Unique recordings provide insight into the sexual activity of the arthropods

Millipedes during sex - the structures of their genitals are clearly visible in UV light. © Stephanie Ware, Field Museum

Researchers have observed millipedes during sex - and looked very closely. Her recordings of the sexual act reveal for the first time in detail how the genitals of males and females of the genus pseudopolydesmus interact with each other. It also turned out: After sex, the millipede lady apparently seals her vulva with a sticky secretion - maybe this way she protects the sperm, the team suspects.

Whether foreplay-loving tardigrade, the underwater act of the dolphins or snow monkeys with extraordinary erotic preferences: The diverse genital organs and sex practices in the animal kingdom always amaze researchers. Even males who mutilate their partner's genitals after mating and species in which the female sex has the penis have already discovered biologists.

At the same time, there are many animals that have not yet been observed during lovemaking. How they do it is therefore largely a mystery - this also applies to the millipedes. These multi-legged arthropods have produced thousands of species in the course of evolution, and each of them is likely to reproduce in its own way, as Xavier Zahnle from the University of California at Davis and his colleagues explain.

Love game in the petri dish

In order to find out more about the genitals and sex practices of the millipedes, the team of scientists has now devoted itself to representatives from the genus Pseudopolydesmus. “The problem with millipedes is that they do many things underground. If you take them out, you disturb them and then they stop,” says co-author Petra Sierwald of the Field Museum in Chicago. Not so pseudopolydesmus millipedes: "These animals have sex even in bright light in the petri dish."

This exhibitionistic disposition was just right for the researchers: they observed the arthropods during reproduction and took a large number of photos of the act. They used UV light, among other things, because the genital organs of the arthropods shine under the influence of this radiation and the individual tissues can be better distinguished. Computed tomography (CT) studies provided additional insights into the structure of the genitals.

View of the female genital organs on the second pair of legs © Stephanie Ware, Field Museum

Ejaculation and mating members separated

The recordings of the millipede genitals - both individually and combined in the sexual act - now reveal for the first time how sex works in the genus pseudopolydesmus. Specifically, it turned out that, as already known from other millipedes, the testicles of the males are not directly connected to the mating extremities. "The male must therefore ejaculate and immerse his so-called gonopods in the bluish ejaculate," reports Sierwald.

For the actual sexual act, the female then turns her vulva outwards, as the scientists found. "She has two openings between her second pair of legs," says the researcher. When the male penetrates, tiny pliers hook it into the end of his gonopods in the female genitals. Gonopods and vulva fit together like a key to the lock - this is the only way the mating act works, as the team suspects.

Sealing of Vulva after sex

Also interesting: After sex, the vulva is sealed with a sticky secretion and the sperm is enclosed in the female's body. If the millipede later lays eggs, they come into contact with the stored sperm on the way out. But who actually closes the external female genital organs?

"Before the study, I thought that the secretion came from the male, who wants to use this method to prevent the female from mating again," reports Sierwald. “But our CT images revealed glands inside the vulva. This suggests that much of the secretion could come from the female. Whether it wants to protect his genitals or the sperm, it is an exciting question for further research."

In addition to giving us a better understanding of the mechanics of millipede sex, Sierwald hopes the project will enable scientists to better understand the relationships between different millipede species, which could shed light on how they evolved.



"This study will be important for understanding how millipedes in this genus are related and how they're distributed," says Sierwald. "They can tell us about the geologic history of North America. As mountain ranges and rivers formed, groups of millipedes would get cut off from each other and develop into new species." And, she notes, Pseuopolydesmus is just the tip of the iceberg.

"There are 16 orders of millipedes in the world, and for most of them, we have only faint ideas what the vulvae look like."

Bibliography: Genital morphology and the mechanics of copulation in the millipede genus Pseudopolydesmus (Diplopoda: Polydesmida: Polydesmidae). Xavier J. Zahnle, Petra Sierwald, Stephanie Ware, Jason E. Bond Arthropod Structure & Development, 2020; 54: 100913 DOI: 10.1016/j.asd.2020.100913