A new spider genus (Araneae: Linyphiidae: Erigoninae) from a tropical montane cloud forest of Mexico

A new genus and species of spider (Araneae, Linyphiidae, Erigoninae) from a tropical montane cloud forest of Mexico is described from both male and female specimens, Xim trenzado gen. et sp. nov. A phylogenetic parsimony analysis situates Xim gen. nov. as a distinct genus among the distal Erigoninae. Xim gen. nov. is sister to a clade including Ceratinopsis, Tutaibo and Sphecozone, but differs from those genera by having a high cymbium, large paracymbium, short straight embolus, male cheliceral stridulatory striae widely and evenly spaced, both sexes with a post-ocular lobe, male with two series of prolateral macrosetae on femur I, and the female by having strongly oblong, u-shaped spermathecae.


Introduction
enumerated 18 genera and 67 species for the family Linyphiidae Blackwall, 1859 in Mexico. Currently, 24 genera and 105 species of Linyphiidae are cited from Mexico (Banks 1898;Chamberlin 1924;Gertsch & Davis 1937, 1946Dondale 1959;Millidge 1980Millidge , 1987Prentice & Redak 2013;WSC 2020) whereas the rest of North America (Canada and USA) has 174 genera and 937 species (Draney & Buckle 2017). Llorente et al. (1996) estimated the arthropod fauna of Mexico as three times as rich as that of the rest of North America, but in the case of Linyphiidae, the available information gives an opposite perspective. Miller (2007) reviewed the taxonomy of the Erigoninae Emerton, 1882 subfamily of Linyphiidae for the Neotropical region including 50 genera, of which he considered 39 as endemic to the Neotropics, but he stated that "Central American and Mexican linyphiids remain poorly for the tracheal system. Palpi and epigyna were examined with a compound microscope using methyl salicylate as a temporary clearing agent. SEM images were made with a TOPCON SM-510 scanning electron microscope at El Colegio de la Frontera Sur (ECOSUR), Unidad Tapachula. Specimens used for obtaining SEM images were previously dehydrated in an ethanol series, dried to critical point, mounted on aluminum stubs and then coated with gold/palladium.

Phylogenetic analysis
In order to explore the phylogenetic placement of Xim trenzado gen. et sp. nov. among the Erigoninae, we scored its morphology based on the character matrix of Miller & Hormiga (2004) (Table 1). Characters 135 (Femur I prolateral macrosetae), 136 (Tibia I proximal dorsal macroseta) and 138 (Tibia II proximal dorsal macroseta) were coded as polymorphic (among brackets) because they differed for each sex. We appended the character codings of this species to the matrix of Frick et al. (2010, available from TreeBASE M5022), which is based on the matrix of Miller & Hormiga (2004). The resulting matrix (176 characters by 112 taxa) was analyzed using TNT ver. 1.5 (Goloboff & Catalano 2016), with multistate characters treated as non-additive, memory space set to 10 000 trees, performing a traditional search (1000 replications of TBR, saving 10 trees per replication and collapsing trees after the search). Bremer support was calculated in TNT based on all most parsimonious trees, with TBR search and retaining the suboptimal trees up to 16 steps longer. Winclada (Nixon 2002) was used to generate the character optimization tree.

Etymology
The genus is named after the word spider in the Mam language, whose native speakers inhabit the border area between Mexico and Guatemala. The gender is masculine.

Diagnosis
Monotypic genus, see genus diagnosis.

Etymology
The specifi c epithet is a noun in apposition taken from the Spanish word meaning 'braided', referring to the interweaved macrosetae on the femur I of the male.

Male
HABITUS. Carapace, chelicerae and endites light brown with some suffused grey on thoracic part, labium and sternum brown, distal half of labium light brown. Legs, coxae to femora yellow-cream with distal half of femur I and II light-brown, tibiae to tarsi yellow brown. Opisthosoma light brown with sparse yellow spots and slender yellow chevrons on the rear half of dorsum ( Fig. 1A- PROSOMA. Carapace with conical post-ocular lobe, about as high as clypeus in lateral view, apex crowned with few hairs. Eye diameters and interdistances: AME 0.039, ALE 0.052, PME 0.039, PLE 0.039, AME-AME 0.026, AME-ALE 0.065, PME-PME 0.065, PME-PLE 0.065; AER in frontal view almost straight, slightly recurved, PER in dorsal view almost straight, slightly recurved. Clypeus 0.13 high with few short setae below AME. Chelicerae with fi ve promarginal (Fig. 1G) and four retromarginal teeth; lateral face of chelicerae with scaly striae, widely and evenly spaced. Sternum 0.40 long, 0.44 width, produced between coxae IV posterior tip truncated; coxae IV 1.4 times their width apart (Fig. 1C).

