﻿A new genus of bamboo culm tarantula from Thailand (Araneae, Mygalomorphae, Theraphosidae)

﻿Abstract Bamboo plays an important role in the animal world, including providing a nutritious food source, shelter, and habitat. Inside of bamboo culm, we discovered a new genus of tarantula, which we describe here as Taksinusgen. nov. (♂♀). Specimens of this new tarantula were collected from Mae Tho, Mueang Tak district, Tak province, in Thailand, making it geographically distant from any other arboreal genera. Genital morphology was used to diagnose its genus, which is supported by distributional data, natural history, morphological characters, and photographic illustrations of the male and female. Diagnosis of the new genus was determined by distinguishing its different characters from those of other arboreal theraphosid spiders distributed throughout Southeast Asia. This tarantula’s specialization is that it lives in the stalks of the Asian bamboo Gigantochloa sp.


Introduction
Theraphosidae Thorell, 1869, which are commonly known as tarantulas, comprises the most diverse family among Mygalomorphae Pocock, 1892, with over 1,000 species currently described (World Spider Catalog 2021). Asian tarantulas in the subfamily Ornithoctoninae Pocock, 1895, which are commonly known as earth tigers, were originally established by Pocock (1895). In this subfamily, four arboreal theraphosid genera have been recognized: Omothymus Thorell, 1891, Lampropelma Simon, 1892Phormingochilus Pocock, 1895, and Melognathus Chamberlin, 1917. Tarantulas have not been observed to disperse aerially and thus have limited dispersal capabilities and habitat fidelities (Hendrixson et al. 2013). The type of localities and distribution of the Southeast Asian arboreal tarantula differ, as some species are restricted to a specific island; therefore, the spider's location can be used to alongside morphology to diagnose species (Gabriel and Sherwood 2019). Lampropelma has been found in Indonesia (Sangihe Island and Sulawesi); Omothymus was reported in Malaysia, Singapore, and Sumatra; Phormingochilus currently contains four species, which are restricted to Borneo. The type locality of Melognathus is unclear. It was formerly reported as "East Indies? Philippines?" (Chamberlin 1917). In this study, we report a new genus, Taksinus gen. nov., discovered in northern Thailand, which is geographically distant from any previous record of the Asian arboreal species (Figs 1, 2). Historically, the taxonomy of arboreal Ornithoctoninae has been based primarily on unstable taxonomic characters, resulting in highly problematic identification characters being used before 2019. Pocock (1895) distinguished the genus Phormingochilus from Omothymus based on the anterior narrowness of the sternum. In the key of the Ornithoctoninae, Raven (1985) distinguished Phormingochilus based on the breadth of the ocular tubercle and the low caput. He defined Lampropelma from Cyriopagopus by the presence of brush setae on the retrolateral palpal femur. Smith (1994) proposed that the genus Phormingochilus could be distinguished by an absence of tibial apophyses and brush setae on the retrolateral face of the palpal femur, while exhibiting twin spermathecae. Smith and Jacobi (2015) reported that Lampropelma, Omothymus, and Phormingochilus share the following characteristics: a wide ocular tubercle, spermathecae with twin seminal receptacles, and low caput, whereas terrestrial tarantulas, such as Cyriopagopus have an extremely raised caput on the carapace. Recently, Gabriel and Sherwood (2019) revised the arboreal Ornithoctoninae and proposed stable taxonomic features, which clearly delineated the subfamily Ornithoctoninae, especially based on the male palpal bulb and tibial apophyses, which remain constant and relevant for characterizing arboreal species and genera. The comparative female leg measurements and geographic distribution can be used to further elucidate these taxa. Thailand currently has two genera of the tarantula subfamily Ornithoctoninae: Ornithoctonus Pocock, 1892and Cyriopagopus Simon, 1887(World Spider Catalog 2021. The newly recorded arboreal Taksinus gen. nov. was discovered in an extraordinary habitat, namely, bamboo culms with silken retreat tubes covering the stem cavity (Fig. 1A, B). The type localities of Taksinus gen. nov. and some Southeast Asian arboreal Ornithoctoninae are shown in Figure 2. We describe and provide illustrations of the body and copulatory organs, as well as information on the natural history and morphological characteristics that distinguish the new genus from other arboreal Ornithoctoninae.

