A new species of Tridactylogonus Jeekel, 1982 from South Australia (Diplopoda, Polydesmida, Paradoxosomatidae)

Abstract Tridactylogonus warrenbenensis sp. n. is described from Warrenben Conservation Park at the southern end of the Yorke Peninsula in South Australia. Like T. obscurus Jeekel, 1982 and T. rugosissimus Jeekel, 2002, the new species has prominent cellular sculpturing on the prozonites and granulose sculpturing on parts of the metazonites. Unlike its congeners and most species in the subfamily Australiosomatinae, the new species lacks a femoral process or tubercle on male leg 1.


Introduction
I collected the new species described here during two recent visits to the lower Yorke Peninsula in South Australia (Fig. 1). It was the only native polydesmidan species I found in the area, much of which has been heavily colonised by the introduced Portugese millipede Ommatoiulus moreleti (Lucas, 1860). It is a remarkably atypical species within the Australian Paradoxosomatidae, as it lacks a femoral process on male leg 1 and has prominent cuticular sculpturing on the prozonites and metazonites.

Materials and methods
All specimens are stored in ethanol in the South Australian Museum (SAM). Body measurements were estimated with a Nikon SMZ800 binocular dissecting microscope using an eyepiece scale. Stacks of colour images were manually generated using a Canon EOS 1000D digital SLR camera mounted on the Nikon SMZ800 fitted with a beam splitter, then focusstacked with Zerene Stacker 1.04 software. One gonopod was cleared in 80% lactic acid, temporarily mounted in a 1:1 glycerol:water mixture and imaged using an eyepiece video camera mounted on an Amscope binocular microscope. Preliminary drawings were traced from printed copies of the images, then corrected by reference to the actual gonopod. Scanning electron microscope images were acquired digitally using a Hitachi SU-70; body parts were examined after air-drying and sputter-coating with platinum, and later returned to alcohol. Figures were composed using GIMP 2.8 software. Parts of the backgrounds of the colour photomicrographs have been edited to remove distracting highlights and artifacts.
Locality details are given with latitude and longitude in decimal degrees based on the WGS84 datum. The estimated uncertainty for a locality is the radius of a circle around the given position in metres. Abbreviations: SA = South Australia, Australia; SAM = South Australian Museum, South Australia, Australia. Diagnosis. Differs from T. obscurus in having variably rugose rather than smooth metatergites, and in the anteromedial process of the gonopod telopodite being flattened rather than lanceolate. Differs from T. rugosissimus in having one process extending from the base of the solenomere rather than two. Differs from both T. obscurus and T. rugosissimus in lacking a femoral process or tubercle on male leg 1.
Male with vertex bare, frons sparsely setose, clypeus moderately setose; vertigial sulcus distinct, ending just above level of antennal sockets; post-antennal groove shallow; antennal sockets separated by ca 1 socket diameter. Antennae clavate, reaching dorsally to rear of ring 2; antennomeres with relative lengths 6>(2=3)>(4=5); 6 thickest. Collum ( Fig. 2A) half moon-shaped, strongly convex, rear margin straight, corners rounded and slightly upturned. Head slightly wider than collum; collum to ring 18 nearly uniform in width but rings 2 and 3 slightly narrower. Ring 2 paranotum ( Fig. 2A) thin, dorsally concave, set lower than collum corner and ring 3 paranotum, extending slightly past posterior ring margin. Paranota on rings 3 and 4 ( Fig   at anterior end of narrow, ovoid depression at rear of paranotal margin. Spiracles on diplosegments above and just anterior to leg bases, rims thin; anterior spiracle ( Fig. 4D) with rim slightly elevated, filter slightly emergent dorsally, filter elements flattened and with forked tips. Midbody sternites about as long as wide, sparsely and finely setose, cross impressions about equal in depth; no cones or other projections on any sternites. Midbody legs (Fig. 2D) with relative podomere lengths (femur=tarsus)>prefemur>(post femur= tibia). Pre-anal ring (Fig. 2B) with a few long setae; epiproct extending well past anal valves, in dorsal view slightly tapering and truncate, tip ca 1/3 width of pre-anal ring; hypoproct paraboloid. Spinnerets in square array, dorsal setae longer than ventral, each pair set in narrow, shallow, transverse concavity on posterior surface of epiproct.
Female more robust than male; epigyne thickened but barely protruding; cyphopods not examined.
