Decoding the Feeding Mystery of Adoretus sinicus: A Comparative Study of Mouthpart Morphology Among Four Species of Rutelinae (Coleoptera: Scarabaeidae)

Mouthparts exhibit diverse morphological features in scarab beetles, resulting from the adaptation to the various living and feeding habits. Adoretini is a small tribe of Rutelinae, remarkable for bearing a beak-like projection on the labrum. However, the correlation between peculiar structures and feeding habits remain unsatisfactory. In this study, the adult mouthparts of Proagopertha lucidula, Anomala corpulenta, Popillia quadriguttata and Adoretus sinicus were compared using scanning electron microscopy, to investigate the structural basis underlying the distinctive feeding behavior of A. sinicus. Based on our investigation, the adult incisors of A. sinicus could not meet when closed. Instead, a beak-like labral projection fits precisely between the paired incisors, which is a configuration markedly different from the other three species. Our scanning electron micrographs reveal distinct wear marks on lateral sides of the labral projections and the mesal corners of mandibular incisors, supporting the hypothesis that both structures are involved in the chewing process. These findings help explain the characteristic perforated damage observed on leaves attacked by Ad. sinicus and also provide a potential functional basis for the labral projection unique to adult Adoretini.