Abstract: hree rice grasshoppers, Oxya japonica, O. chinensis formosana and O. Yezoensis are extremely similar in morphology. Here we studied their reproductive isolation mechanisms by interspecific cross experiments. For sympatric species, ethological isolation was found to be strong, but not complete, in reciprocal crosses between O. japonica and O. chinensis formosana, and between O. Japonica and O. yezoensis; however, even where mating had occurred there was no sperm transfer. For allopatric species, asymmetric ethological isolation was apparent in crosses between O. chinensis formosana and O. yezoensis: females of O. Chinensis formosana readily accepted males of O. yezoensis, but females of O. yezoensis show reluctance to males of O. chinensis formosana; however, in either reciprocal cross sperms were transferred. These results supported the conclusion that reproductive isolation evolves slower in allopatric species or populations than sympatric species or populations.

Key words: Rice grasshoppers, Oxya, reproductive isolation, mating choice, sperm transfer, relationship