HORN AND WING ALLOMETRY AND MALE DIMORPHISM IN GIANT RHINOCEROS BEETLES (COLEOPTERA, SCARABAEIDAE) OF TROPICAL ASIA AND AMERICA

In populations of four rhinoceros beetle species (Chalcosoma caucasus F. and C. atlas L. from Asia, and Dynastes neptunus Quonsel and D. hercules L. from America), quantitative analyses of intraspecific variations were conducted with emphasis on the allomorphosis of horns and wings. In all female populations, body length was isometric to all other morphological traits measured, indicating that large females are in all aspects magnified versions of small females. Males were larger and more variable than females. In all male populations, head and thorax horns were remarkably and positively allometric, whereas hind wing and width were negatively allometric to body size. Thus, smaller males had comparatively large wings and rudimentary horns, and larger males had greatly enlarged horns and comparatively small wings. Males of C. caucasus, C. atlas, and D. neptunus diverged into two clearly distinguishable morphs: minor with comparatively small body and shorter horns, and major with larger body and longer horns; in each morph the body and horn size relationship was subject to a different formula. In D. hercules, divergence of males into two morphs was less clear-cut; yet distinct body and horn size relationships could be detected in the miners and majors. The positive allometry of horns and the negative allometry of wings persisted across as well as within morphs. These results suggest that allomorphosis of horns and wings is general and widespread among rhinoceros beetles of different phylogenies, and male dimorphism is also common, but its rigidity varies among different phylogenetic groups.