Copulatory Mechanics Reveals a Self-Bracing Mechanism via a Femoral Apophysis in Funnel Weavers (Araneae, Agelenidae).

Spiders utilize an indirect method of sperm transfer via specialized male palpal structures. In entelegyne spiders, these structures exhibit a remarkable complexity, comprising various sclerites that interlock with the female genitalia to provide stability and facilitate sperm transfer. Among the four primary coupling mechanisms recognized in entelegyne spiders, one, termed self-bracing, involves interactions between structures stabilizing the expanded copulatory organ during mating. Such interactions can involve elements that are not part of the copulatory organ. The retrolateral tibial apophysis (RTA), a characteristic of the largest group of spiders (RTA clade), is the most prominent structure for this purpose. However, recent research has demonstrated that in spiders that have lost the RTA, other parts of the palp, specifically femoral apophyses, can be involved in self-bracing. The presence of a femoral palpal fapophysis is uncommon in spiders, and only a few taxa possess apophyses on multiple palpal articles, i.e., tibia and femur, the interaction and evolution of which remain to be elucidated. This study investigated the function and interaction of apophyses on different palpal structures for the first time using the funnel weaver