Mating dynamics of ghost spiders (Amaurobioidinae, Anyphaenidae) reveal independent regressive evolution of important genital properties via functional substitution

During evolution, animal organ morphology can vary dramatically, including reductions in structure and loss. This phenomenon recurs in many spider male mating organs. Many spiders usually have well-developed structures but are missing or marked in some taxa. One example is the posterior tibial dermatospread (RTA) of male pedipalp in the rta clade, the most diverse lineage of spiders. Perform important functions frequently during genital coupling. This is the first major mechanical involvement (i.e., primary lock) during genital coupling. However, in some strains, RTA is lost or reduced, raising questions about how genital binding is achieved in these taxa, and what is the potential driver for RTA regression. To address these questions, we used the subfamily Amaurobioidinae (Anyphaenidae) as a model to study nine genital mechanisms with varying degrees of RTA size. Genital coupling is reconstructed using microcomputed tomography (Micro-CT) data of frozen mating pairs, and with the exception of species with prominent RTA, the main lock is achieved only by inserting a conductor (male genital sclera) into one of the female fat releases. Our phylogenetic analysis shows that RTAs were independently lost or reduced six times in Amaurobioidinae, and that at least one functional substitution of RTA by conductors occurred within the subfamily. Using conductors for primary locks assumes that they will replace the main function of RTA, allowing for repeated regression.