I. senegalensis 9 9 exhibit colour dimorphism as andromorphs and gynomorphs, to which males seem to switch their mate preference according to prior copulation experience. In the field where andromorphs were dominant, the binary choice experiments were conducted both in the early morning, which marks the onset of daily copulation activity, and in the afternoon, which marks the end of the copulation activity. During the former period, S 6 showed fair selectivity, while they preferred the andromorphs in the afternoon, suggesting that S mate preference to each 9 morph switched in relation to copulation experience; i.e. the mating attempts of <J 6 were biased to the dominant 9 morph. Mating attempts in the afternoon were considered to inhibit 9 oviposition behaviour, resulting in a decrease of her reproductive success. Therefore, biased <J mate choice toward the dominant morph in the afternoon might be a selective force to maintain the 9 colour dimorphism.