Materials and methods
Specimens were collected in Tak province, Thailand, on 21 July 2020. All tarantulas were preserved in 95% ethanol. Specimens were transferred to the Department of Entomology and Plant Pathology, Khon Kaen University, Khon Kaen, Thailand (ENTOKKU), for dissection and identification. The total body length, including cephalothorax and abdomen without spinnerets and appendage segments, were measured using digital vernier calipers. Diagnostic features and genitalia were photographed using a digital camera mounted to the phototube of a Nikon SMZ745T stereomicroscope, and the NIS-Elements D program was employed for measurement and counting morphology. Appendage measurements were made dorsally along the central axis of the measured structures from the left side and recorded in millimeters (mm). The length ratio of leg I-leg IV multiplied by 100 (von Wirth and Striffler Figure 2. Distribution records of Taksinus bambus sp. nov. from Tak province, Thailand, and some arboreal Ornithoctoninae (Gabriel and Sherwood 2019), which demonstrate the separation of the genera by color. The topographic map of Southeast Asia and data were obtained from OpenStreetMap and OpenStreetMap Foundation OpenStreetMap (https://www.openstreetmap.org).
2005) was used to calculate the relation factor (RF), which was compared with data from Gabriel and Sherwood (2019) who proposed the difference between the total lengths of leg I and IV to show the distinct characteristics of Phormingochilus, Lampropelma, and Omothymus. The leg formula, with the leg length in decreasing order, is also provided. Unless otherwise noted, the color of the morphological character was observed in ethanol-preserved specimens. Female genitalia were dissected and cleared in 3 M aqueous KOH solution. Specimens were identified by comparing them to related species (Smith and  If all the spines in the apical part were apically positioned, the term "apical" would be used to refer to their position.

Other material examined
Omothymus sp. 2 ♂ Surat Thani and Chumphon, Thailand. Omothymus sp. 1 ♀ specimen was donated from an unknown locality. Cyriopagopus albostriatus (Simon, 1886) 2 ♀ Saraburi, Thailand. Cyriopagopus minax (Thorell, 1897) 3 ♀ Chiang Mai, Thailand. Cyriopagopus lividus (Smith, 1996)  Diagnosis. The characteristics of Taksinus gen. nov. that differ from Ornithoctonus and Cyriopagopus are: a low caput, a clypeus that is less than the width of the median ocular quadrangle ( Etymology. The generic name was named Phraya Tak (governor of Tak province), which is in honor of Taksin the Great, king of the Thonburi Kingdom, in commemoration of his early career.
Description. Carapace longer than wide, low caput. Fovea deep, straight (males) or slightly procurved (females). Clypeus short, less than width of median ocular quadrangle in males and females. Eight eyes arranged on tubercle, anterior eye row slightly procurved and the posterior row straight. Outer cheliceral on lower surface from margin with five slightly curved pad of plumose setae on the retrolateral chelicerae. Maxillae longer than wide with >155 cuspules (male) or 149-183 (females), two horizontal rows of 10-11 stout thorn-like spines on the lower half of prolateral maxillae (below suture) and one horizontal row of six stout thorn-like spines on the upper half of prolateral maxillae (above suture). Spines of varying lengths, with the longest being at the top of the series; combined to form a stridulating organ. Labium wider than long, with 75 cuspules (male) or 125 (females). Sternum longer than wide, with two pairs of ovoid sigillae; Posterior sigilla is significantly remote from the edge, middle sigilla is close to the margin, and anterior sigilla is indistinguishable. Legs: formula 1423 (males); ± Total lengths of legs I and IV = 0.48, 4123 (females) ± Total lengths of legs I and IV = 2.41-3.33, RF = 101 (males) or 90.6-93 (females). Scopulae distinct, thickly set on tarsus; ventral surface not divided. Tibial spur capped with multitude of thin, short black spines, with no single megaspine on the inside of the tibial apophyses. Palpal bulb is ellipsoid and partly concave, embolus short compared to palpal bulb length (1:1), moderately curved, rounded apex, with single retrolateral keel. Spermathecae have twin seminal receptacles, rounded tombstone receptacles, fused in the basal region.
Distribution. Tak province, Thailand Etymology. The species name bambus refers to the species, which was discovered in a bamboo plantation and lives in Asian bamboo stalks.

