Horn Dimorphism of Allomyrina dichotoma septentrionalis (Coleoptera: Scarabaeidae) Affected by Larval Nutrition

I reared larvae of a dimorphic horned beetle, Allomyrina dichotoma septentrionalis (Kono), under 2 different conditions and measured the horn length and body length of male adults obtained from those larvae. Adults from well-nourished larvae showed a bimodal frequency distribution of horn length, but adults from ill-nourished larvae did not. The allometric relationship between horn length and body length differed significantly between ill-nourished and well-nourished larvae. The results suggest that adults from ill-nourished larvae develop their bodies more fully than their horns, and that adults from well-nourished larvae develop their horns more fully than their bodies. I conclude that the horn dimorphism of this beetle arises from differences in the level of larval nutrition.