Mechanisms of flight loss in Island endemic stag beetle Prosopocoilus hachijoensis (Coleoptera: Lucanidae): a comparative analysis of flight and walking traits among congeners

Abstract

Flight is a crucial locomotion strategy in insects, facilitating predator avoidance, foraging, and mating. However, when its energetic costs outweigh the benefits, flight capabilities can be lost secondarily. Flight loss has been studied in species exhibiting intraspecific polymorphisms, whereas both proximate and ultimate factors remain poorly understood in monomorphically flightless species. This study investigates the mechanisms of flight loss in Prosopocoilus hachijoensis, a flightless stag beetle, through comparison with flight-capable Prosopocoilus species. We hypothesized that flight muscle atrophy is a proximate factor, whereas reduced dependence on flight and increased reliance on terrestrial locomotion are the ultimate factors. We conducted field sampling in the Izu Islands, Japan, to examine morphological and behavioral traits related to both flight and walking. All species attempted flight, including the flightless P. hachijoensis. Wing reduction was observed in P. hachijoensis females and flight muscle atrophy was exclusive to this species. Allometric analyses revealed that P. hachijoensis exhibits reduced relative wing size compared with flight-capable congeners. While both male and female P. hachijoensis exhibited a smaller ratio of tarsal length to total leg length, the males’ walking speed did not differ from their congeners. In contrast, females of P. hachijoensis walked faster and showed a lower immobility frequency. We concluded that flight muscle atrophy is a proximate factor for flight loss in P. hachijoensis, and increased reliance on walking as locomotion may represent a potential ultimate factor underlying flight loss.