Genus Dibolostethus Hoffman, 2009

Dibolostethus Hoffman, 2009: 151 .

Type species

Dibolostethus sicarius Hoffman, 2009, by monotypy.

Diagnosis

The genus differs from all other chelodesmid genera by the following combination of characters: body rings constricted between pro- and metazonites and paranota reduced, resulting in a moniliform body outline (Figs 1, 2A); spiracles reduced as compared to other members of the family, oval (Fig. 2B, red circle); gonopores located on subterminal convexity on the mesal side of 2 nd coxae (Fig. 3A); in males, sternite of 4 th body ring prominent, with a pair of acute projections curving anteriad (Figs 2B, 4); in males, femora of 4 th pair of legs with large acute process (Fig. 3B); in males, femora of 5 th pair of legs incrassate (Fig. 3C); gonopods large, straight; solenomere long, slender, and unbranched; prefemoral process widening at midpoint and terminating in several well-defined lobes (Figs 5–7).

Description

Body composed of 20 rings. Length of males between 25 mm ( D. sicarius) and ~ 30 mm ( D. inopinatus sp. nov.). Coloration (preserved in 70% isopropanol/ethanol): body rings yellowish ( D. inopinatus) to dark reddish brown (nearly purple; D. kattani sp. nov., Fig. 1); antennae, head, legs, and paranota same color as the body ( D. inopinatus) or light yellow, contrasting with the body; telson same color as the body ( D. inopinatus) or reddish with posterior margin yellowish. Head: smooth, with epicranial setae 2–2, interantennal 1–1, and frontal 2–2; other facial setae sparse and irregular, becoming more dense closer to the mouthparts. Gnathochilarium without modifications, but narrower in relation to other members of Chelodesmidae, covered by few setae (Fig. 2C). Mandibular bases prominent. Incisura lateralis rounded, not completely closed. Antennae: sixth antennomere with an invagination possibly containing sensilla basiconica, obscured by debris in examined specimen; seventh antennomere having two well-demarcated invaginations and one slight invagination between the sensory cones; sensilla basiconica arranged into a small pocket, connected to the cones by a slit (Fig. 2D, red arrow). Body ring cuticle smooth, appearing slightly rugose at high magnification, with a median sulcus on the metazonites (Fig. 2A, red rectangle). Collum: small, more slender than the head; posterior margin concave, anterior margin convex; corners rounded. Paranota reduced, distantly separated, rounded, and without projections, not developed on 19 th body ring, and body rings constricted between pro- and metazonites, all resulting in a moniliform appearance (Figs 1, 2A). Stigma oval (Fig. 2B, red circle). Sternites of males: 4 th body ring with two slender acute projections (Figs 2B, 4); 5 th body ring with two pairs of short, blunt projections (only in D. kattani; Fig. 3D, red oval); post-gonopodal sternites unmodified. Ozopores: posteriorly situated on paranota (Fig. 2A); peritremata indistinguishable; arrangement typical for Polydesmida (5, 7, 9, 10, 12, 13, 15–19); without a well-demarcated rim. Legs of males: femur of 4 th pair produced into a large acute process (Fig. 3B); femur of 5 th pair of legs incrassate and glandular; prefemur of 5 th pair of legs with a small basal pore (Fig. 3C, E, red arrow); tibia of all legs with a ventro-apical translucent acute projection, reducing in size posteriorly (Fig. 3B–C, red arrows). Podosterna of 4 th –6 th body rings notably elevated (Figs 2B, 3D). Telson: subtriangular, with three pairs of macrosetae dorsolaterally and two pairs in the apical region. The apical pairs of macrosetae separate from and equidistant to the mesal lips of the paraproct, forming the four corners of a rectangle.

Male characters

Gonopores without modifications, located on the coxae of the 2 nd leg-pair, with a subterminal pore. Gonopod aperture: on 7 th body ring; oval; rim notably projecting above sternite, with a U-shaped concavity on posterior margin, posterior edge without folds. Gonopods: gonocoxae length variable relative to telopodite length; largely obscured by telopodite in ectal view; without a spiniform process (Figs 5–7). Cannula hook-shaped, without modifications (Figs 5C, 6C, 7A). Solenomere long, slender, unbranched, apex falciform (Figs 5–7). Prostatic groove visible along the entire length of the solenomere in mesal view, without a trace of torsion (Figs 5C, 6C, 7A, dashed lines). Prefemoral region (Figs 5–7): small, about ⅓ the length of the telopodite; a slight cingulum demarcates the transition zone between the prefemoral and solenomere regions (Figs 5–7, red ovals). Prefemoral process: long (similar in length to solenomere); apex broad, with a series of distinct lobes and lamellate flanges (Figs 5–7).

Female characters

Known only for D. sicarius . Vulvae: small, oval-shaped, unmodified (Fig. 8A); held against coxae of 2 nd leg pair within deep emargination of 3 rd segment, not protruding beyond U-shaped vulvar rim (Fig. 8B); composed of two subequal valves and a small operculum; setose (Fig. 8A, C).

Remarks

Although the structure of Dibolostethus gonopods (i.e., presence of a prefemoral process and a hook-shaped canula) shows definitive placement within the Chelodesmidae, all known Dibolostethini exhibit striking similarities with members of Paradoxosomatidae . These include the presence of a transverse metatergal sulcus, reduction of paranota and constriction between the pro- and metazonites giving the body a moniliform appearance (shared with several other groups of Chelodesmidae), elongated legs and antennae, and the positioning of the dorsal pair of the macrosetae in relation to the mesal lips of the paraproct (Fig. 2). In the Paradoxosomatidae, these macrosetae are positioned far away from the mesal lips of the paraproct, and located directly above the ventral pair of macrosetae, forming the four corners of a rectangle (Nguyen & Sierwald 2013: 1147). In the vast majority of the Chelodesmidae, the dorsal pair of macrosetae is located directly on the mesal lips of the paraproct, although this character is somewhat variable. For example, in the Lepturodesmini Hoffman, 1975, the dorsal pair of macrosetae is immediately adjacent to the mesal lips of the paraproct but not mounted on them. In species of Dibolostethus, the two pairs of macrosetae are positioned far away and equidistant from the mesal lips of the paraproct as in the Paradoxosomatidae . The elongated antennae and legs (similar to the chelodesmid Trichomorphini), and the presence of an adenostyle-like femoral process on leg pair 4 also resemble many species of Paradoxosomatidae .

Distribution

Known from the Andean regions of Colombia and Ecuador (Fig. 9).

Composition

Dibolostethus sicarius Hoffman, 2009, D. inopinatus sp. nov., and D. kattani sp. nov.