Name. For the type locality, Warrenben Conservation Park. Distribution. Known from six localities over ca 4 km 2 in Warrenben Conservation Park at the southern end of the Yorke Peninsula, South Australia (Fig. 1A, B). Found in bark litter under dead she-oak trees (Allocasuarina stricta) and in Eucalyptus sp. bark litter in shrubby, mallee-type vegetation on limestone at 20-30 m elevation. The area has an annual rainfall of ca 440 mm (Bureau of Meteorology 2017).
Remarks. Taxonomic affinities. Although its gonopod telopodite is "bidactylous" rather than "tridactylous", T. warrenbenensis sp. n. closely resembles the other two Tridactylogonus species in its small size and gonopod form. The genus was thought by Jeekel (1982, p. 128) to "stand rather isolated" within the tribe Antichiropodini, and its discovery "might seem to narrow the taxonomic disjunction between this tribe and the Australiosomatini". However, as in other Australian Antichiropodini, especially Aethalosoma Jeekel, 2006, Aulacoporus Verhoeff, 1924, Brochopeltis Verhoeff, 1924, Pseudostrongylosoma Verhoeff, 1924and Walesoma Verhoeff, 1928, the gonopod telopodite in Tridactylogonus has a narrow, straight femoral portion arising from a small, setose prefemoral portion, with the femoral portion clearly demarcated from the solenomere and any other apical processes. Also, as in Aethalosoma, Dicranogonus Jeekel, 1982, Notodesmus Chamberlin, 1920and Pogonosternum Jeekel, 1965(fig. 2 in Mesibov 2009, fig. 4B in Decker et al. 2017, the spiracular filter elements in the new species are flattened and have forked tips. Within the genus, T. warrenbenensis sp. n. is most similar to T. rugosissimus in gonopod form and in metazonite sculpturing (see below), which Jeekel (2002, p. 65) thought could distinguish the latter species "from all other known Australian Paradoxosomatidae". The new species is remarkable in lacking a femoral process or tubercle on male leg 1, a character almost universally present in Australiosomatinae. Jeekel reported that the leg 1 femur in T. obscurus had "the usual ventral tubercle" (Jeekel 1982, p.130), and in T. rugosissimus "a small ventral tubercle" (Jeekel 2002, p. 64). Unfortunately I have not been able to examine the type and only known specimen of T. rugosissimus; it was not deposited in the South Australian Museum as proposed (Jeekel 2002, p. 60) and has not been found among material in the late Dr Jeekel's study collection in the Naturalis Biodiversity Center in Leyden, the Netherlands (K. van Dorp, in litt., 17 September 2014).
Surface sculpture. Jeekel (1982, p. 129) noted "a fine, but quite conspicuous cellular structure" on the prozonites and "subgranulose" sides in T. obscurus. In T. rugosissimus the prozonites were said to have either "pronounced reticulate structure or minute and regular granulation", while the metatergites were "coarsely and densely rugose, with irregular longitudinal or somewhat oblique or undulate rugae", and the sides "coarsely granulose to subrugulose" (Jeekel 2002, p. 63). Cellular prozonite sculpturing seems to be a constant character state in T. warrenbenensis sp. n. and closely resembles the prozonite sculpturing in another Australian paradoxosomatid, Desmoxytoides hasenpuschorum Mesibov, 2006(fig. 5A in Mesibov 2009). "Rugose" sculpturing of the metazonites is more variable. It is always present on the sides as irregular microscopic ridges, but is variably prominent on the metatergites as coarse, narrow wrinkles. Each microscopic ridge appears to project from one cuticular scale, but ridges on adjoining scales sometimes appear to be confluent (Fig. 4E), in contrast to the discretely spaced projections seen in some other recently examined Polydesmida (Mesibov 2009, Akkari andEnghoff 2011).
Biogeography and conservation. The three Tridactylogonus species occur around Spencer and St Vincent Gulfs in southern South Australia (Fig. 1B) and form an allopatric species mosaic. T. warrenbenensis sp. n. is so far known only from Warrenben Conservation Park (Fig. 1A), which covers ca 4000 ha and is managed for nature conservation. Over several days of searching in 2016 and 2017 I was unable to find any native millipedes in the nearby and larger Innes National Park (Fig. 1A), despite the presence there of apparently suitable woodland and scrub habitats. The National Park instead has a dense and almost ubiquitous population of the introduced Portugese millipede Ommatoiulus moreleti (Lucas, 1860). O. moreleti is also the most common millipede elsewhere on the Yorke Peninsula in both partly natural and entirely agricultural habitats. If O. moreleti has displaced native millipedes in Innes National Park, it may displace T. warrenbenensis sp. n. in future in Warrenben CP, which is linked to Innes NP by several large blocks of privately owned bushland. During my 2016 and 2017 visits I saw no O. moreleti in Warrenben CP.