Female
HABITUS. Carapace, chelicerae and endites light brown with some suffused grey on thoracic part, a narrow diffuse grey band on border of thoracic part, labium and sternum brown. Legs yellow cream, slightly darker from distal half of femur (I and II) or tibiae (III and IV) to tarsi. Opisthosoma yellow cream on the dorsum with a diffused central longitudinal light brown stripe over the 4/5 anterior part, sides and venter light brown; epigastrium and spinnerets yellow-cream ( Fig. 1D-F, H).
EPIGYNUM. With large ventral plate with scattered hairs, with anterior semicircular concavity (ACV) whose rear margin forms a sclerotized border (Fig. 5A, G-H). Dorsal plate in ventral view small, rectangular and transverse, occupying only posterior border of epigynum, with a big anterior lobe, fl ush with ventral plate, ovoid and slightly wider than long, with a narrowing forming a neck at the level of anterior end of copulatory opening slits, where the anterior lobe joins the ventral plate ( Fig. 5A-B, G). Ventral and dorsal plates slightly darker than venter; anterior lobe of dorsal plate yellow-whitish, membranous in appearance (Figs 1F, 5G). Copulatory openings as narrow slits between ventral plate and anterior lobe of dorsal plate (Fig. 5A-B, G). Spermathecae visible ventrally through integument (Figs 1F, 5G). In dorsal view, copulatory duct wide and very short, connected to an asymmetrical, u-shaped, oblong spermatheca with one mesal and one ectal branch, both branches of spermatheca anteriorly directed, ectal branch longer and larger than mesal one, which is almost round (Fig. 5C-F, H). Spermathecae covered by numerous small glandular pores, with bigger pores in base of ectal branch (Fig. 5C-F). Fertilization duct barely visible, arising from postero-mesal part of ectal branch and posteriorly oriented (Fig. 5H).

Subadult morphology
Subadult males show enlarged palpal cymbium, palpal tibia with a dorsal protuberance corresponding, in position and shape, to forthcoming prolateral apophysis; carapace with a low post-ocular conical lobe, similar in height to that of adult females; femur I with distal half enlarged and bent to ventral side (less than in adult males), with one series of 9-10 prolateral recumbent macrosetae pointing distally (not interweaved among them), and some subadult males with another series of 2-3 macrosetae located dorsally to fi rst series. In subadult females, carapace shows the post-ocular area slightly higher than the ocular area, and the epigastrium slightly swollen.

Distribution and ecology
Known only from a small area in the south of Chiapas State, Mexico, the type locality and from Cerro Boquerón, Municipio de Motozintla. One male and one female of this species were collected for the fi rst time by pitfall trap (in January and March 2007, respectively) in a year-round sampling for soil spiders in the type locality. Two years later (2009), when sampling the understory vegetation in the same locality, numerous specimens of both sexes were collected by beating low understory, showing that this species is associated normally to low understory vegetation and was found accidentally on the soil in 2007 (Ibarra-Núñez et al. 2011;Maya-Morales et al. 2012; cited as "Erigoninae sp1"). They were collected from January to August but were more abundant during the dry and cold season (January to beginning of March), when adult numbers were about twice that of juveniles. Human disturbances also affect negatively the abundance of this species (Maya-Morales et al. 2012). Other surveys for spiders were made later in three other cloud forests in the same mountain range (Sierra Madre de Chiapas), but only one female of this species was collected in the nearest of the three studied localities (Cerro Boquerón, 27 km distance from the type locality, by beating low understory). This makes Xim trenzado gen. et sp. nov. a micro-endemic species, and consequently highly vulnerable to habitat destruction.

Remarks
In the ventral plate of the epigynum of Xim trenzado gen. et sp. nov., the anterior semicircular concavity (Fig. 5A, G) is very similar to that of Tutaibo velox (Keyserling, 1886) as described and illustrated by Miller (2007: 179, fi g. 138f).
The matrix of Miller & Hormiga (2004) does not include some characters present in Xim trenzado gen. et sp. nov. that could be interesting to compare with other Erigoninae, paracymbium size (large), shape of the spermathecae (u-shaped), and the difference between female and male in the occurrence of prolateral macrosetae on femur I (male with two series, lacking in female). Miller & Hormiga (2004) found that the absence of prolateral femoral macrosetae on legs I was one of the characters that gives unambiguous support to Erigoninae. Among the Erigoninae, several species of Erigonoplus Simon, 1884 have macrosetae on femur I, but these are located on the ventral side (Millidge 1975). Another Erigoninae, Labullinyphia tersa (Simon, 1894) also has macrosetae on femur I, but these are concentrated in the basal region (Benjamin & Hormiga 2009). Males of Xim gen. nov. are unique by having macrosetae on the prolateral, distal half of femur I. Lack of these macrosetae in females implies its presence as a sexual character, as in the Erigonoplus spp. and Labullinyphia tersa. The series of macrosetae woven as a braid in adult males is a very rare character, not known until now in any other spider species; it is also puzzling how this braid is formed in the process of the last molt.
The taxonomic knowledge of Linyphiidae species in Mexico has been very limited. Most of the Mexican species were described or recorded from the end of the 19 th century to the fi rst half of the 20 th century (Banks 1898;Pickard-Cambridge 1902;Gertsch & Davis 1937, 1946 and later, only two publications by Millidge (1983Millidge ( , 1987 added another important number of species. Since then, only fi ve species have been added to the Mexican Linyphiidae fauna (Ibarra-Núñez et al. 2011;Prentice & Redak 2013;Gómez-Rodríguez et al. 2014;Jiménez et al. 2018;Campuzano Granados et al. 2019). The discovery of a new genus recorded here suggests that the knowledge of the Mexican linyphiids is defi cient in general, and particularly from habitats barely explored like the montane cloud forests. It is likely that additional studies of other cloud forests discover new species in this genus, and perhaps other new linyphiid genera.