Review on the Genus Stylophoronychus (Acari: Tetranychidae), with Description of a New Species

Simple Summary The spider mite family Tetranychidae includes 85 genera and more than 1300 species worldwide, and is the group of phytophagous mites that has the greatest impact on the agricultural economy. Stylophoronychus is a small genus of six species that belongs to the tribe Aponychini which was reinstated as a valid taxon containing three genera (Aponychus, Paraponychus and Stylophoronychus). Here, a new species Stylophoronychus wangae Pan, Jin & Yi sp. nov. from Majiang Country, Guizhou Province, China (the Oriental realm) is described based on the deutonymph and adults. Two species, S. guangzhouensis (Ma and Yuan, 1980) and S. lalli (Prasad, 1975) are considered junior synonyms of S. vannus (Rimando, 1968). The taxonomy of the genus Stylophoronychus is reviewed and the studies on its ontogenetic development are discussed. Abstract Only two species of the genus Stylophoronychus, S. baghensis (Prasad, 1975) and S. guangzhouensis (Ma and Yuan, 1980), have been recorded in China. Herein we describe a new species Stylophoronychus wangae Pan, Jin & Yi sp. nov. based on characteristics of the deutonymphs and adults. The synonym of S. guangzhouensis (Ma and Yuan, 1980) and S. lalli (Prasad, 1975) with S. vannus (Rimando, 1968) is proposed. A redescription of S. vannus (Rimando, 1968) based on the adults of both sexes, deutonymphs and a protonymph is given. The ontogenetic changes of leg chaetotaxy in two species are given and discussed. The updated key to the species of Stylophoronychus of the world is provided.


Introduction
The genus Stylophoronychus belongs to the tribe Aponychini of the subfamily Tetranychinae and contains six species (S. baghensis, S. guangzhouensis, S. insularis, S. lalli, S. nakaoi and S. vannus) [1][2][3][4]. Most of these species are distributed in the Oriental realm, primarily found on Bambusa spp., with the exception of S. insularis which has been reported only from Araliaceae sp. in the Ethiopian realm [1,3,5]. Stylophoronychus was originally erected as the subgenus of Aponychus by Prasad to accommodate S. baghensis based on two strong lobes of the stylophore [1,6]. Later, the subgenus was elevated by Meyer [7] to the generic status due to the key character of nine pairs of dorsal setae on the hysterosoma, and all the species of Stylophoronychus were reclassified based on this character [1,7]. For more detailed changes in the classification of the genus refer to Zhang et al. [1].

Materials and Methods
The mite specimens studied were examined using a Leica DM 5000B microscope with differential interference contrast. Line drawings were prepared with the aid of a drawing tube attached to the microscope. Photographs were taken under oil immersion using a camera (Nikon DS-Ri 2) attached to the microscope (Nikon Ni E). Measurements were obtained using software (Nikon NIS Elements AR 4.50) and are provided in micrometers (µm). Length of the idiosoma was measured from the center of the setal base of v 2 to h 1 , while width was measured from the center of the setal base of c 1 to c 3. The measurements are presented for the holotype followed by the range of paratypes in parentheses. Morphological terminology follows that of Lindquist [10].

