Use of mandibular tusks as weapons in the aggressive behavior of the burrowing mayfly <i>Rhoenanthus coreanus</i> (Yoon and Bae, 1985) (Ephemeroptera: Potamanthidae)

In larvae of burrowing mayflies (Ephemeroidea), mandibular tusks are generally associated with burrowing and/or feeding. This study reports, for the first time, the use of mandibular tusks as weapons in the aggressive behavior of the burrowing mayfly <i>Rhoenanthus coreanus</i> (Yoon and Bae, 1985) (Potamanthidae). The larvae used their tusks as weapons in combat behaviors such as tusking, which was observed in 60.81% and 62.98% of the studied males and females, respectively. Most cases involved combat by tusking between females (70.45%), followed by combat between males (21.60%), and combat between male and female individuals (3.97%). Aggressive behavior was more common in female larvae with longer mandibular tusks. Positive relationships were observed between the number of cases of aggressive behavior (i.e., tusking) and the size of mandibular tusks in both male and female larvae (<i>p</i> &lt; 0.001). Female larvae with large mandibular tusks (5.00 ± 0.50 mm) could use their tusks to lift their opponents in a manner similar to horned beetles. Contests typically occurred as a result of territorial conflict and served as a means to find or retain occupied shelters for feeding and hiding.