Male dimorphism and mate guarding behavior in the harvestman Phalangium opilio (Arachnida: Opiliones: Phalangiidae)

Male dimorphism, the presence of two different forms of males within a population, has been observed in several species and is often accompanied by distinct reproductive strategies. Among arachnids, harvestmen are known to frequently exhibit this phenomenon. However, previous research has largely focused on species of the suborder Laniatores from the Neotropical region. In order to address this issue, an investigation was conducted into the male morphology and reproductive behavior of Phalangium opilio (suborder Eupnoi), a species of harvestman that is widely distributed throughout the northern hemisphere and New Zealand. We identified male dimorphism for the first time in P . opilio , specifically in the chelicerae and pedipalpal femurs. Furthermore, we found the presence of mate guarding behavior after mating, a behavior that had not been previously reported in this species. It is noteworthy that this behavior was more prevalent in males with large chelicerae or pedipalpal femurs, but minor males also exhibited guarding behavior when the opponent's traits were smaller. This pattern suggests that the relative size, rather than the absolute size, of these traits may be crucial in determining guarding behavior. Furthermore, we reported the first instances of cannibalism caused by lethal aggression in this species.