A synopsis of the spider beetles (Coleoptera: Ptinidae) of Socotra, with the description of a new genus and two new species

The previously known spider beetle fauna of Socotra comprises four endemic species (Mezium erinaceus Belles, Sphaericus hirsutus Belles, Ptinus bertranpetiti Belles, and Silisoptinus inermicollis Belles) and Dignomus mesopotamicus (Pic), which is also known from Arabia and Mesopotamia. The study of new material has resulted in the discovery of a new genus and two new species, and many new localities of the previously known species. Here, a synopsis is presented that brings together all information available on the fauna of spider beetles of the Socotra archipelago, including updated distributions and ecological and biogeographical considerations of all species. http://www.zoobank.org/urn:lsid:zoobank.org:pub:3EB99992-B352-42AD-A33D-C01EF494666C

Recently, I received from Jiří Hájek a series of 65 spider beetles from Socotra deposited in the collections of the National Museum of Prague. The study of these new materials has resulted in the discovery of a new genus and two new species that are described in the present paper. In addition, the study has led to more details on the distribution area of several previously known species. This paper brings together all known information on the spider beetles fauna of Socotra Archipelago, including a number of ecological and biogeographical considerations regarding the species recorded.

Material and Methods
The new material of spider beetles examined in the present work consisted of six specimens from Socotra collected by Aidas Saldaitis in 2010, and 59 specimens collected during the three latest Czech expeditions of 2010 and 2012, described in detail by Bezděk and Hájek (2017). The participants and insect collectors of these expeditions were Vladimír Hula and Jana Niedobová (in July 2010); Jan Batelka, Jan Bezděk, Jiří Hájek, Peter Hlaváč, and Luboš Purchart (in November 2010); and J. Bezděk, J. Hájek, V.Hula, Petr Kment, Igor Malenovský, J. Niedobová, and L. Endemic to Socotra, this Mezium is related to those distributed in the Mediterranean area, including North Africa (Belles, 2009). The new material indicates that the distribution of M. erinaceus is scattered throughout the eastern half of the island, in low woodland localities, at altitudes between 240 m (Go'o) and ca. 500-900 m (Homhil, Dixam) (Figure 1), in some areas with Boswellia and Dracaena trees. The fact that most of the specimens have been collected by sifting suggests that M. erinaceus lives in forest leaf litter. Belles, 2009 Sphaericus hirsutus Belles, 2009. -Zahradník (2015.

