Barydesmus truncatus, Golovatch & Korotayeva & Martínez-Torres, 2024

Golovatch, Sergei I., Korotayeva, Alyona M. & Martínez-Torres, Daniela, 2024, Three species of the millipede family Platyrhacidae from Costa Rica: a new Barydesmus Cook, 1896 and two oldest Nyssodesmus Cook, 1896 (Diplopoda Polydesmida), Zootaxa 5538 (5), pp. 424-438 : 426-428

Barydesmus truncatus sp. nov.

Figs 1 , 14

Material examined. Holotype male (in alcohol, ZISP MYR_DIP_0000192), Costa Rica, San José Province, Carara, Macaw Lodge , tropical rainforest, 9.728528°N, - 84.518639°W, 11–18.VI.2018, D. Logunov leg.

Paratype: 1 female (in alcohol, ZISP MYR_DIP_0000192), same data as for holotype .

Name. To emphasize the truncate tip of the gonopodal lateral branch.

Diagnosis. Differs from congeners primarily by a peculiar, truncated tip of the gonopodal lateral branch (lb) ( Fig. 1C, D ).

Description. Length 71 mm (male) or 63 mm (female), width of midbody pro- and metazona 6.0 and 13.5 (male) or 8.5 and 12.8 mm (female), respectively. General coloration dark brown with venter, antennae, legs, paraterga and most of epiproct contrasting beige ( Fig. 1A, B ) (male) or blackish brown with venter, antennae, legs, paraterga and most of epiproct contrasting beige (female). Bases of paraterga light brown, gradually turning into beige on paraterga. Tegument almost dull to very poorly shining, metazona microtuberculate-granulate, prozona finely microgranulate. Ventral sides of metazona a little more strongly granulate than both prozona and strictures. Head granulate all over, with an axial, V-shaped, microtuberculate structure on occiput and similarly granulate mesal, anterior and lateral margins of antennal sockets ( Fig. 1B ). Epicranial suture very deep, broad and distinct, ending up anteriorly between antennae; 3+3 tufts of setae on sides of clypeus, 2+2 tufts of supra-labral setae, and about 5+5 tufts of labral setae. Vertex and occiput bare. Antennae in situ extending past ring 2 dorsally ( Fig. 1B ) (male, female). In length, antennomeres 6>2–5>1>7. Distal part of antennomere 6 infuscate, brown. Antennomeres 1 and 2 placed inside a groove each side. Genae rounded.

In width, ring 6–15 >> 4 = 5> 3> 2 >> collum> head. Starting with ring 16, body clearly tapering towards telson. Paraterga very well-developed ( Fig. 1A, B ) (male) or well-developed (female), low, set at about half midbody height, mostly regularly declined and sloping like dorsum. Collum with a small, rounded, granulate flap each side. Paraterga in caudal 1/3 body increasingly projecting past posterior tergal margin. Anterior shoulders/parts of paraterga small, but evident, slightly convex near junctions to paraterga, narrowly rimmed ( Fig. 1A ). Posterior corners of paraterga 2–17 increasingly acute, dentiform and directed caudad, vs broadly rounded and multidenticulate on paraterga 18 and 19; caudal margins of paraterga straight, only starting with paraterga 16 increasingly, but still slightly concave; paraterga mostly only faintly and regularly narrowed distad. Lateral margins of paraterga parallel-sided, irregularly dentate to denticulate, with>2 more or less distinct teeth. Metaterga very densely and irregularly granulate, with up to three transverse rows of a little larger grains often discernible, caudal row (4–5 + 4–5) usually being more readily visible ( Fig. 1A ). Strictures faint. Limbus thin, short and entire. Pore formula normal (5, 7, 9, 10, 12, 13, 15–19), ozopores inconspicuous small rings located in central parts of poriferous paraterga. Pleurosternal carinae small, rounded, increasingly obliterated bulges visible on rings 2–4 (male, female).

