Retrorsia Shelley, 2003

Shear, William A. & Marek, Paul E., 2021, Three new genera and eighteen new species of miniature polydesmid millipedes from the northwestern United States (Diplopoda, Polydesmida, Polydesmidae), Zootaxa 4975 (1), pp. 81-126 : 93

Genus Retrorsia Shelley, 2003

Retrorsia Shelley, 2003:3 . Type species, Retrorsia leonardi Shelley, 2003 .

Species included: Retrorsia leonardi Shelley, 2003 ; Retrorsia benedictae Shelley, 2003 ; Retrorsia richarti , n. sp.; Retrorsia simplicissima , n. sp. and Retrorsia gracilis , n. sp.

Diagnosis. The diagnosis provided by Shelley (2003) is actually a description and does not compare Retrorsia to other polydesmid genera. However, he implied that the retrorse branch of the terminal zone of the gonopod distinguished Retrorsia species from all others. This branch is very obvious in the two species he described, but less so in the three new species described below. Nevertheless it is present and serves as a diagnostic character.

Distribution. Northwestern Oregon and western Washington, in or west of the Cascade Ranges.

Notes. Retrorsia species made up the first component of the diverse minute polydesmid fauna of the Pacific Northwest to be recently described. The general appearance of the two previously described species is typical of the tiny polydesmids, with large, setose heads, relatively small collums with scattered setae, and metazonites with toothed margins and with rows of setae on prominent sockets. Alveolate cuticle is confined to the head, epiproct and sometimes the anterior margin of the collum. The epiproct is short and not swollen. The gonopods are somewhat variable from species to species; the two described by Shelley (2003), R. leonardi and R. benedictae , have a prominent subterminal branch in the terminal zone, the lateral one of which Shelley termed the endomerite and the mesal one the tibiotarsus. In the terminology used here, the gonopod consists of a simple acropodite (ac, Fig 27 ) bearing the pulvillus, and a terminal zone with two processes, one of which recurves anteriorly (rp, Fig. 28 ) and is the source of the generic name. A further uniting character is a rounded notch just distal to the pulvillus, although this is not obvious in R. leonardi , the type species of the genus. Shelley’s illustrations of the gonopods show the course of the seminal canal as lacking a loop and a seminal vesicle. Close examination reveals that a loop is indeed lacking, but that the seminal canal at the very least expands just before the pore ( Figs 121, 122 ), so it can be argued that a vesicle is indeed present and perhaps has been reduced, or is in the process of developing. The three new species described below have simpler gonopods than the two described by Shelley (2003) and the retrorse process of the terminal zone is less distinct.

The detailed descriptions of the nonsexual characters of the two species named by Shelley (2003) can serve as well for the three additional species described here. The anterior setal row on the collum consists of 16–20 setae, varying according to species. The anterior metazonites have four rows of 14–18 setae set on stout, discrete tubercles; the number of rows increases to five midbody and may or may not decrease again to four at the posterior end.

Shelley (2003) did not mention Utadesmus as possibly related to Retrorsia , but there are similarities in the gonopods, especially in the division of the terminal zone into two processes.

Figures

FIGURES 27, 28. Retrorsia leonardi Shelley gonopods. 27. Gonopods in situ, posterior view. 28. Gonopod tips. SEM stub WS32-15. Abbreviations: ac, acropodite; rp, retrorse process; sm, solenomere; tz, terminal zone.

FIGURES 121–126. Drawings of gonopods of polydesmid species. 121. Left gonopod of Retrorisia leonardi, mesal view. 122. Left gonopod of Retrorsia benedictae, mesal view. 123. Left gonopod of Retrorsia richarti,n. sp., mesal view. 124. Right gonopod of Retrorsia gracilis, n. sp., lateral view. 125, Gonopod tip of Retrorsia gracilis, mesal view. 126. Right gonopod of Retrorsia simplicissima, n. sp., lateral view.