Photos: B. A. Huber
The Panjange nigrifrons group on Borneo includes beautiful spiders whose palps appear pitch-black in life specimens (above). They have been assigned to the SE-Asian genus Panjange from the very beginning (i.e. when the first species was described together with the genus in 1983). However, relationships are far from clear.
Bornean representatives of Panjange (i.e., representatives of the Pa. nigrifrons group) are more similar to species currently placed in Pholcus from Sumatra, the Malay Peninsula, and Sri Lanka than to other species groups of Panjange in the Philippines (incl. the type species) and east of the Wallace Line (Huber 2011; Huber & Nuñeza 2015). For example, the pointed prejections from the eye stalks in some Bornean Panjange species (above) strongly remind of certain Pholcus species from Sumatra.
One surprising finding of this study is that Panjange seems to be absent from Sabah (above). We sampled with the same methodology and diligence at six localities west of Niah (sea level to 930 m a.s.l.) as we did at nine localities east of Niah (sea level to 1650 m a.s.l.). Panjange was found at all six localities west of Niah (and in Niah), but in none of the nine localities east of Niah.
Even more surprising, Aetana shows the exactly opposite distribution pattern in northern Borneo. We found various species of Aetana at eight of nine localities east of Niah (and in Niah), but in none of the six localities west of Niah (Huber et al. 2015).
Obviously, large sampling gaps remain, and a meaningful interpretation will have to build on a much more intense sampling including Kalimantan. However, data on better studied organisms such as birds have also supported a biogeographical divide between Sabah and Sarawak despite continuous rainforest (Gawin et al. 2014, http://dx.doi.org/10.1525/bio.2011.61.1.10; Sheldon et al. 2015, http://dx.doi.org/10.1007/s10336-015-1188-3). These authors mainly suggest historical reasons for this pattern, such as early Pleistocene rainforest refugia in Sabah versus dry habitats in Sarawak (at that time part of central Sundaland). Whether the same explanations apply to spiders as to birds remains to be established.