In most plants and animals, a consistent relationship exists between the DNA content of a cell and its metabolic activity. The male-haploid sex determination of Hymenoptera and other arthropods may therefore impose a particular selective pressure upon males, which must evolve adaptations to cope with a genomic DNA reduced by half compared with that of females. Here, we show that a nuclear DNA content similar to that of females is restored in muscles of males in all hymenopteran lineages tested except the most basal one (Xyelidae). This doubling of DNA content in males does not occur in other haplodiploid insects, such as thrips (Thysanoptera) and whiteflies (Sternorrhyncha). These results indicate that this adaptation probably occurred early in hymenopteran history, possibly because males acquired strong flying and dispersal abilities.