Retrorsia Shelley 2003, new genus

Retrorsia, new genus

Type species. Retrorsia leonardi, new species.

Diagnosis. A genus of minute, pallid Polydesmidae with 19 segments including epiproct, adult body dimensions ca. 5.6 mm in length and 0.7 mm in width; body noticeably broader with wider metaterga from segments 10–14. 6th antennomere distinctly swollen. Collum slightly narrower than succeeding tergites, not covering epicranium, setae scattered subuniformly across surface. Dorsum slightly convex, metaterga lightly papillate with dense coverings of parallel­sided setae, latter arranged in four rows on segments 2–9, five rows on segments 10–17, and four rows again on segments 18–19; paranota present on all segments, margins variably dentate. Sterna without modifications. Legs with prefemora noticeably swollen through segment 10, becoming progressively narrower caudad; tarsal claws inconspicuous. Gonopodal telopodite relatively long, curving ventrad for most of length, divided distad into tibiotarsus and endomerite, with variable expansion proximal to division point and rounded or oblong pulvillus proximal to expansion; tibiotarsus curving variably dorsad, countering axis of stem; endomerite located laterad, length and configuration variable.

Species. Two are known, R. leonardi and R. benedictae, n. spp., but others may exist in nearby areas.

Distribution. Western Oregon and Washington, USA, most sites being in the lower Columbia River Valley (Figs. 1 [6], 2).

Etymology. The generic name denotes the retrorse curvature of the tibiotarsus, the most distinctive feature.

Remarks. The species of Retrorsia are the smallest known polydesmids in western North America, smaller than even the species of Bidentogon, which are over 6 mm in length (Buckett and Gardner, 1968). They are thus comparable in size to representatives of the Pyrgodesmidae and Fuhrmannodesmidae, and their gonopods are only microns in length, mere specks in a watch­glass of alcohol.

Retrorsia is related to Utadesmus, whose closest known locality is in the Henry Mountains, Garfield County, Utah (Shelley, 1996), some 770 mi (1,232 km) to the southeast (Fig. 1 [3]). The telopodal division point is distal in both genera, and the endomerites arise there along with long, terminal tibiotarsi. This configuration contrasts with that exhibited by Scytonotus and Calianotus, in which the endomerites arise basally and are subequal in lengths to the tibiotarsi (Shelley, 1993, 1997). The tibiotarsus is coaxial with the telopodal stem in Utadesmus, whereas it curves dorsad and counters the general curvature in Retrorsia. The pulvillus in Utadesmus is a cupulate, perpendicular projection located distad at the bases of the endomerite and tibiotarsus; it is located near 1/3 length in Retrorsia. The oblong structure in R. benedictae is intermediate between the condition in Utadesmus and the small, rounded structure in R. leonardi. This affinity between Retrorsia and Utadesmus suggests that isolated populations of related polydesmids may inhabit intervening areas in the Great Basin Physiographic Province, particularly the Blue Mountains in eastern Oregon, the Ruby and Shell Creek Mountains and Wheeler Peak (Great Basin National Park), Nevada, and the Deep Creek Mountains in western Utah, which are forested and relatively moist at high elevations.