Opiona Chamberlin 1951

Shear, William A., 2011, Cave millipeds of the United States. XI. Opiona graeningi, n. sp., a troglomorphic caseyid milliped from Siskiyou County, California, with comments on the genus Opiona Chamberlin 1951 (Diplopoda, Chordeumatida, Caseyidae), Zootaxa 3114, pp. 50-56 : 51

Opiona Chamberlin 1951

Opiona Chamberlin 1951 , p. 8. Gardner and Shelley, 1989, p. 203.

Gardner and Shelley (1989) give a complete description of Opiona , although some modifications to their account of the gonopods are required. Species of Opiona may readily be diagnosed as follows: small to medium-sized caseyids (8–14 mm long) with (fig. 1) or without obvious lateral striations of the pleurotergites, males with the first legpair robust, set with special setae (figs. 3, 4), second legpair reduced, with long gonopophyses (fig. 5), third legpair with greatly elongate and swollen coxae and reduced and modified telopodites which attach laterally at the midpoints of the coxae or more distally (fig. 5), tenth coxae enlarged, with anterior gland openings and a large coxal hook (fig. 7); females with vulvae unmodified (modified in the related sympatric genera Caseya and Ochrogramma ).

Some species of Opiona fall into two distinct groups. Opiona casualis Gardner & Shelley 1989 (Gardner & Shelley is hereafter abbreviated to G&S), O. goedeni G&S 1989, and O. exigua G&S 1989 conform to the gonopod pattern of the type species, O. columbiana Chamberlin 1951 , with a flagellocoxite divided into many hollow tubes, most of them individually sheathed in a folded structure derived from the angiocoxite, a remnant telopodite (see below for a discussion of gonopod structure in the genus), and without bilaterally paired posterior projections of the sternum which meet in the posterior midline. On the other hand, Opiona siliquae Causey 1963 , O. distincta G&S 1989, O. bifurcata G&S 1989, O. beresseyi G&S 1989, and O. communis G&S 1989 differ in lacking the folded sheathing structure and the remnant telopodite, and show the bilaterally paired posterior sternal projections. One might think that the two groups could be recognized as separate but related genera, except for the fact that three additional species, O. facetiae G&S 1989, O. scytonoides G&S 1989, and O. fisheri G&S 1989 are intermediate: they have remnant telopodites but lack both the sheathing structure and the posterior sternal projections ( Opiona graeningi n. sp. falls in this intermediate group). Additionally, all species of Opiona have very similar modifications of legpairs 1–3 and 10 (figs. 5, 7). Thus the composition of the genus is open to question. This problem will be treated in greater detail when more of the newly discovered species of Opiona are described.

As with most other members of the Caseyidae , males are easily recognized in the field by the laterally projecting telopodites of the ninth legpair, which look like large, white buttons (fig. 1). Preserved males show curious double anterior flexures of the body, one between segments seven and eight and another between segments two and four. These flexures are due to the contraction of powerful muscles associated with the gonopods and with legpair three respectively.