Bidentogon Buckett & Gardner, 1968

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 : 87

Genus Bidentogon Buckett & Gardner, 1968

Bidentogon Buckett & Gardner 1968:198 . Type species, Bidentogon helferorum Buckett & Gardner 1968 .

Bidentogon, Shear, 1972:489 ; Shelley, 2003:9 .

Species included: Bidentogon californicus Buckett & Gardner, 1968 ; B. expansus Shelley, 2003 ; B. norcal n. sp.; and B. buttensis n. sp.

Diagnosis. The simple gonopods of Bidentogon species distinguish them from any other small polydesmids; the gonopod acropodite consists of a long solenomere and a simple, unbranched terminal zone ( Fig. 5 ). The gonopod prefemorite is reduced and strongly flattened. Males have the femur of the third leg enlarged ( Fig. 11 ), a character not seen in other polydesmids. From the other genera discussed here, Bidentogon species can be diagnosed by the flattened seta-bearing tubercles of the metazonites that somewhat resemble what Sierwald et al. (2020) called “blisters” in Pseudopolydesmus and by the presence of alveolate cuticle on the metazonites between these tubercles ( Figs 1, 2 ). In addition, Bidentogon species never have more than three rows of setae on the metazonites; the other small polydesmid genera described here may have three rows on the anteriormost rings but transition to four or five rows posteriorly.

Distribution. California, from San Mateo Co. and Sacramento north to Shasta Co. (see map in Shelley [2003] and new records below).

Notes. Bidentogon is a rather anomalous genus whose inclusion in Polydesmidae sensu strictu is questionable. The genus was at first placed in the family Vanhoeffeniidae (Buckett & Gardner 1968) , then in Trichopolydesmidae due to the synonymy of Vanhoeffeniidae with that family ( Shear 1972). Shelley (2003) included it in Polydesmidae , a better position for the present time. At the least, Bidentogon probably represents an undiagnosed tribe in the family, and at most a family of its own. The simple gonopods ( Figs 5, 6 , 10, 12 , 15, 16 , 18, 19 ), in which the acropodite consists of a solenomere (s, Fig 18 ) and an unmodified terminal zone (tz, Fig. 18 ), give few clues to the affinities of the genus. Further complicating things are the absence in the gonopods of its species of the three most reliable polydesmid synapomorphies: a pulvillus, vesicle, and loop in the seminal canal. The gonopod prefemorite is curiously reduced and flattened (pf, Fig. 10 ))

However, SEM examination reveals that the solenomere is weakly sclerotized, which has not been observed using the optical microscope. In fact, there appears to be a continuum from a very lightly sclerotic solenomere with projecting cuticular scales (in Bidentogon norcal , n. sp.; Figs 18–20 ) to a solenomere that is more sclerotized ( B. californicus ) but still with thinner, wrinkled cuticle near its base. We hypothesize that the solenomere may have developed from a typical pulvillus that became elongated and eventually more sclerotized. The cuticular scales in B. norcal would be homologous to the fimbriae surrounding the seminal pore on other species. A similar trend occurs in species of the genus Calianotus ; C. yosemitensis (Causey) has a flat pulvillus, while that of C. sastianus (Chamberlin) is elongated into a tube and set with hair-like scales, and the solenomere of C. bituberculatus (Loomis) is a tube without scales ( Shelley 1997). If supported, this hypothesis would argue for keeping Bidentogon in Polydesmidae .

Mikhaljova (2005) has speculated that the northeast Asian genus Uniramidesmus Golovatch & Mikhalova, 1979 may be related to Bidentogon . At least some species of the central Asian genus Turanodesmus Lohmander, 1932 strongly resemble Bidentogon in their metazonital ornament as well as gonopod anatomy ( Spelda et al. 1998). Finally, there is a close resemblance of the gonopods of species of Utadesmus Chamberlin & Hoffman (Utah, New Mexico) to those of Bidentogon species , differing only in the presence of a pulvillar process and a flattened, shelflike pulvillus; Shelley (1996) described the ornament of the metazonites as being much like that of Bidentogon species.

Nonetheless, the general appearance of Bidentogon species is polydesmid. An apparently unique character is the large, blunt tubercles that bear the setae of the metazonites—flattened and expanded to resemble the dorsal areas seen in Polydesmus and Pseudopolydesmus —but separated by alveolate cuticle; in the other genera discussed here, alveolate cuticle does not occur on the dorsa of the metazonites and the seta-bearing tubercles are not flattened. In addition, the third legpair of males is unusually modified in Bidentogon species , with a distinctly swollen, almost spherical femur. No sphaerotrichomes were observed on the legs of the males.

Figures

FIGURES 1–6. Bidentogon californicus Buckett & Gardner. 1. Head and anterior rings, dorsal view. 2. Midbody ring, dorsal view. 3. Epiproct, lateral view. 4. Anterior end of male, ventral view. 5. Gonopods in situ, ventral view. 6. Gonopod tips, ventral view. Abbreviations: sm, solenomere; tz, terminal zone. SEM stub WS33-8.

FIGURES 7–12. Bidentogon expansus Shelley 7. Collum and first ring, dorsal view 8. Midbody ring, dorsal view. 9, Epiproct, dorsal view. 10. Gonopods in situ, ventral view. 11. Right leg 3, ventral view. 12. Part of right gonopod, ventral view.Abbreviations: ac, acropodite; f3, femur of third leg; pf, prefemorite; sm, solenomere; tz, terminal zone. SEM stub WS33-7.

FIGURES 13–16. Bidentogon buttensis, n. sp. 13. Head and anterior rings, dorsal view. 14. Midbody ring, dorsal view. 15. Gonopods in situ, ventral view. 16. Gonopod tips, ventral view. Abbreviations: ac, acropodite; pf, prefemorite; sm, solenomere; tz, terminal zone. SEM stub WS33-9.

FIGURES 17–20. Bidentogon norcal, n. sp. 17. Midbody ring, dorsal view. 18. Rght gonopod, lateral view. 19. Left gonopod, mesal view. 20. Detail of left gonopod, mesal view. SEM stub WS33-10. Abbreviations: sm, solenomere; tz, terminal zone.