Sphaericus hirsutus
Material. Sphaericus hirsutus was described from Wadi Dineghen ("Wadi Danegan"), on the basis of 2 specimens collected in 2000 by A. van Harten and H. Pohl. Those are the only known specimens.
Endemic to Socotra, this Sphaericus is related to those of Canary Islands, especially to S. gibbicollis Wollaston (Belles, 2009). The type specimens were collected in Wadi Dineghen at low altitude (90 m) (Figure 1), using barber pitfall traps. This suggests that S. hirsutus is a ground-dwelling species. Soqotranus gen. n. Type species. Soqotranus niptoides sp. n. Description. Small, 2-3 mm. General habitus globose in both sexes; pubescence consisting of golden setae, distributed in all body parts ( Figure 2A). Head partially visible from above; eye round and convex, projecting laterally; interantennal space narrow, forming a sharp keel; antenna 11-segmented, filiform; last segment of maxillary and labial palpi fusiform. Pronotum granulose, with respective very apparent lateral prominences, and constricted in posterior quarter, showing a clearly delimited transversal groove; scutellum triangular, small but very apparent ( Figure 2A); prosternal process narrow, slightly extending past posterior margin of procoxae; mesosternal process short, not extending posteriorly beyond mesocoxae; legs with femur progressively widened distally, tibiae slender, tarsi with five tarsomeres in all legs. Elytra oval with humeral angles broadly rounded but still prominent, and longitudinally aligned rows of small punctures; pubescence composed of setae inserted along puncture rows and in the intervals ( Figure 2A). Abdomen with five visible ventrites, first of a length equivalent to a fifth of all segments, second almost twice as long as first, third and fifth only slightly longer than first, fourth the shortest of all, approximately half as long than third or fifth. Aedeagus simple, elongate and subsymmetrical ( Figure 2B); male genital segment composed of two simple branches. Sexual dimorphism slight, females are somewhat more robust than males. Differential diagnosis. The new genus is distinguished by a set of characters, such as convex and prominent eyes, narrow interantennal space, pronotum with respective lateral prominences and clearly constricted in the posterior quarter, femora progressively widened distally, and very apparent scutellum. The head, with convex eyes and the interantennal space narrow, and the pronotum clearly constricted posteriorly, are reminiscent of the genus Ptinus L. For example, the structure of the head and pronotum is similar to that of Ptinus spitzyi Villa & Villa, known from central and southern Europe (Pic, 1912). However, the species of Ptinus s. str. are sexually dimorphic, the males having elongated and subparallel elytra, whereas they are shorter and oval shaped in the females. On the other hand, the oval shape of the elytra in both sexes is similar to that seen in species of the genus Niptus Boieldieu, which is practically cosmopolitan (Reitter, 1884;Pic, 1912). However, the species of Niptus s. l. have slightly convex eyes, interantennal space relatively wide and flat, pronotum without large lateral prominences, and femora sharply widened in the distal part. The general appearance of Soqotranus also resembles that of the genera Tipnus Thomson (distributed in Europe and North America), and Pseudeurostus von Heyden (distributed in Europe, Japan and North America). However, the species of these two genera have a different pronotum, without lateral prominences and with less pronounced posterior constriction, and the scutellum is either invisible (Tipnus) or it is very small and transverse, appearing to be part of the anterior border of the elytra (Pseudeurostus). Finally, Niptodes Reitter (=Microptinus Kiesenwetter, by homonymy) and the subgenera Paraniptus Kiesenwetter and Pseudoniptus Reitter (Reitter, 1884;Pic, 1912;Zahradník & Háva, 2014) might also be related to Soqotranus because both sexes have oval elytra. However, the species of these groups, among other minor differences, have the eyes smaller and less convex, the pronotum simple, without lateral prominences, and the elytra without humeral angles. Distribution. Endemic to Socotra island ( Figure 1). Etymology. The generic name refers to the gentilic of Socotra, using the Arabic transliteration "Soqotra".  Description. 1.9-3.1 mm long. Globose in both sexes, generally brown. Head partially visible from above; surface with scattered erect golden setae; eyes round and slightly convex, projecting laterally; antenna filiform but robust, slightly shorter than body length; first segment large and robust, second more graceful and half as long as the first, from second to tenth progressively increasing in length, last the longest of all ( Figure  2A). Pronotum with granulations convex and well delimited, except in the anterior part, which is smoother and with two thin transverse grooves, with two prominent, subconical lateral lobes, and with the posterior constriction consisting of a well delimited wide transverse groove wide and also covered with granulations; scutellum small and forming an equilateral triangle ( Figure 2A); legs slender, tarsi with five tarsomeres, first one a quarter as long as the entire tarsi, second, third and fourth progressively decreasing in length, fourth approximately the same length as first. Elytra with longitudinally aligned rows of small, rounded punctures, slightly smaller than intervals; pubescence composed of golden setae, short and recumbent, inserted in the punctures, and similar but longer setae, recumbent, inserted in the intervals. Aedeagus elongate, subsymmetrical, parameres longer than median lobe, with a series of long erect setae inserted in the apical region and in the anterior third of the external part of each paramere ( Figure 2B). Sexual dimorphism slight, females have antennal segments slightly shorter, eye somewhat smaller and elytra slightly more convex than males. Differential diagnosis. Soqotranus niptoides is the only species of the genus, and can be distinguished from other spider beetle species according to the differential diagnosis of Soqotranus (see above). Distribution. Known from 13 specimens from the typical locality, in the vicinity of Peak Scant, the highest in Socotra, reaching 1,526 m, as well as from a specimen from the Homhil protected area (open woodland with Boswellia & Dracaena trees), at 360-500 m (Figure 1).

