It is empirically well known that adults of both sexes of the Japanese rhinoceros beetle Trypoxylus dichotomus septentrionalis (Kôno) copulate repeatedly under laboratory conditions. However, details of the fertilization success of respective males involved in multiple mating are unknown. To determine the degree of sperm displacement in successive mating in this beetle species, we conducted a male-swapping crossing experiment under laboratory conditions. A mutant line with white compound eyes in the adult stage, of which the white-eye state shows simple recessive Mendelian inheritance to the wild type regardless of sex, was employed in the experiment. Individual adult female mutants were consecutively mated with a mutant and a wild-type adult male in a different order. Although the sample sizes of our experiment were limited, the eye color of all adults of the next generation was inherited from the first-mated males. Therefore, our results strongly suggest that the degree of sperm displacement is very low or absent, or even that sperm transfer does not occur in secondary matings, in this beetle species.
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