Mandible dimorphism in males of a stag beetle, Prosopocoilus dissimilis okinawanus (Coleoptera : Lucanidae)

The dimorphism of weapon size in a stag beetle, Prosopocoilus dissimilis ok inawanus Nomura was examined. The relationship between weapon size and body size became curvilinear when head length was included as a part of body si ze. However, when it was used as a part of weapon size a sigmoid relationsh ip was obtained. Males could not be separated into two groups when nine tra its were analyzed using principal component analysis (PCA). However, when t he angle of the mandible (AN) was used in addition to the nine traits, male s could be separated into two distinct groups (i.e. S and L types). When th e size of each of the nine traits was plotted against the AN, males of the S type could be separated into two sub-groups in all combinations (i.e. S1 and S2 groups). The relationship between body size and weapon size showed t hat weapon size increased with body size in a quadratic manner in the S typ e, whereas it increased in a diminishing quadratic manner with body size in the L type. The difference in the relationship between weapon size and bod y size in the S and L types is discussed in relation to their combat behavi ors.