Revisiting the functional morphology of mandibular tusks in burrowing mayfly larvae (Insecta: Ephemeroptera): analysis using micro-computed tomography and 3D morphometrics

The superfamily Ephemeroidea is a diverse group of burrowing mayflies known for the morphological and behavioural uniqueness of their mandibular tusks during their larval stage. In this study, we investigated the functional morphology of mandibular tusks across all six tusked families of Ephemeroidea using 3D morphometrics from micro-computed tomography combined with an innovative method of automatic landmarking via point-cloud registration. Our findings quantitatively support and refine the hypothesis of Bae and McCafferty in 1995 revealing that curvature and robustness are functionally linked to tusk adaptations. A principal component analysis of the mandibular tusks and forelegs substantiated the grouping of the three dwelling morphotypes (interstitial, burrowing, and hard-substrate burrowing dwellers). Evolutionary trends based on the current Ephemeroidea phylogeny suggest that interstitial dwellers with flattened bodies might have evolved in the basal lineages of Potamanthidae and Euthyplociidae, whereas burrowing dwellers with cylindrical bodies, such as that in most basal lineages of Polymitarcyidae and Ephemeridae, as well as hard-substrate dwellers, such as Povilla—Ashenopus (Polymitarcyidae) and Cheirogenesia—Palingenia (Palingeniidae), appear to have evolved in parallel, with enhanced burrowing efficiency with the forelegs. These findings provide comprehensive insights into the functional and ecological adaptations of these unique structures among benthic macroinvertebrates.