Geographical variation in Japan in egg development of the mayfly, Ephoron shigae (Ephemeroptera: Polymitarcyidae)

1. The effect of temperature on embryonic development was compared in four populations, two bisexual and two unisexual, of Ephoron shigae , including one each near the northern and southern periphery of the species range in Japan. 2. Eggs from every population were chilled at 4, 8 or 12 °C for diapause development after 50 days at 20 °C for pre‐diapause development (experiment I). Some eggs hatched during chilling at 8 °C or 12 °C, whereas no eggs hatched at 4 °C. The rate of hatching in a given condition of chilling was higher for the eggs from warmer winter environments. 3. Chilling at 4 or 8 °C effectively facilitated diapause development. Chilling at 12 °C was, in general, not so effective, but relatively effective for the eggs from warmer winter environments. 4. Eggs were incubated at 8, 12, 15 or 20 °C after chilling at 4 °C to examine the effect of temperature on post‐diapause development (experiment II). The eggs incubated at higher temperature after chilling hatched quicker and more synchronously and had higher hatching success. 5. The relationship between temperature and the days required for hatching after chilling was well described by the power function. There was no significant difference in the slope of the regression lines (i.e. temperature dependency) among local populations. However, a longer time was required for hatching at a given temperature for the population from the colder winter environment. 6. There was no detectable difference in the observed intraspecific variations between unisexual and bisexual populations.