Mandibular tusks vs. cephalic horns: a comparative study of functional behaviour in three flat-bodied mayfly larvae Polyplocia orientalis Nguyen and Bae, 2003, Rhoenanthus coreanus (Yoon and Bae, 1985), and Jappa furcifera (Eaton, 1871) (Ephemeroptera)

Most Ephemeroidea larvae possess mandibular tusks (‘tusks’), while larvae of the genus Jappa Harker, 1954 (Leptophlebiidae) feature cephalic horns (‘horns’). These structures are considered analogous, likely due to convergent evolution. This study compares the functional morphology of these tusks and horns, along with their associated behaviours, using three representative species of interstitial, flat-bodied larvae: Polyplocia orientalis Nguyen and Bae, 2003 (Euthyplociidae), Rhoenanthus coreanus (Yoon and Bae, 1985) (Potamanthidae), and Jappa furcifera (Eaton, 1871) (Leptophlebiidae). Despite similarities in shape, distinct differences were observed in their behavioural roles. All species required a ‘holding element’ to initiate burrowing, with burrow depth correlating to body length. Differences in burrowing, feeding, and aggressive behaviours were noted: P. orientalis and R. coreanus exhibited complex burrowing and feeding behaviours involving their forelegs, while J. furcifera showed rudimentary burrowing behaviour, primarily relying on head movements and using horns. Aggressive behaviours were observed across all species, with J. furcifera being the first to use its horns as weapons. These findings suggest that tusks in ephemeroid species serve multiple functions, likely shaped by evolutionary radiations within the diverse Ephemeroidea. In contrast, the horns of Jappa exhibit a more basic form of multifunctionality, possibly representing a simplified evolutionary adaptation restricted to this genus, which is geographically confined to Australia.