Hypotheses about the functions of the male genitalia and the male scape in insects were tested by measuring the slopes of allometric relations in six populations of Chauliognathus scutellaris. All allometric relations used elytron length as the indicator of overall body size. Male genitalia have lower slopes than male pronota (a structure not involved in reproduction), male scapes (secondary sexual characters) have higher slopes than male pronota, and female scapes have slopes that are not different from the slopes of female pronota. These results support Eberhard's one-size-fits-all hypothesis regarding the size of male genitalia in insects, and they raise questions about the role of the male scape in reproductive activities.