Latitudinal changes in fitness-related traits. (a) The mean length of abdomen increased with latitude (t = 4.618, df = 24, P < 0.001), although no significant effect of morph or interaction between morph and latitude are found (t = 1.244, df = 24, P = 0.227, t = -1.255, df = 24, P = 0.223). The error bars indicate ± S.E. (b) The average volume of mature eggs in andromorphs was significantly smaller than that in gynomorphs (t = 6.158, df = 22, P < 0.001). Error bars indicate ± S.E. (c) The number of ovarioles varied significantly with the abdomen length (GLM: Z = 16.119, df = 59, P < 0.001) and morph (GLM: Z = 2.621, df = 59, P = 0.009). The interaction between morph and abdominal length was also significant (GLM: Z = -2.808, df = 59, P = 0.005). (d) The potential fitness of gynomorphs was higher than that of andromorphs in the south, and the reverse was true in the north, crossing at approximately 36° latitude (equation (3)). The estimated equations are: f(A,l) = 0.0027 exp(0.584 + 0.176l) and f(G,l) = 0.02 exp(0.190 + 0.084l).