Epipharynx shape as a tool to reveal differentiation patterns between insect sister species: insights from Onthophagus taurus and O. illyricus (Coleoptera, Scarabaeidae)

We evaluated the potential of the adult epipharynx to reveal interspecific differentiation patterns in closely related Onthophagus species. Although easy to analyse, this structure has received little attention in Scarabaeidae taxonomy, probably because a qualitative inspection of its shape often does not yield differences between congeneric species. The polyphenic sister species Onthophagus taurus Schreber, 1759 and O. illyricus Scopoli, 1763 were chosen as a case study. They are extremely similar, to the extent that in some cases they cannot be identified unambiguously without the help of biomolecular analysis. In this study, a combination of linear measurements and a landmark-based approach was employed to quantify inter- and intraspecific shape variation in the epipharynx of specimens sampled at the same study site. Our results showed that the epipharynx is a monomorphic structure: its shape does not vary as a function of sex or male phenotype. In males, epipharynx shape does not change with head shape or horn length. The close proximity of the epipharynx to the horns and the synchronous developmental patterns of these two structures suggest that a developmental trade-off may act between them. Despite these predisposing conditions, however, our results suggest that epipharynx size is not subject to costs associated with horn development, and that the trait is highly canalised. Surprisingly, when using geometric morphometrics the epipharynx appears to be a better tool than genitalia for discriminating between the two sister species.