Increase by duplication and loss ofinvariance of segment number in thecentipede Mecistocephalus microporus(Chilopoda, Geophilomorpha,Mecistocephalidae)

Intraspecific variability in the number of trunk segments is demonstrated for the first time in a representative of the Mecistocephalidae, a basal clade within the geophilomorph centipedes. This destabilization of an otherwise species-constant segment number is interpreted as a result of the recent quasi-duplication of segments shown by Mecistocephalus microporus Haase, 1887, with respect to its most recent ancestor. The new finding fits well with a recently proposed biphasic model of centipede segmentation. The species is redescribed and two new synonymies are proposed: Megethmus Cook, 1895 as junior synonym of Mecistocephalus Newport, 1843 and Megethmus pluripes Chamberlin, 1920 as a junior synonym of Mecistocephalus microporus Haase, 1887.