Pandirodesmini Silvestri 1932

Shelley, Rowland M. & Smith, Jamie M., 2015, The enigmatic milliped genus Pandirodesmus Silvestri 1932 and description of a new species from Tobago represented by males (Polydesmida: Leptodesmidea: Chelodesmidae: Chelodesminae: Pandirodesmini), Insecta Mundi 2015 (444), pp. 1-15 : 2-3

Tribe Pandirodesmini Silvestri 1932

Pandirodesminae Silvestri 1932: 11–12 . Jeekel 1971: 366. Hoffman 1975 (text): 143; 1980: 153. Pandirodesmidae: Attems 1940: 483. Jeekel 1963 (list): 5.

Pandirodesmini : Hoffman 1975 (text): 144; 1980: 153. Adis and Golovatch 2000 (text): 98.

Type- and only component genus. Pandirodesmus Silvestri 1932

Diagnosis. A tribe of small bodied Chelodesminae characterized by lightly sclerotized, smooth, grayishwhite exoskeleton and alternating long and short legs, exposed parts usually covered with densely cemented but loosely attached sand grains imparting dark beige to black coloration and obliterating setae. Metaterga narrow, without sulci, with pair of moderately large, subconical/digitiform projections, ozopores opening subapically on latter. Paranota absent. Stigmata opening apically on slender, inconspicuous tubules narrowly segregated from, and slightly shorter than, adjacent coxae. Anterior (longer) segmental legs arising ventrolaterad, posterior (shorter) legs arising submediad, anterior halves of sterna thus broader than caudal halves; leg lengths alternating along most of length of body, posterior pair ~2/3 as long as anterior on segments 5–18; femora and tarsi substantially longer than remaining podomeres on all legs, postfemora short, swollen, and nodular, articulating extremities of tibiae and tarsi also swollen and nodular. Claws absent from legs 1–3, narrow and “pin-like” on remaining legs, surrounded and overhung by long, filiform setae, those on coxae to proximal end of tarsi apically ramose and dendritic. Gonopods minute; coxae without apophyses, joined by strong, distinct sternum; telopodite triramous, prefemoral process (medialmost) expanded, folded, and laminate, partly enveloping other branches, primary (lateralmost) long, curving broadly mediad and partly overhanging other branches, divided distad into medial solenomere and subequal tibiotarsus; secondary (inner) branch subupright, narrowing and expanding throughout length, tip bilobed. Prostatic groove opening apically on solenomere.

Distribution. Known only from west-central Guyana and the northern tip of Tobago, Trinidad and Tobago ( Fig. 4 ).

Relationships. Based primarily on the shared minuteness of their tarsal claws that are overhung and partly concealed by apical setae, Silvestri (1932), Hoffman (1975), and Adis and Golovatch (2000) suggested affinity between Pandirodesmini and Trachelodesmini . As RMS has conducted two studies on Trachelodesmini ( Shelley 1981, 1999) and has closely examined P. rutherfordi , we do not perceive such an affinity; among other differences, the exoskeleton of P. rutherfordi is as smooth as baby’s skin, not rough or tuberculate as in most trachelodesminines. To us, the partly volvated juvenile ( Fig. 5 ) superficially resembles an oniscodesmid ( Polydesmida : Polydesmidea), but we concur with the tribe’s present placement. Only three leptodesmidean families inhabit South America – Chelodesmidae , Platyrhacidae , and Aphelidesmidae – and with a triramous telopodite, Pandirodesmini is not assignable to the last two. From an anatomical standpoint, only two possibilities remain: assignment to Chelodesmidae , the only family with deeply bi- or tripartite telopodites, or placement in a monotypic taxon. Furthermore, rather than the apical tarsal trait, chelodesmid affinities are best revealed by gonopodal synapomorphies, but these appendages were not available to prior authors, and we do not find them helpful. Aside from the above works on trachelodesminines, we have no experience with chelodesmids and no thoughts as to Pandirodesmini’s affinities. As Chelodesmidae was a focus of the late R. L. Hoffman, we perused his papers and others in RMS’ library without finding any remotely similar gonopods; consequently, we leave Pandirodesmini in the nominate subfamily. Until another worker develops encyclopedic knowledge of Chelodesmidae , Pandirodesmini’s affinities will probably remain obscure, and our only present thought is that Pandirodesmus /ini may be the sole surviving remnant of an ancestral dichotomy that perhaps antedates development of the predominant diplosegment burrowing mechanisms. With two species and without obvious relatives remotely near its area, Pandirodesmus /ini are truly enigmatic taxa and similar in this regard to Choctella Chamberlin ( Spirostreptida : Choctellidae ) in the Cumberland Plateau of Tennessee and Alabama, United States ( Hoffman 1969, Shelley and Floyd 2014).

Remarks. Silvestri (1932) originally assigned Pandirodesminae to Polydesmidae , and after Attems (1940) elevated the taxon to familial status, Hoffman (1980) reduced it to a tribe of the nominate chelodesmid subfamily.

Figures

Figure 4. Distributions of Pandirodesmus and Pandirodesmini. Red dot, P. rutherfordi. Red star, P. disparipes.

Figures 5–10. Male paratypes of P. rutherfordi. 5) Partly volvated juvenile showing the uniformity of the thinner coating. 6) “Naked,” fragmented, and probably freshly molted adult showing the natural exoskeletal color. 7) Anterior view of the head showing the clear, uncoated frons/clypeus/labrum and distal antennomeres. 8) Dorsal view of anterior tergites showing a ridge of accumulated sand (arrow) between the physically closer spines/ozopores on that metatergum. 9) Ventral view of the caudal end showing sand coatings on the sterna and legs (excepting tarsi), the alternating leg lengths, the more lateral origins of the anterior segmental legs, and the wider sterna between them. 10) Segment 7 of NMNH paratype showing protruded gonopods, encircled by dashed green line, in situ.