A new species of Bicoxidens Attems , 1928 ( Diplopoda , Spirostreptida , Spirostreptidae ) from northern Zimbabwe

Bicoxidens aridis sp. n. is described from fi ve male specimens collected from northern Zimbabwe. Besides being the smallest member of the genus, B. aridis has a hook-shaped distal telocoxite, a telopodite with a long postfemur and a proximal clockwise coil. Th e discovery of this species suggests that the geographical range of the genus may extend further north into Zambia.


Introduction
According to Mwabvu et al. (2007), many southern African millipedes are unknown to science because several habitats have not been surveyed and because large quantities of material held in museums have not been studied.Our knowledge of the region's millipede diversity and distribution is therefore incomplete (Mwabvu et al. 2007).
Th e genus Bicoxidens is endemic to southern Africa (Mwabvu 2000).It has been recorded in savanna woodland, riverine vegetation and forests south of the Zambezi River (Mwabvu et al. 2007).Th e distribution of brincki Schubart, 1966 extends southwards from Zimbabwe across the Limpopo River into the Kruger National Park, South Africa, and fl avicollis Attems, 1928 extends eastwards from Zimbabwe to Maguge in western Mozambique (Mwabvu et al. 2007), but otherwise the genus seems to be restricted to Zimbabwe, particularly the central, eastern and southern regions (Fig. 3).Details of localities and synonymies of known species are provided in Mwabvu et al. (2007).
Th e genus is characterized by a telopodite which has neither antetorsal process nor torsotope, but which may have one or two lobes at the femur and terminally.Until now Bicoxidens had not been recorded from the Zambezi Valley, which led to the suggestion that the hot and arid conditions in the valley are less habitable and, therefore, restricted the spread of the genus northwards (see Mwabvu et al. 2007).However, a recent study of millipedes held in the Natural History Museum, uncovered a new species from the Zambezi Valley in northern Zimbabwe.

Material and methods
Th e material examined is housed in the Royal Museum of Central Africa, Tervuren, Belgium (MRAC) and the Natural History Museum, Bulawayo, Zimbabwe (NMZ).
Methods follow Mwabvu et al. (2007), and terminology follows Hoff man (2008).Specimens were studied using a Carl Zeiss Stereo microscope (Stemi DV4) and photographed using auto montage software (Leica Microscope MZ12s with 3 CCD Toshiba Camera).Adobe Photoshop CS (version 8) was used to manipulate the images and prepare the plate.In addition, images of the telopodite were produced using a JEOL JSM-6480LVscanning electron microscope.Material for electron microscopy was prepared following Barnett et al. (1991).ArcGis (Arcmap 9.1) was used to prepare the distribution map of species.Etymology.Specifi c name refers to the fact that the species occurs in a dry, low rainfall region.

Bicoxidens aridis
Diagnosis.Distal telocoxite folded towards apical proplica, producing a hook shape and a median cavity (Figs 1a,1b); lateral margins of proplica and metaplica setose; postfemur of telopodite as long as proplica, with a proximal clockwise coil distal to the femoral lobes, and without looping at the extremity (Fig. 2).
Colour.Body light to dark brown; head light brown; antennae dark brown to black; posterior part of metazonite dark brown or black; prozonite cream; legs light brown.
Gnathochilarium.With long setae along distal margin of the stipes and lingual lamella; one small papilla behind and between the larger two.Collum.Anterior corner rounded or square in shape, not produced into a lateral lobe, with 2-6 submarginal folds (Fig. 1c).
First pair of legs.Tarsal claws straight, but gently curved apically; prefemoral processes proximally parallel, but not touching medially, with laterally directed nipple-shaped apical lobes that are widely separated distally (Fig. 1d).
Sigilla.One row of large circles.Midbody ring.Diameter of metazonite greater than prozonite, prozonte with up to 16 striae that are further apart posteriorly; limbus with straight edge.
Proximal half of telopodite broader than distal (postfemoral) half.Length of postfemoral region of telopodite equal to that of proplica.Telopodite bends laterally and produces a tight clockwise coil distal to the femoral lobes, above level of the paracoxite  2b).Postfemur of telopodite extends distally towards the telocoxal apex and crosses the post knee part proximal to the coil and femoral lobes (Fig. 1a).Telopodite without apical looping, but with two unequal lobes at the extremity (Fig 2a).
Distribution.Appears to be widespread in the Zambezi Valley and northern Zimbabwe, a dry savanna region characterised by high temperatures and low, unreliable rainfall (Nyamapfene, 1991).

Comparisons
In terms of body width, B. aridis is the smallest among the known species in the genus.Other major diff erences relate to the structure and shape of the apical gonopod, and the clockwise coil of the telopodite at the femur.
Although the apical folding of the metaplica is more pronounced and produces a median cavity in aridis, folding of this structure is not unique to aridis; varying degrees of folding of the distal telocoxites was reported by Mwabvu et al. (2007) in brincki, friendi Mwabvu, 2000, gokwensis Mwabvu, 2007, matopoensis Mwabvu, 2007and nyathi Mwabvu, 2007.However, the characteristic median cavity and telopodite coil are present in aridis only.
Distally the proplica has a heart-shaped lateral lobe which covers the telopodite at the knee (Fig. 1a); which appears to prevent the telopodite from slipping out of the gonoschisma.A similar function was suggested for the apical lobe of the proplica which is found in fl avicollis and grandis (see Mwabvu et al. 2007).
Unlike other species, the aboral surface of the metaplica of aridis does not have a lobe or projection at the level of the apical proplica.Additionally, the lateral margins of the metaplica and proplica are setose, while in other species only the apical region of the proplica is setose.
Th e post knee region of the telopodite of aridis is approximately twice as long as the proplica, in addition, the post knee telopodite is not L-shaped nor defl ected medially at the level of the paracoxites or sternite (Figs 1a, 1b) as is the case in congeners (see Mwabvu et al. 2007).Th e telopodite typically produces a single clockwise coil just after the femoral lobes and lacks terminal bending or looping, making the shape of the telopodite unique to aridis.