An unusual cryptogeobiid from southeastern Brazil revisited (Opiliones, Laniatores, Gonyleptoidea)

The monotypic genus Bunostigma Mello-Leitão, 1935 is herein revisited. A detailed redescription of its type-species, Bunostigma singulare Mello-Leitão, 1935 from Rio de Janeiro state, eastern Brazil, is herein presented, providing in depth knowledge of this genus. Bunostigma is most closely related to Cryptogeobius Mello-Leitão, 1935, and as this genus, it has a stout complex apophysis on male coxa IV, lacking, however, a second complex on the stigmatic area. Bunostigma has an oblique elongate malleus, similar to that of Zalanodius Mello-Leitão, 1936 (as opposed to a globular malleus in Cryptogeobius). Bunostigma singulare has short legs, with some sexual dimorphism on its stoutness, a dorsal scutum entirely unarmed and an ocularium with an accessory central mound, although unarmed.


INTRODUCTION
Amidst the soil-dwelling Neotropical harvestmen, the recently described family Cryptogeobiidae includes 55 valid species (Kury et al., 2021) distributed in the eastern Brazilian Atlantic Forest, extending somewhat to the interior to Paraguay (Kury, 2003). Cryptogeobiids are small laniatorean Gonyleptoidea with male genitalia characterized by a hammer + lamina parva as opposed to other such as cosmetids and gonyleptids (which possess a ventral plate). They are typically short-legged, although Pseudopachylus and immediate related genera sport sexual dimorphism on the length of femur IV, which resembles the Mitobatinae (Kury, 2014). Bunostigma singulare possesses (1) an unusual bicolor pattern of body (in most cryptogeobiids, when the body is bicolor, the scutal areas are darker than the rest), (2) a greatly developed retroventral apical apophysis on Cx IV which distorts overall outline of stigmatic area as to make it roughly triangular (as opposed to T-shaped in most members of the family), (3) a stout Ti IV on male, armed with thick spiniform apophyses. All those features set B. singulare apart from the run-of-the-mill Cryptogeobiidae.
Abbreviations are: MNRJ (Museu Nacional, Rio de Janeiro) RJ = Rio de Janeiro state. Other abbreviations used: CL = carapace length, CW = carapace width, AL = abdominal scutum length, AW = abdominal scutum width, Cx = coxa, Tr = trochanter, Fe = femur, Pa = patella, Ti = tibia, Mt = metatarsus, Ta = tarsus. FAp = Fasciolate hyaline apophyses (as described in Kury, 2014). Megaspines of pedipalpal cage: i = small, I = large, Î = extremely large, much longer than the others. Tarsal formula: numbers of tarsomeres in tarsus I to IV, when an individual count is given, order is from left to right side (figures in parentheses denote number of tarsomeres only in the distitarsi I-II). All pre-2018 material was destroyed in the huge fire at the palace of MNRJ.

Systematic background of Bunostigma
Mello-Leitão (1935a: 10) described in Gonyleptidae Pachylinae the monotypic genus Bunostigma Mello-Leitão, 1935 along with the type species Bunostigma singularis [sic] Mello-Leitão, 1935, from "Distrito Federal: Bico do Papagaio", providing an informative albeit far from complete illustration (authored by his assistant Roger P. Arlè) of the male habitus in dorsal view. Kury (2003: 201) transferred this genus to the Gonyleptidae Tricommatinae. Kury & Alonso-Zarazaga (2011: 58) noted that στίγμα (stígma = mark, spot) is neuter, making the genus also neuter, so the specific epithet should be accordingly declined as singulare. Finally, Kury (2014: 8) transferred Bunostigma along with several other genera from Tricommatinae to the new family Cryptogeobiidae in the immediate vicinity of Cryptogeobius Mello-Leitão, 1935. In the present paper, we deepen the available morphological information on Bunostigma singulare Mello-Leitão, 1935, a species of cryptogeobiid endemic from the Atlantic Forest of Rio de Janeiro state, southeastern Brazil.
Placement: Bunostigma was originally in Pachylinae. Transferred to Tricommatinae by Kury (2003: 201) and to Cryptogeobiidae by Kury (2014).   and posterior margin also arched backwards, with small V-shaped cleft. All areas and free tergites entirely smooth and unarmed. Posterior border of dorsal scutum gently convex, with laterals merging into articular membrane along with free tergite I ( Figs. 2A-B).

Diagnosis
Venter (Figs. 1D, 2B, D-F): Coxae I-IV and stigmatic area only finely granular, with transverse rows of setiferous tubercles stouter anteriorly. Free sternites each with row of minute setiferous tubercles. Cx I to III transversal to main body axis; Cx II curved around Cx I, as long as Cx I, much longer than Cx III. Cx II and III delimit a narrow sternum. Maxillary lobe of Cx II as a small triangle. Cx I movable. Cx II linked to Cx III by four pairs of lateral tubercular bridges; Cx III linked to Cx IV by six to seven pairs of large lateral tubercular bridges. Cx IV oblique, as large as all others combined. Stigmatic area roughly triangular, only separated from Cx IV by a difference in height, fused to sternite II, with forked frames around the stigmata (Fig. 2D). Sternite II strongly thickened. Stigmata small, entirely longitudinal and framed by a complex formed by a forked structure + the retroapical apophysis of Cx IV (Fig. 2F). Cx IV with a well-developed wrench-shaped retroapical apophysis flanking the stigma and deeply embedded on the sternite II. Ventral complex without any lobes, expansions, fasciolate hyaline apophyses or cluster of modified setae.

Mouthparts (Figs. 3A-C):
Basichelicerite with well-developed bulla, separated from peduncle by a narrow waist. Its mesal surface covered with patches of small denticles, but without any plectra. Cheliceral hand weak, monomorphic. Pp Tr with 1 large ventral megaspine, Fe with 2 small megaspines, Pa short, with one meso-distal setiferous tubercle, all of those with slender setae. Pedipalps stunt; Pp Fe cylindrical, only gently convex dorsally and moderately compressed, without stridulatory grid and with stout meso-distal setiferous tubercle. Spination of Pp cage: Ti mesal IiIi, ectal IÎI, Ta mesal IIi, ectal iIi.    Pap. Avulsos Zool., 2021;v.61: e20216194 6/7 podomeres in leg IV, while clade B contains the species mostly with a huge, hooked spine on the ocularium and a mitobatine-like sexual dimorphism on leg IV, where the podomeres in males, especially the femur IV, are straight and much elongate. Bunostigma belongs to the clade A, along with five other genera (Cryptogeobius Mello-Leitão, 1935, Heteromeloleptes Mello-Leitão, 1931, Paratricommatus Piza-Jr., 1943, Pseudophalangodes Roewer, 1912and Zalanodius Mello-Leitão, 1936, and some undescribed terminals. Although Bunostigma was resolved as sister-group to Cryptogeobius, there is a wide morphological gap between both genera, mostly because of the unique scutum outline in Bunostigma and the strong armature of Cx and Tr IV in Cryptogeobius. Although monotypic genera are a noxious heirloom of the Roewerian System, this gap makes it more intuitive to leave Bunostigma singulare to stand alone in its own genus. There are stumbling blocks hampering the attainment of a more refined phylogeny hypothesis for the Cryptogeobiidae: (1) most species are poorly known, and (2) there is a great number of undescribed species. Therefore, refining the morphological description of the type species of a genus is an important step towards this goal.