Genital morphology and the mechanics of copulation in the millipede genus Pseudopolydesmus (Diplopoda: Polydesmida: Polydesmidae)

Mate choice, copulation, genital morphology, and sperm storage are not
very well understood in millipedes. The use of three-dimensional x-ray
computed tomography (µCT) provides new morphological data regarding
millipede reproductive systems in both the female and male, including
chitinous sclerites and membranes, muscles, glands, oviducts, and sperm
conduits. Here we present a complete integrated account of the morphology
and function of the female genital organs in the family Polydesmidae
(Diplopoda: Polydesmida) using µCT, UV fluorescence imaging, and scanning
electron microscopy. These data allow us to consider competing hypotheses
regarding millipede vulva formation. We additionally present the
morphology of copulatory interface in Pseudopolydesmus Attems, 1898 using
images of a mating pair in copula and by simulating the interface of the
organs using 3D models from µCT, allowing us to tentatively identify a
lock-and-key-like mechanism. Finally, we use µCT to reveal the topology of
the seminal canal in the gonopod of male Pseudopolydesmus, a topic that
has remained unresolved for nearly 80 years.