Dicranogonus pix Jeekel, 1982

Mesibov, Robert, 2014, The Australian millipede Dicranogonuspix Jeekel, 1982 (Diplopoda, Polydesmida, Paradoxosomatidae): a species with and without paranota, ZooKeys 454, pp. 29-39 : 31-35

Taxon classification Animalia Polydesmida Paradoxosomatidae

Dicranogonus pix Jeekel, 1982 Figs 1, 2, 3, 4, 5

Dicranogonus pix Jeekel 1982: 209; Fig. 4 (p. 206). Shelley et al. 2000: 97. Mesibov 2004: 42; 2009: Fig. 2B (p. 534). Nguyen and Sierwald 2013: 1155.

Morphology.

Gonopods. The gonopod telopodite varies only slightly in details over the Dicranogonus pix range (Figs 1-3). There are two small tabs (Jeekel: “lappets”) on either side of the solenomere tip in the holotype, and one or both tabs (more often the basally directed one) are reduced or missing in some populations (Fig. 2). The most divergent male examined is from the northeast corner of Flinders Island in the Furneaux Group (Fig. 2C); the tip of the solenomere in this specimen is abruptly curved basally and the subapical process of the telopodite (Jeekel: “tibiotarsus”) is thinner and closer to the solenomere than in most males.

Paranota. In agreement with the original description of Dicranogonus pix , the diplosegments of a nearly topotypical male have obvious paranota (Figs 3, 4A, 4B). Similarly well-defined paranota are present on almost all Dicranogonus specimens from eastern Victoria and small islands in the northeast portion of Bass Strait. In contrast, all specimens from islands in the southeast portion of Bass Strait (i.e., the Furneaux Group) and the Tasmanian mainland lack paranota (Figs 3, 4C, 4D), although the paranotal area on diplosegments is usually marked by lighter colour, and on some rings there is a very slight lateral bulge at the level of the ozopore.

The only exceptions to this simple geographical pattern in the material examined are three males and two females lacking paranota from the Buchan district in eastern Victoria, collected in 1907 (Figs 3, 4E, 4F).

Other characters. I add here only a few minor details to the very clearly written, 1600-word description by Jeekel (1982) of the typical Dicranogonus pix . Spiracles on diplosegments located just above and anterior to leg bases (Fig. 5A); anterior spiracle (Fig. 5B) ovoid with long axis nearly vertical, anterodorsal portion of rim extended as thin cowl and directed slightly posteriorly; posterior spiracle nearly round, rim slightly raised and rounded; anterior and posterior spiracular filters composed of numerous thin, forked tabs with blunt tips (Fig. 5B), the dorsal half of the filter produced in the posterior spiracle and emergent in the anterior spiracle. Paranota on diplosegments well-defined to ring 16, then progressively diminishing to a very slight lateral bulge on ring 19. Spinnerets in square array.

Biogeography.

Dicranogonus pix and Notodesmus scotius Chamberlin, 1920 are the only Polydesmida so far known to occur naturally on both sides of Bass Strait (see Notodesmus scotius distribution records and KML file at http://www.polydesmida.info/millipedesofaustralia/localities.html). The Notodesmus scotius material I have examined is uniform throughout the species’ range in Tasmania, Victoria and southeast New South Wales, and I have not detected any morphological discontinuity in Notodesmus scotius in Bass Strait.

For other poorly vagile animals with trans-Bass Strait distributions, I have not yet found any documentation of discontinuities congruent with the paranota/no-paranota divide in Dicranogonus pix between the Kent and Furneaux Groups. A possible evolutionary parallel is in the rhaphidophorid cricket genus Cavernotettix Richards, 1966. Cavernotettix flindersensis (Chopard, 1944) is known only from the Furneaux Group, and Cavernotettix craggiensis Richards, 1974 is known only from Craggy Island (ca 40 ha), located between the Kent and Furneaux Groups. ( Cavernotettix records from the online Atlas of Living Australia, http://www.ala.org.au, accessed 17 September 2014.)

Within Victoria the known distribution of Dicranogonus pix is a zone ca 150 km long and up to 60 km wide, running north from East Gippsland over the Great Dividing Range, from near sea level to ca 600 m. On the Tasmanian mainland all but one of the locality records are less than ca 2 km from the sea, the exception being a 1964 collection from Gladstone, a small town. “Gladstone”, however, may only represent the nearest named place to a coastal collecting site on Ringarooma Bay, ca 15 km distant. I have collected Notodesmus scotius , but not yet Dicranogonus pix , in the dry eucalypt forests beginning ca 10 km inland from Dicranogonus pix localities along the northeast Tasmanian coast.

Ecology. Jeekel (1982: 212) wrote of Dicranogonus pix at the holotype and paratype localities in Victoria: "This elegant little creature was locally quite common, occurring numerously in the upper litter layer of the dry Eucalyptus forests, and, judging from the number of specimens seen, mass appearances may occasionally happen". Adults have so far been collected in every month of the year except May, and in August 1998 I found a mixed Notodesmus scotius - Dicranogonus pix 'mating swarm’ during the day in coastal heathland near Blackmans Lagoon in northeast Tasmania.

Surprisingly, Dicranogonus pix was missing from pitfall samples collected in coastal heathland within the Dicranogonus pix range in northeast Tasmania in 1986-88. The sampling was carried out by T.B. Churchill, who trapped paradoxosomatids (as by-catch) in three 9 × 9 m pitfall arrays (nine evenly spaced traps per array) located at each of four 90 × 90 m sampling sites, with the traps emptied once a month for 14 months ( Mesibov and Churchill 2003). The traps yielded 9754 specimens of Notodesmus scotius and 116 specimens of an undescribed Pogonosternum species.

Type specimens.

Jeekel (1982: 209) lists the following type specimens for Dicranogonus pix , all collected on 14 November 1980 by Dr Jeekel and A.M. Jeekel-Rijvers:

Holotype male: "Sta. 86. 4 km ESE Bruthen... Eucalyptus forest, State forest, under logs" [My location estimate for the type locality near Bruthen, Vic is 37°43'18"S 147°52'24"E ± 1 km, probably along the Bruthen-Buchan Road.]

Paratypes: 3 males, 6 females, details as for holotype; 38 males, 29 females, "Sta. 85. 13 km SE Buchan... Eucalyptus forest, State forest, under logs" [37°36'S 148°11'E ± 2 km, probably along a forest road]; 46 males, 73 females, "Sta. 87. Mt Taylor, 11 km NNW Bairnsdale...fragment of Eucalyptus forest, along roadside between grassland, under logs and litter" [37°45'28"S 147°35'55"E ± 1 km, possibly along Bullumwaal Road].

Dicranogonus samples from the three localities listed above have recently been located in the Naturalis Biodiversity Center (K. van Dorp, in litt., 17 September 2014), following a long period during which their location was uncertain. The three samples, which I have not examined, presumably contain the holotype and the published paratypes.