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SUMMARY In wing polyphenisms that produced alternative wing morphs depending on environmental conditions, the developmental regulations to balance between flight and reproductive abilities should be important. Many species of aphids exhibit wing polyphenisms, and the development of wing and flight muscles is thought to incur costs of reproductive ability. To evaluate the relationship between flight and reproduction, the fecundity and the wing- and ovarian development in the parthenogenetic generations were compared between winged and wingless aphids in the vetch aphid Megoura crassicauda. Although no differences in offspring number and size were detected, the onset of larviposition after imaginal molt was delayed in winged adults. The comparison of growth in flight apparatus revealed that, after the second-instar nymphs, the flight-apparatus primordia of presumptive wingless aphids were degenerated while those of winged nymphs rapidly developed. In the ovaries of winged line, the embryo size was smaller and the embryonic stages were delayed from third to fifth instars, although these differences had disappeared by the time of larviposition. It is therefore likely that the delay in larviposition in winged aphids is due to the slower embryonic development. The correlation between embryo size and developmental stage suggests that the embryos of winged aphids are better developed than similarly sized embryos in wingless aphids. These heterochronic shifts would facilitate the rapid onset of larviposition after the dispersal flight. This developmental regulation of embryogenesis in the aphid wing polyphenism is suggested to be an adaptation that compensates the delay of reproduction caused by the wing development.