Taxonomic Discussion in S. guangzhouensis, S. lalli, S. vannus
These three species were compared based on the following characteristics: (1) Idiosoma. The body shapes of the adult females for all three species have the idiosoma nearly oblong, slightly longer than wide, and margins on both sides approximately parallel ( Figure 1).
(2) Dorsal setae. For the three species shown in Figure 1 and Table 1, the dorsocentral setae are long, linear, inserted on tubercles and pubescent. They are characterized such that their length, with the exception of the first row, is approximately equal or shorter than distances between their bases. The key provided by Zhang et al. [1] uses the relative length of e 1 subequal and less than the distance between their bases to distinguish among the three species, but this character is unreliable. According to Hernandes and Feres [2], the shape of setae h 1 is the only character that can distinguish these three species reliably but only when a large series of specimens is examined. However, after examining 19 female specimens of S. guangzhouensis, we found the shape of setae h 1 is variable, ranging between spatulate, fan-like or palmate. Figures 2 and 3 show that S. guangzhouensis shares the shapes of h 1 setae with the other two species. This variation of the shape of h 1 is most likely intraspecific polymorphism and cannot be used as a character to distinguish the three species. (2) Dorsal setae. For the three species shown in Figure 1 and Table 1, the dorsocentral setae are long, linear, inserted on tubercles and pubescent. They are characterized such that their length, with the exception of the first row, is approximately equal or shorter than distances between their bases. The key provided by Zhang et al. [1] uses the relative length of e1 subequal and less than the distance between their bases to distinguish among the three species, but this character is unreliable. According to Hernandes and Feres [2], the shape of setae h1 is the only character that can distinguish these three species reliably but only when a large series of specimens is examined. However, after examining 19 female specimens of S. guangzhouensis, we found the shape of setae h1 is variable, ranging between spatulate, fan-like or palmate. Figures 2 and 3 show that S. guangzhouensis shares the shapes of h1 setae with the other two species. This variation of the shape of h1 is most likely intraspecific polymorphism and cannot be used as a character to distinguish the three species.        (3) Patterns of hysterosomal median protuberance. S. guangzhouensis, S. lalli and vannus have a protuberance on the hysterosomal median that arched upward, shaped l the outline of butterfly wings, densely covered with irregular patterns of circles or fusion of many circles ( Figure 4). (3) Patterns of hysterosomal median protuberance. S. guangzhouensis, S. lalli and S. vannus have a protuberance on the hysterosomal median that arched upward, shaped like the outline of butterfly wings, densely covered with irregular patterns of circles or the fusion of many circles ( Figure 4).
(4) Patterns of integument on medial prodorsum. S. guangzhouensis, S. lalli and S. vannus have a nearly square bulge on the prodorsal median area with a conspicuous pattern of highly wrinkled ornamentation that resembles a brain cortex (described by Hernandes and Feres) ( Figure 5).
(5) Stylophore. There are similar apicodorsal granulations on the two strong lobes of the stylophore with a median convex area in S. guangzhouensis, S. lalli and S. vannus ( Figure 6).   Figure 4B refer to the outline shape of protuberance. "Black line" in Figure 4B refer to the shape of the hysterosomal median protuberance of S. lalli female, because the outline of the photo is not clear.
(4) Patterns of integument on medial prodorsum. S. guangzhouensis, S. lalli and S. vannus have a nearly square bulge on the prodorsal median area with a conspicuous pattern of highly wrinkled ornamentation that resembles a brain cortex (described by Hernandes and Feres) ( Figure 5).    Figure 4B refer to the outline shape of protuberance. "Black line" in Figure 4B refer to the shape of the hysterosomal median protuberance of S. lalli female, because the outline of the photo is not clear.
Based on the above common typical characteristics analysis, S. guangzhouensis, S. lalli and S. vannus are considered to be synonymous, the latter species having priority. The variations on tarsus III-IV of Stylophoronychus vannus as shown in Table 2.
Based on the above common typical characteristics analysis, S. guangzhouensis, S. lalli and S. vannus are considered to be synonymous, the latter species having priority. The variations on tarsus III-IV of Stylophoronychus vannus as shown in Table 2.
Gnathosoma ( Figures 10F and 15). Stylophore with slightly or well-developed bilobed horn-like anterior projections. Ornamentation of integument similar to that of fe- Four setae are added to the legs of the deutonymph of this species during ontogeny: l′ is added to femur I. v′ on trochanter I-III, respectively. A total of 50 setae are suppressed on legs I-IV in the deutonymphal stage of this species: two on femur I, four on genua I, four on tibia I, eight on tarsus I, four on genua II, four on tibia II, three on tarsus II, one on femur III, three on genua III, four on tibia III, three on tarsus III, one on femur IV, three on genua IV, four on tibia IV, and two on tarsus IV. Protonymph (n = 1) Dorsum (Figures 17 and 18). Idiosoma oval without protuberance on hysterosoma, 198 long, 148 wide.
