Larval nutritional environment determines adult size in Japanese horned beetles Allomyrina dichotoma

The effects of larval nutrition and parental size on offspring horn (male) and body size (male and female) were examined in the Japanese horned beetle Allomyrina dichotoma L. (Coleoptera: Scarabaeidae). Offspring‐parent regressions for both horn size and body size of males show no heritable effect, and the magnitudes of these traits were primarily determined by the larval nutritional condition. Male Allomyrina dichotoma also displayed dimorphic horn size‐body size allometry, that is, larger males had longer horns relative to their body size and vice versa. Because it has been suggested that males of different body sizes adopt different reproductive tactics, the dimorphic horn size–body size allometry and male reproductive tactics are also a result of the larval environment. Similarly, female body size was determined by larval nutrition, and, thus, larval condition might influence future female fecundity. Females under low nutrition treatment spent longer duration of the third larval instar than females under high nutrition. Females under poor nutrition treatment probably attempted to be as large as possible by the extent of larval duration. Since horn and/or body sizes of males and females affect their fitness, this suggests the evolution of female choice for better oviposition site.