the anterior portion of dorsocentral area on dorsal opisthosoma.

Later, Saito et al. (2004) reinstated the genus Stigmaeopsis and distinguished it from Schizotetranychus and other genera of Tetranychidae by having six setae / structures on the palp tarsus in the female (instead of seven); dorsal striations between c 1 and d 1, clearly longitudinal, forming a trapezoidal shape instead of having mostly transverse or longitudinal irregular without forming a trapezoidal shape in Schizotetranychus . Also, the bases of setae e 1, d 1, and c 1 gradually become further apart than the bases of f 1 setae; if hypothetical lines connecting their bases are drawn, they form a V-shaped pattern vs these lines being almost parallel in Schizotetranychus and related genera, as described by Saito et al. (2004). Based on these characteristics, Flechtmann (2012) transferred two Schizotetranychus species, S. malkovskii Wainstein, 1956 and S. meghalensis ( Gupta & Gupta, 1994) to Stigmaeopsis . Although S. meghalensis has transverse striations between setae c 1, d 1, and e 1, and does not satisfy several characters of Stigmaeopsis , despite the fact that Saito et al. (2016, 2018, 2019) provisionally included this species in Stigmaeopsis because of its six setae / structures on the palp tarsus.

Morphological diagnostic features previously used for grouping Schizotetranychus species

Flechtmann (2012) categorized world 106 Schizotetranychus species into 17 groups based on following female morphological characters: body length: width ratio, dorsal setal length, shape of peritremes, number of tactile setae on tibia I. As a result, numerous species groups in the genus Schizotetranychus based on variable morphological characters are causing confusion and misunderstanding in species identification.

Peritremes distally are variously developed in Schizotetranychus species, straight in most species, and either making a U-shape, ring, or looped distally others. Peritremes distally are anastomosing in two species S. cajani Gupta, 1996 , S. prosopis Tuttle, Baker & Abbatiello, 1976 . Flechtmann (2012) used this character to arbitrarily propose different groups for Schizotetranychus species. We consider the shape of the peritreme at species level a misleading character because it is variously developed distally even in different specimens of the same species, and described and illustrated differently for one species by various authors. Also, this character was already causing confusion while attempting to separate the species groups of Schizotetranychus created by Flechtmann (2012).

Mite body shape is either oval (longer than wide in most of species), or orbicular (as long as wide in few species), or elongate (more than 2 × longer than width of body in few species). This character was used by Flechtmann (2012) to develop groups in Schizotetranychus . However, it caused confusion in the identification of those groups because some species lie on the borderline in length and width ratios. Also, the length and width ratios could be affected by the mounting of specimens on glass slides.

Dorsal and ventral idiosoma is entirely striated in almost all Schizotetranychus species, either widely or closely spaced, except S. reticulatus Baker & Pritchard, 1960 with reticulations on the propodosoma medially, and the hysterosoma is entirely striated and rugose. The dorsal hysterosoma between setae c 1, d 1, e 1, and f 1 with transverse striations entirely in all Schizotetranychus species except six, namely S. hidayahae Yusof & Zhang, 2003 , S. baltazarae Rimando, 1962 , S. spiculus Baker & Pritchard, 1960 , S. brevisetosus Ehara, 1989 , S. rhodanus Baker & Pritchard, 1960 , and S. colocasiae Ehara, 1988 (as in Ehara & Tho, 1988), in which the striations between setae e 1 and d 1 form a V-shaped pattern or is irregular.

Dorsal body setae are usually setiform in Schizotetranychus species. However, few species have awl-shaped dorsal setae with slightly expanded bases. Flechtmann (2012) used this character to develop species groups in Schizotetranychus . However, dorsal setae were not properly illustrated or described in detail for many Schizotetranychus species, so for those species it is very difficult to discern the exact shape (awl or setiform) of the dorsal setae. This character can be considered as supporting species level character.

Taxonomic notes about two Schizotetranychus species having six setae / structures on the palp tarsus as in the genus Stigmaeopsis

As mentioned earlier, the genus Stigmaeopsis differs from Schizotetranychus by having six setae / structures on the palp tarsus in females instead of seven; dorsal striations lie between c 1 and d 1 are clearly longitudinal and forming a trapezoidal shape instead of being mostly transverse or irregularly longitudinal between setae d 1 and e 1 in six Schizotetranychus species, namely, S. hidayahae , S. baltazarae , S. spiculus , S. brevisetosus , S. rhodanus and S. colocasiae without forming a trapezoidal shape. Also, the bases of setae e 1, d 1, and c 1 gradually widen further apart than the bases of f 1 setae if hypothetical lines connecting their bases are drawn. They form a V-shaped pattern vs almost parallel lines are in Schizotetranychus and related genera, as described by Saito et al. (2004).

In the present study, it was found that two Schizotetranychus species, S. gausus Baker & Pritchard, 1960 and S. luculentus ( Tseng, 1990) have six setae / structures including spinneret and solenidion on palp tarsus. The original description of these species lacking information of palp setae. So, relying on the original illustrations, these two Schizotetranychus species having six setae on palp tarsus are provisionally transferred to Stigmaeopsis . Also, dorsum is entirely reticulated in S. luculentus ( Tseng, 1990) . However, dorsum with transverse striations between setae c 1, d 1 and irregular longitudinal between setae e 1 and d 1 in S. gausus . Moreover, 16 known species of Stigmaeopsis have five tactile setae on tibia II except S. gausus having seven setae on tibia II.

