The coevolution of genitalia in Diplopoda Eugnatha

Abstract

The genitalia of Diplopoda Eugnatha are characterised by a remarkably high degree of functional-morphological disparity. The differences between major taxa concern not only a rather superficial 'ornamentation', such as the shape, proportion and number of certain parts, but there are also substantial disparities in the fundamental functional morphology and in the configuration of female and male structures in relation to each other. The evolutionary pathways and selective mechanisms that led to this disparity remain poorly understood. Here, a brief narrative review on the genital mechanics in Diplopoda Eugnatha is provided and differences between individual species are illustrated using simplified diagrams. Two contrasting hypotheses regarding the genital mechanics of the last common ancestor of Eugnatha are presented. The telopodite hypothesis assumes that this ancestor had already fully differentiated the eighth pair of male legs into gonopodes, and that it would have transferred sperm with the modified telopodite of this gonopods. In contrast, the coxal sac hypothesis assumes that the ancestor would have transferred sperm using the coxal organs of nearly unmodified walking legs. It is currently unclear which of the two narratives is more likely to be true. A comparative approach highlights a particularly striking evolutionary trend, namely a mutual fitting between male and female structures that is made possible by female cooperative structures, complementary to male structures. Female and males structures together stabilise the mutual genital entanglement and increase the chances of successful sperm transfer.