Epiproct as usual, shovel-shaped, with 6 tufts of setae near or at a rounded and slightly undulate caudal margin ( Fig. 1A, B ). Hypoproct subtrapeziform, with 1+1 distinct rounded knobs flanking an emarginate caudal margin, each knob bearing a tuft of setae ( Fig. 1B ). Paraprocts with 2+2 very small knobs, basal ones more strongly apart that distal ones, both pairs also supporting tufts of setae. Legs densely setose, ca 1.2 (male) or 1.1 times (female) as long as midbody height. In length, femur> tarsus> prefemur> coxa = postfemur = tibia; claw very small and simple.

Coxae 1 and 2 contiguous medially, sternum very narrow. Sternum between coxae 3 narrow, but coxae clearly separated. Sternum between coxae 4–6 a little broader, with deep cross-impressions. Sternum between coxae 7 flattened (male). Sterna between following coxae with deep cross-impressions, axial and transverse impressions being subequal and deep. A very small setigerous knob near each coxa, but all setae abraded.

Gonopodal aperture crater-shaped, transversely oval, slightly shifted onto prozonum 7, margins clearly elevated, lateral margin also a little thickened( Fig.1B ).Gonopod ( Fig.1C, D ) simple, typical of the genus.Coxite subcylindrical, short, bare, with a usual, well developed and unciform cannula. Telopodite slender, gradually attenuated distad, a usual densely setose and clearly demarcated prefemorite (= densely setose part) about as long as acropodite, the latter in situ regularly curved and directed anteromesally, in apical part divided into a slightly sigmoid, much shorter and pointed solenomere (sl) and a stronger, longer and truncate lateral branch (lb) (“tibiotarsus”). Seminal groove as usual, running entirely on mesal side of telopodite before moving onto sl.

Remarks. Of the four formal Barydesmus species so far recorded from Costa Rica ( Carl 1902; Pocock 1909; Brölemann 1905, 1911; Recuero & Sánchez-Viales 2018), three are known from male material: B. pococki ( Brölemann, 1911) , B. propinquus ( Carl, 1902) and B. riparius ( Carl, 1902) ( Carl 1902; Pocock 1909; Brölemann 1911). The species B. stenopterus ( Brölemann, 1905) was described based on a female holotype ( Brölemann 1905). The gonopodal structure of B. truncatus sp. nov. is unique not only among the three congeners from Costa Rica in which the gonopods have been described and depicted, but also in the entire genus, by the truncate, not regularly attenuating and sharp, tip of the gonopodal lateral branch (lb), coupled with a slightly sigmoid solenomere (sl). Both gonopods of B. truncatus sp. nov. being identical, this dismisses a belief that a typical flagelliform tip of the gonopod could have been broken off.

Barydesmus truncatus sp. nov. differs readily from B. stenopterus in the body being slender (vs stout), most paraterga well-developed even in the female (vs poorly developed), and paraterga 18 and 19 well rounded (vs not rounded) ( Brölemann 1905).

An occasionally truncated lateral branch of the gonopodal acropodite seems to occur only in a few members of the platyrhacid genus Psammodesmus Cook, 1896 , with 11 described and up to 11 unnamed species that range from Panama to Peru and Brazil ( Recuero & Sánchez-Vialas 2018; Golovatch & Korotaeva 2023). Psammodesmus differs drastically from Barydesmus in that the gonopodal acropodite is bent not cephalad, but caudally ( Hoffman 1953).

Figures

FIGURE 1. Barydesmus truncatus sp. nov., male holotype (ZISP MYR_DIP_0000192). A & B. Habitus, dorsal and ventral views, respectively. C & D. Right gonopod, mesal and lateral views, respectively. Abbreviations: lb, lateral branch; sl, solenomere. Photographs not taken to scale.

FIGURE 14. Distributions of Barydesmus truncatus sp. nov. (yellow triangle), Nyssodesmus python (Peters, 1865) (black square), N. cf. python (Peters, 1865) (blank square), and N. alboalatus Cook, 1896, type locality (yellow circle) and additional record (orange circle).