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The proportion of melanism (mean ± SD) under different photoperiods. The proportion of melanism increased when the light period become longer. The proportion of melanism under 16:08 was significantly higher than that under 12:12 (P = 0.0341) or 8:16 (P = 0.0279). The same letter represent there is no significant differences in statistics. The capital letters represent the level of 1%, the lower-case letters represent the level of 5%.
