Geometric morphometric analysis of gonopods in Bicoxidens flavicollis populations (Diplopoda, Spirostreptida, Spirostreptidae)

Male gonopods are useful in taxonomic diagnoses and descriptions of millipedes, although they may vary intraspecifically in shape and size. To assess this intraspecific variation, we used geometric morphometric analysis to compare gonopod morphology among eight isolated populations of the colour-polymorphic southern African millipede Bicoxidens flavicollis. Our results showed that gonopod shapes vary significantly across the examined populations, and elucidated subtle variations. CVA cross-validation test indicates an average classification rate of 75% for the five populations for which we had more than one specimen. Although we had a small number of replicates for three populations, our results still illustrate the importance of applying quantitative approaches to what would otherwise be qualitative and subjective gonopod shape categories in millipedes. As such, the taxonomic assignment of the populations of B. flavicollis may require further investigation, and further revisions would be required with an integrative approach, including molecular data, in order to re-evaluate the taxonomic diversity and distribution data of this species. Finally, we highlight the conservation potential of divergent populations as evolutionary insurance against a dynamic and unpredictable climate, whether or not they undergo full speciation.