Legs ( Figure 11A-D). Empodial claws absent. One pair of duplex setae on tarsus I, solenidion ω 8-9, one additional ventral solenidion (vω) at the same transverse level with u setae, 10-14 long, tectal seta (tc ) unpaired, thicker than other tactile setae on tarsus I; tibia I with one solenidion ϕ 10-12 long; tarsus II without duplex setae, solenidion ω 10-13 long, tectal seta (tc ) unpaired, thicker than other tactile setae; tarsus III without solenidion, tectal setae paired; tarsus IV with one proximal solenidion ω 6-7 long, tectal setae paired. Number of tactile setae on leg (I-IV) segments: trochanters 1-1-1-1, femora 6-5-3-1, genua 1-1-1-1, tibiae 3-1-1-1, tarsus 7-7-6 (or 7) -6 (or 7). Number of eupathidia on tarsus I-IV: 3-3-0-0. Legs I-IV setation and notation as shown in Figure 11A- Number of tactile setae on Tarsus III-IV varies among specimens, and differs between right and left legs in the same specimen (Table 2). Among 23 adult females, 12 (one from Thailand, one from India, six from Yunnan Province, China and four from Guangdong Province, China) with six tactile setae (u , u , ft , ft , pv , pv ) on both right and left tarsus III; six (two from Thailand, four from Yunnan Province, China) with seven tactile setae (u , u , ft , ft , pv , pv , v 1 ) on both right and left tarsus III; three (two from Yunnan Province, China and one from Guangdong Province, China) with seven tactile setae on right tarsus III and six tactile setae on left; one (from Guangdong Province China)with six tactile setae (u , u , ft , ft , pv , pv ) and one solenidion (ω ) on right tarsus III and six tactile setae on left; one (from Yunnan Province, China) with seven tactile setae on left tarsus III and unknown the right side due to the broken tarsus III. Among 23 adult females, 15 (one from Thailand, one from India, eight from Yunnan Province, China and five from Guangdong Province, China) with six tactile setae (u , u , ft , ft , pv , pv ) and one solenidion (ω ) on both right and left tarsus IV; four (two from Thailand, two from Yunnan Province, China) with seven tactile setae (u , u , ft , ft , pv , pv , v 1 ) and one solenidion (ω ) on both right and left tarsus IV; two (from Yunnan Province, China) with seven tactile setae and one solenidion on left tarsus IV and six tactile setae and one solenidion on right; one (from Guangdong Province, China) with seven tactile setae on left tarsus IV and six tactile setae and one solenidion on right; one (from Yunnan Province, China) with six tactile setae and one solenidion on left tarsus IV and unknown the right side due to the broken tarsus IV. The variations in the setal count of tarsus III and IV are here considered intraspecific in nature, and attributed to the geographical position of the samples and different host plant species. In order to express the ontogenetic development of leg chaetotaxy conveniently, tarsus III with six tactile setae and tarsus IV with six tactile setae and one solenidion are regarded as normal setal count.
Setal counts (solenidion in parentheses following tactile setae) on legs I-IV are: femora 6-5-3-1, genua 1-1-1-1, tibiae 3(1)-1-1-1, tarsus 7(2)-7(1)-6(0)-6(1). There is a significant amount of setal suppression on the legs in this species, with a total of 15 setae being added to the legs in the adult female stage of this species: pair l 1 on femur I, pair v 1 on tarsus I, pair l 1 on femur II, pair v 1 on tarsus II, v on trochanter III, v and l 1 on femur III, l on genua III, v on trochanter IV, l on genua IV and ω on tarsus IV. According to the normal ontogenetic setal additions for the family [10], seven of thirteen additional setae are delayed additions: v 1 on tarsus I suppressed on protonymph stage, v 1 on tarusus I, v 1 on tarsus II, v on trochanter III and IV, l on genua III and IV are suppressed on deutonymph stage.
A total of 26 setae were added to the legs in the adult male stage of this species, and 11 additional setae are delayed additions: v on Genua I and II suppressed in larva stage, v 1 on tarsus I suppressed on protonymph stage, and v 1 , ω 1 on tarusus I, v 1 on tarsus II, v on trochanter III and IV, l on genua III and IV, and ω on tarsus III are suppressed on deutonymph stage. Tarsus III in male of S. vannus does not express standard adult seta v 1 , replaced by v 1 .
Aedeagus ( Figure 13A,B). Aedeagus dorsally curved, gradually narrowing and bent distally to form a somewhat right angle.
Legs (Figure 16A-D). Empodial claws absent. One pair of duplex setae on tarsus I, sometimes setal bases of ft and ω separated, solenidion ω 5-6, one additional ventral solenidion (vω) at the same level with u setae, 7-10 long, tectal seta (tc ) unpaired, thicker than other tactile setae on tarsus I; tibia I with one solenidion 5-8 long; tarsus II without duplex setae, solenidion ω 8-10 long, tectal seta (tc ) unpaired, thicker than other tactile setae; and tarsus III and tarsus IV without solenidion. Number of tactile setae on leg (I-IV) segments: trochanters 1-1-1-0, femora 4-3-1-1, genua 1-1-0-0, tibiae 3-1-1-1, tarsus 5-5-6-6. Number of eupathidia on tarsus I-V: 3-3-0-0. Legs I-IV setation and notation as shown in Figure 16A Four setae are added to the legs of the deutonymph of this species during ontogeny: l is added to femur I. v on trochanter I-III, respectively. A total of 50 setae are suppressed on legs I-IV in the deutonymphal stage of this species: two on femur I, four on genua I, four on tibia I, eight on tarsus I, four on genua II, four on tibia II, three on tarsus II, one on femur III, three on genua III, four on tibia III, three on tarsus III, one on femur IV, three on genua IV, four on tibia IV, and two on tarsus IV.

Key to species of Stylophoronychus (females)
Author Contributions: Conceptualization, all authors; methodology, all authors; software, X.P.; data curation, R.O., D.J. and T.Y.; writing-original draft preparation, X.P.; writing-review and editing, all authors; supervision, T.Y.; project administration, T.Y.; funding acquisition, T.Y. All authors have read and agreed to the published version of the manuscript.