Opiona gonopods

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 : 51-52

Opiona gonopods

Shear and Leonard (2007) pointed out that the interpretation of caseyid gonopods given by Gardner and Shelley (1989) was based on a contemporary understanding of chordeumatid gonopods which has since been superseded. Shear and Leonard (2007) reinterpreted the gonopods of the genus Caseya according to new information on their homologies. Species of Caseya have very complex gonopods and the derivation of some of the parts remains obscure. In contrast, Opiona gonopods are relatively simple. The following discussion, however, applies to the gonopods of O. graeningi only; additional work will be needed to understand the gonopods of all Opiona species, especially those with evident sternal elaborations.

See figs. 8–10. The sternum (s) of the gonopods of O. graeningi is a band-like structure that extends laterally and posteriorly, but does not close behind the coxae, which are partially fused to it. The coxae (cx) are flattened and laterally carry an articulated, elongate process that bears setae, which Gardner & Shelley (1989) called the “sternal process.” However, examination of scanning electron micrographs of this species and of O. goedeni G&S 1989 show clearly that the “sternal process” articulates with the coxa. Thus it strongly resembles the remnant telopodites of some members of the Anthroleucosomatidae , even to the setation. Since the sterna of chordeumatidans do not bear setae, it seems to me unlikely that this process arises directly from the sternum. Instead it seems more consistent to refer to it as a telopodite (t) arising from the coxa. This vestigial distal appendage is found only in Opiona among caseyids; in O. facetiae G&S 1989, it is elongate, acute, and lacks setae, but otherwise it is a short, monoarticulate, curved rod with apical setation. The group of species around O. siliquae do not have telopodites.

Also of coxal origin are angiocoxites (ac), deeply divided in this species and others, with a posteriorlateral branch (lb) that sheaths the flagellocoxite and a smaller, curved mesal branch (mb). The flagellocoxite (fc) is divided into a number of branches, each of which is a very narrow hollow tube; the flagellocoxite is likely also of angiocoxal origin. What Gardner and Shelley called “telopodites” are now understood to be the colpocoxites (cc). In Opiona they are saclike and poorly sclerotized so that they collapse under the drying process needed to prepare specimens for SEM work. In the drawings of Gardner and Shelley, made with light microscopy, they appear as posterior lobes.

As with all caseyids, the ninth legpair is drastically reduced and modified, the coxae bearing processes that support the gonopods , and the telopodites reduced to large, button-like objects that project laterally (figs. 1, 6). The first through third legpairs (figs. 3–5) and legpair 10 (fig. 7) are modified in ways typical of the genus (see above).