The enigmatic milliped genus Pandirodesmus Silvestri 1932 and description of a new species from Tobago represented by males (Polydesmida: Leptodesmidea: Chelodesmidae: Chelodesminae: Pandirodesmini).

Rowland M. Shelley, Jamie M. Smith

Abstract


Pandirodesmus rutherfordi, n. sp., represented by 18 individuals including eight adult males, occurs in secondary forests near Charlotteville and Speyside, Tobago, Trinidad and Tobago. Along with the type and second species, P. disparipes Silvestri, from Guyana and known only from females, the segmental legs of P. rutherfordi alternate between long (anterior pairs) and short (posterior ones), spiracular openings are on straw-like tubules, and ozopores are located on paramedian metatergal spines. These features appear to be adaptations for biotopes of loose sand, detritus, or frass, and 17 specimens, including the six juveniles, exhibit coatings of “sand grains” that are loosely cemented together and to the smooth, translucent, grayish-white exoskeleton. The tubules and spines elevate the spiracles and ozopores above the coating, thereby ensuring that they remain open and functional. The coating, which provides camoufl age and lends strength and rigidity to the poorly sclerotized exoskeleton, is a subuniform “pavement” that covers the entire animal except the labrum/clypeus, tarsal and antennal apices, prozonae, paraprocts, and the gonopods in males. Ramose/dendritic setae, particularly on narrowly rounded podo-/antennomeres, trap “sand grains,” and the ozopore secretions apparently constitute the “glue” that cements the coating, as evidenced circumstantially by layers of “sand” between the spines on the anterior metaterga, where they are physically closest. The alternating segmental leg lengths, in part due to differing ventrolateral and ventromedial origins, appear to be an adaptation for lateral/sideways motion in which the long (anterior) legs extend laterally and pull the body to the level of the short (posterior) ones, which continue the motion while the anterior legs extend to begin the next stroke. The opposing legs perform the complementary pushing motion a fraction after the long legs initiate the pulling stroke and hence are slightly and purposefully out of sync. An adult male paratype lacks the coating, probably because it had just molted and lacked time to amass it; the juvenile female paratype of P. disparipes also is “naked,” as was, according to Silvestri, the now lost adult female holotype. Until fresh material is collected, coatings cannot be confi rmed for P. disparipes even though it shares the anatomical modifi cations that seem adaptions for such. The minute, triramous gonotelopodites of P. rutherfordi are unlike any known for a chelodesmid, so the current generic placement, in a monotypic tribe in the nominate chelodesmid subfamily, is retained. With species in both South America and the southern Antilles, Pandirodesmus/ini had to exist on both the “proto-Antillean” terrane and the adjoining part of Pangaean Gondwana before the former rifted in the Cretaceous/Paleocene, ~66 million years ago, and P. rutherfordi is a remnant of the former population that became isolated on present-day Tobago when the terrane fragmented. Affi nity between Guyanan and southern Antillean platyrhacid millipeds (Polydesmida: Leptodesmidea) suggest that Pandirodesmus/ini may occur sporadically as far north in the island chain as St. Lucia.

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