The Japanese horned beetle Trypoxylus dichotomus population in Fukue Island in western Japan has a smaller body size than Honshu populations in the wild. However, factors underlying this size variation are unclear. In this study, we examined genetic and phenotypic differences in body size under common laboratory conditions, by comparing the Fukue Island population with Kumamoto (in similar latitude to Fukue Island) and the Honshu (Tsukuba, Tokyo, and Yamaguchi) populations. Individuals from the Fukue Island and Kumamoto populations were indistinguishably smaller than those from Honshu populations even when reared under common laboratory conditions. However, in wild-caught individuals, the Fukue Island population had a smaller body size than the Kumamoto population. Therefore, the small body size of the Fukue Island population is likely attributable to both genetic and environmental factors. We characterized body size variation within the Fukue Island population with respect to sexual dimorphism in body size and its variation, and their annual fluctuation. The Fukue Island population showed significant annual fluctuation in body size, but consistently had smaller size variation within each sex, and smaller sexual size dimorphism than the Honshu populations in each year. These results suggest that although larval food availability for T. dichotomus fluctuates annually in Fukue Island, the feeding conditions there are often poorer compared to those on the mainland.

All but two specimens of geometrid moths collected in 1964 were heavily infested by carpet beetles. Two specimens of Arichanna gaschkevitchii remained free from carpet beetle damage for over 50 years, suggesting that toxic substances ingested during the larval stage have a sustained deterrent effect against carpet beetles.