X. Belles
Biology. Species found in montane evergreen woodland, in slight to mid elevation, between ca. 300 and 1500 m. A series of specimens were collected by sifting, which suggests that it lives in forest litter. Etymology. The specific name refers to the general similarity with the species of Niptus.
Material. Ptinus bertranpetiti was described with a specimen from Dixam plateau (H. Pohl campaign of 1999) and two specimens from "Zemhon area, 270-350 m 12º30′58″N 54º06′39″E" (Czech expedition, 2010), which corresponds to Aloove village in the center-east of the island (Bezděk et al., 2012). These three specimens are the only known material.
Endemic to Socotra, P. bertranpetiti is related to the Palearctic Ptinus belonging to the subgenus Gynopterus (Iablokoff-Khnzorian & Karapetian, 1991;Belles, 2012). It would be localized in medium to relatively high altitude localities (250-1000 m) in the central part of the island (Figure 1). Nothing is known about the biology of this rare species.
Dignomus mesopotamicus (Pic, 1894) Ptinus pulverulentus v. mesopotamicus Pic, 1894. Ptinus (Pseudeutaphrus) pulverulentus v. mesopotamicus, Pic. -Pic (1912). Dignomus pulverulentus (Boieldieu). - Belles (1996). Dignomus mesopotamicus (Pic). -Borowski (2002) Dignomus mesopotamicus is known from Arabia and Mesopotamia (Pic, 1912), including the Socotra Archipelago (Borowski, 2002;Belles, 2009). It belongs to the "Dignomus pulverulentus group" as defined by Belles (1996), whose species are distributed on both sides of the Red Sea, reaching the Iranian plateau in the east. The data indicate that D. mesopotamicus is widely distributed throughout most of the island of Socotra ( Figure  3), being present also in the small adjacent island of Abd al Kuri. It seems to be an ecologically versatile species, which lives from coastal dunes areas (0 m), to mid-mountain places, such as Homhil (300-600 m), and wetlands in the Dixam plateau (1020 m). In Homhil, it was collected near a stream with Ficus via light traps. In Haalla and Abd al Kuri, it was captured with Malaise traps. These data suggest that D. mesopotamicus is an efficient flier, which may have facilitated the colonization of Abd el Kuri from Socotra, and even of Socotra from the Arabian mainland. Belles, 2005 Silisoptinus inermicollis Belles, 2005. -Belles (2009), Zahradník (2015.