Furthermore, bases of length dorsal setae c 1, d 1 which is ~ 2 × more widely spaced to the bases of e 1 and f 1 (bases of c 1, d 1, e 1, f 1 forming a V-shaped pattern) in S. Attiah, 1967 as in all known 16 Stigmaeopsis species. Few other Schizotetranychus species have a similar pattern of dorsal setal bases. So, the supporting diagnostic character that hypothetical lines connecting the bases of setae c 1, d 1, e 1, and f 1 forming a V-shaped taken by Saito et al. (2004) for Stigmaeopsis while reinstating this genus to separate it from Schizotetranychus becomes impractical.

Hence it is understood from the above discussion that genus Stigmaeopsis is different from Schizotetranychus by only one character, the presence of six setae / structures on palp tarsus vs seven in Schizotetranychus . All other supporting characters (longitudinal striations between setae c 1 and d 1, bases of setae c 1, d 1, e 1, and f 1 forming a V-shaped pattern) of Stigmaeopsis taken by Saito et al. (2004, 2018) as a generic character could be considered as species level characters.

Species groups and subgroups of Schizotetranychus developed in the current study

In the present research, after comprehensive taxonomic assessment of descriptions and illustrations of all known (116) species of the genus Schizotetranychus , species grouping in this genus is reconsidered based on females using only the number of tactile setae on tibia II and species subgroups based on only the number of tactile setae on tibia I. The number of tactile setae on tibia II is found to be a consistent diagnostic character in Schizotetranychus species and described in 110 Schizotetranychus species, even those which were very briefly described. Flechtmann (2012) used tactile setae on tibia I to separate some Schizotetranychus groups. Pritchard and Baker (1955) and Mushtaq et al. (2021) also used tactile setae on tibia I to develop species groups in the genus Oligonychus . Based on tibial setal counts, species groups of Schizotetranychus can easily be recognized.

In the present study, the genus Schizotetranychus can be divided into five species groups based on the number of tactile setae on tibia II in the female: schizopus group (52 spp.) with five setae, asparagi group with seven setae (20 spp.), bambusae group with eight setae (22 spp.), spireafolia group with six setae (10 spp.) and vermiculatus group with four setae on tibia II (four spp.). Also, keys to the world Schizotetranychus species, species groups, and subgroups are developed for the first time. Eight Schizotetranychus species were not assigned any species group because these have been described and illustrated very briefly without information on the number of setae on tibia I and II.

1. Species group schizopus

Diagnosis. Female: Tibia II with five setae (52 species).

Exemplar species. Schizotetranychus schizopus (Zacher, 1913)

Species group schizopus is further divided into three species subgroups based on number of tactile setae excluding solenidion on tibia I.

i) Species subgroup schizopus

Diagnosis. Female. Tibia I with eight / nine setae (21 species).

Exemplar species. Schizotetranychus schizopus

ii) Species subgroup andropogoni

Diagnosis. Female. Tibia I with seven setae (26 species).

Exemplar species. Schizotetranychus andropogoni ( Hirst, 1926)

iii) Species subgroup taquarae

Diagnosis. Female. Tibia I with six setae (5 species).

Exemplar species. Schizotetranychus taquarae Paschoal, 1971

2. Species group asparagi

Diagnosis. Female. Tibia II with seven setae (20 spp.)

Exemplar species. Schizotetranychus asparagi ( Oudemans, 1928)

3. Species group bambusae

Diagnosis. Female. Tibia II with eight setae (22 spp.).

Exemplar species. Schizotetranychus bambusae Reck, 1941

4. Species group spireafolia

Diagnosis. Female. Tibia II with six setae (10 spp.).

Exemplar species. Schizotetranychus spireafolia Garman, 1940

5. Species group vermiculatus

Diagnosis. Female. Tibia II with four setae (04 spp.).

Exemplar species. Schizotetranychus vermiculatus Ehara & Wongsiri, 1975

Ungrouped species

The following eight species were not assigned any species group because these have been described and illustrated very briefly without information of number of tactile setae on tibiae I and II. Schizotetranychus setariae Meyer, 1987 was not assigned to any species group / subgroup because it was only described / known from the male.

S. graminicola Goux, 1949

S. glabrisetus ( Ugarov & Nikolskii, 1937)

S. tuberculatus (Ugarov & Nikolski, 1937)

S. guatemalae-novae ( Stoll, 1886)

S. hindustanicus ( Hirst, 1924)

S. mustafaii Mustafa & Chaudri, 1972 (as in Mustafa et al. 1972)

S. oudemansi Reck, 1948

S. setariae Meyer, 1987

Moreover, two Schizotetranychus species, S. gausus and S. luculentus , that have six setae / structures including spinneret and a solenidion on the palp tarsus based on original illustrations, are provisionally transferred to Stigmaeopsis . Further studies are required to confirm the clear taxonomic status of these two species.