Discussion and conclusion
Recently, Gabriel and Sherwood (2019) revised some arboreal Ornithoctoninae and proposed various stable morphological features that enable generic-level delineation, including the presence of the embolus rising parallel and sloping to a point at the tip, which distinguishes the genus Omothymus from Phormingochilus; embolus apical swelling in Lampropelma and the difference between the total lengths of leg I and IV (± 2-3 mm difference in Phormingochilus, ± 5 mm in Lampropelma and ± 10 mm in Omothymus) in all cases, leg I is longer than leg IV. Von Wirth and Striffler (2005) proposed a similar concept for terrestrial Ornithoctoninae called leg relation factor (RF), although Leg RF is a mathematical formula that expresses the relationship as a decimal value. We examined both methodologies and included Taksinus gen. nov. in our analysis, using leg measurements from original species descriptions or study type material obtained from the World Spider Catalog (2021), as shown in Table 4. The result indicated that Taksinus gen. nov. had the minimum difference between the total length of leg I minus IV = −3.33 to −2.41 (RF = 90.56-93.04), whereas O. violaceopes had the maximum height at +9.9 (RF = 112.3). According to Gabriel and Sherwood's criterion, Taksinus gen. nov. and Phormingochilus share a leg measuring range ± 2-3 mm, indicating that they belong to the same genus, although their RF are clearly dissimilar. After evaluating Taksinus gen. nov., we conclude that the range based on the generic number of female variations in leg I and IV length cannot be used to classify all arboreal Ornithoctoninae. Furthermore, we plot charts to show the value distribution for each species by comparing the total lengths of leg I-IV and the RF (Fig. 9). This study indicated that species on the left (red area) have a longer leg IV than their leg I (leg formula = 4123), whereas species on the right (blue area) have a longer leg I than their leg IV (leg formula = 1423). The comparative approach used in this study revealed that Taksinus gen. nov. appeared on the left side, with legs varying by −3.33 to −2.41 (RF = 90.56-93.04), whereas Lampropelma and Omothymus appeared on the right side with legs varying by +6.4 to +9.9 of the total length of leg I minus IV and RF greater than 100 (RF = 109-112.3). The variation in the length difference between Phormingochilus legs I and IV is centered on the charts, ranging from 0 to 2 (RF = 100-103.5). A visual leg length comparison of arboreal Ornithoctoninae, using  (Abraham, 1924) Lampropelma violaceopedes (Abraham, 1924 both methodologies is indicated in Fig. 9. Gabriel and Sherwood (2019) employed a generic number that enables rapid assessment for a defined difference of Omothymus, Phormingochilus, and Lampropelma, but whilst useful for those genera, Taksinus gen. nov. cannot be defined using this method. In contrast, the RF can be used to calculate a decimal value for leg proportions. For instance, a value greater than 100, such as O. violaceopes 112.3, indicates that this species has short hind legs, whereas a value less than 100 indicates that this species has long hind legs. The RF, however, was developed primarily to diagnose species rather than for higher-level delineation and needs to be further evaluated before it could be reliably used at the genus level. Evaluation of the geographic distributions of Asia arboreal tarantula currently identified within the Ornithoctoninae subfamily-Lampropelma, Omothymus, and Phormingochilus, and Taksinus gen. nov. (Fig. 2) provided interesting data. We classified T. bambus in a new genus rather than Omothymus, which occurs in Sumatra and peninsular Malaysia due to the consistency in the morphological features of T. bambus sp. nov., which have a rounded, gentle curve and reduced ridges on the embolus (Taksinus gen. nov. Fig. 5A, B; Omothymus Fig. 5F-J; see Gabriel and Sherwood 2019: 139, figs 1-5), and embolus is short compared to palpal bulb length (1:1). T. bambus sp. nov. is closely related to Phormingochilus based on palpal bulb morphology (see Figures 9. Scatter plot illustrating the difference between the total leg lengths I minus IV and the relation factor (RF) of arboreal Ornithoctoninae. The red area contains data indicating that species have a longer leg IV (leg formula = 4123), whereas the blue area has data indicating that species have a longer leg I (leg formula = 1423). Gabriel and Sherwood 2019: 142, figs 9-13), with the palpal bulb's rounded apex in apical view. Based on Gabriel and Sherwood (2019), T. bambus sp. nov. is similar to Phormingochilus in terms of the total lengths of leg I and IV in females ±2.41-3.33 mm (±2-3 in Phormingochilus; see Gabriel and Sherwood 2019); they differ when the leg formula is calculated as the length of leg I minus that of leg IV (Taksinus gen. nov. are −2 to −3 while Phormingochilus indicated that leg I is longer with +2 to +3 of the value). Furthermore, Phormingochilus is found solely on Borneo Island, posing a considerable geographic barrier to northern Thailand (Fig. 2). Conclusively, our findings indicated that Taksinus gen. nov. showed significant differences in its morphology and comparative leg measurements compared to other Ornithoctoninae arboreal tarantulas. Nonetheless, future researchers should investigate and characterize these genera using molecular phylogenetic approaches.