Silisoptinus inermicollis
Material. Silisoptinus inermicollis was described with a male specimen collected at Qalansiyah ("Calanthia"), a coastal locality (5-15 m) in the northwest of the island (Figure 4). Now, I have studied six new specimens collected by A. Saldaitis in 2010, and 14 specimens from the Czech expeditions of 2010 and 2012 (Bezděk & Hájek, 2017). These 20 new specimens are from the Endemic to Socotra, S. inermicollis is the second species of the genus Silisoptinus, after Silisoptinus singularicollis Pic, known from Zanzibar, in continental Africa (Pic, 1917). The new localities extend the distribution of S. inermicollis throughout the northern half of the island, predominating in coastal localities, such as Qalansiyah (5-15 m), Qeysoh (35 m), Di Hamri (6 m), Hadiboh (10-20 m), Wadi Ayhaf (40 m, in the lower part of the valley) (Figure 4). Moreover, S. inermicollis also spreads in the central area of the island, at altitudes between 300 m (Aloove), and ca. 900 m (Kaza Kazihon, at 890 m) (Bezděk et al., 2012) (Figure 4). Although it is not as generalist as D. mesopotamicus, S. inermicollis seems remarkably versatile from an ecological point of view, spreading in coastal habitats, in dune areas, also in the lower river basins, and in slight to mid elevation areas, such as the locality of Aloove, with predominance of Jatropha unicostata shrubland with Boswellia elongata trees.
The specimens from the northern, coastal zone correspond morphologically to the holotype specimen, in particular with regard to elytral pubescence. As in the holotype, all have an elytral pubescence of three types: 1: long, brownish hairs, erect and placed on the intervals; 2: short, brownish-yellow hairs, recumbent and placed in the punctures, and 3: recumbent and dense short pale hairs scattered over anterior third and behind disc (see Figure 5A, showing the holotype, which exemplifies this kind of pubescence). The Aloove and Kaza Kazihon specimens do not clearly show the third type of pubescence that in other localities form discrete spots, but is sparser and more or less distributed throughout the elytron ( Figure 5B). However, the general morphology and the structure of the aedeagus conforms to that of the holotype of S. inermicollis, thus the detail of the virtual absence of well-defined spots of recumbent pale hairs should be considered within the range of variability of the species. Until now, only a male of S. inermicollis was known (Belles, 2005(Belles, , 2009, therefore, the availability of females now makes it possible to assess that sexual dimorphism is minimal. Females are a bit more robust than males, in general, with the posterior third of the elytra slightly more convex, and the eyes somewhat less prominent. Silisoptinus hajeki sp. n. Description. Female ( Figure 5C) 4.6 mm long. Robust, subparallel sided; cuticle blackish. Head densely clothed with fine, short, white, recumbent hairs, and a number of scattered blackish hairs, erect, projecting towards the front; eyes round and moderately convex, antennae filiform, slender, slightly shorter than body length, the first segment, large and robust, second half as long as first, from second to eleven, progressively increasing in length. Pronotum with the disc deeply excavated in posterior third, disc convex and with numerous longitudinal grooves, basal third with a deep transverse groove and with elliptic, spiral shaped structure at each side, which is very sclerotized and has a dense patch of yellowish tomentum on upper part; pronotal disc showing fine, short, yellowish recumbent hairs, and long blackish hairs, erect, projecting towards the head; scutellum triangular, longer than wide; legs slender, tarsi with five tarsomeres, the first one almost a quarter as long as the entire tarsi, second, third and fourth progressively decreasing in length, fourth approximately the same length as first (the tarsi of the three right legs are missing in this specimen: Figure 5C). Elytra subparallel-sided, with apical third quite pointed and humeral angles broadly rounded; basal quarter showing a series of fine grooves starting from the scutellum and diverging to the elytral sides; punctures not very marked, confused by the generally rough surface of the elytra, especially on the disc, which is very convex; pubescence of three types: 1st) very long, blackish hairs, erect and placed on the intervals, 2nd) short, brownish hairs, recumbent and placed in the punctures, and 3rd) recumbent and dense short pale hairs forming a discontinuous band in the basal quarter, and two discrete spots behind disc ( Figure 5C). Male unknown. Differential diagnosis. The shape of the pronotum of S. hajeki is reminiscent of that of S. inermicollis Belles (Belles, 2005(Belles, , 2009. In contrast, the pronotum of S. hajeki is very different from the other species of the genus, S. singularicollis Pic, from Zanzibar, in the Indian Ocean, off the coast of Tanzania (Pic, 1917). S. hajeki can be readily distinguished from S. inermicollis by having the following differentiating characters. The larger size of S. hajeki (4.6 mm) compared with S. inermicollis (2.9-4.3 mm); the scattered long and erect blackish hairs of the head (whitish and shorter in S. inermicollis); the relatively short antennae, which are shorter than body length (longer than body length in S. inermicollis); the prominent elliptic, spiral shaped structure at each side near the pronotum base (less prominent in S. inermicollis); the long and erect blackish hairs of the pronotum (absent in S. inermicollis); the elytra with the apical third quite pointed, the humeral angles broadly rounded, and the disc very convex (elytra with the apical third rounded, somewhat depressed at the apex border forming a convex rim, humeral angles prominent, and the disc practically flat in S. inermicollis); the elytra showing a series of fine grooves at the base, and the punctures not very marked and confused by the generally rough surface of the elytra (elytra without striations at the base, and with the punctures well marked, forming well defined longitudinal puncture lines and intervals in S. inermicollis); the elytral pubescence showing a type of recumbent and dense short white hairs that form a discontinuous band in the basal quarter, and two discrete spots behind disc (in S. inermicollis this type of white pubescence is scattered over anterior third and behind disc, and at the apex, and can be even absent, especially in the specimens from Aloove and Kaza Kazihon). Distribution. Silisoptinus hajeki is endemic to Socotra, localized in the Homhil basin, in the northeast of the island (Figure 4). Biology. The only specimen was collected in an area with open woodland with Boswellia elongata, and Dracaena cinnabari trees (Bezděk et al, 2012). Etymology. Dedicated to my friend and colleague Jiří Hájek, one of the collectors of the holotype, who sent it to me for study, and who already guessed that it was a new species.

Discussion
The new material of spider beetles from the island of Socotra has led to the identification of a new genus and species, Soqotranus niptoides, and a new species of the genus Silisoptinus, S. hajeki. In addition, the distribution of the other known species has been considerably expanded, in particular that of S. inermicollis, until now known only from the locality of Qalansiyah, and at present showing a distribution that includes eight localities distributed throughout most of the island. With these data and those of Table 1. Abundance and altitudinal occurrence of spider beetles in Socotra Island. Indicated for each are the number of specimens examined, the number of collecting localities and the altitude, which is classified into three main categories: "Sea level" (coastal areas), "Mid altitudes" (250-900 m) and "High altitudes" (>900 m). All species are endemic to Socotra archipelago, except D. mesopotamicus. All localities refer to Socotra Island, except in those of D. mesopotamicus which include also a record of Abd al Kuri.

Funding
This work was supported by the Catalan Government (grants 2014 SGR 619, and 2017 SGR 1030).

Disclosure Statement
No potential conflict of interest was reported by the author.