Micro-CT analysis reveals directional genital asymmetry in a mygalomorph spider with two different sperm-storage organs (Araneae: Mecicobothriidae)

Sexual selection is a pervasive evolutionary force shaping the extraordinary diversity of animal genitalia, often driving rapid morphological divergence and complex interactions between male and female reproductive structures. In spiders, sexual selection acts through both pre- and postcopulatory mechanisms, frequently resulting in elaborate male intromittent organs, specialized female sperm-storage structures, and intricate mating behaviours. Despite this diversity, mygalomorph spiders have traditionally been regarded as morphologically conservative, with comparatively simple genitalia and stereotyped reproductive patterns. In this study, we provide the first integrative analysis of genital functional morphology, copulatory mechanics, and mating behaviour in a mygalomorph species, Mecicobothrium thorelli. Our results reveal a surprising degree of anatomical and behavioural complexity. Most notably, we document the first known instance of directional genital asymmetry in a mygalomorph spider. Female genitalia are characterized by two distinct spermathecal types, with one being asymmetric. This asymmetry is matched behaviourally by differential, selective use of the symmetric male copulatory organs. The consistent preference for specific male–female genitalia combinations suggests finely tuned mechanical and functional coordination during copulation. By uncovering these hidden complexities, our study expands current understanding of sexual selection and genital evolution in early-diverging spider lineages.