<br> Adult body size often varies geographically in response to local climatic conditions. The Emma field cricket (<br> Teleogryllus emma<br> ) exhibits a reverse Bergmann’s rule pattern, with smaller adults occurring at higher latitudes. This trend was confirmed across populations sampled from 33°N to 41°N. One unique population inhabits the periodically exposed reservoir slopes (RS) of the Oishi Dam, Niigata, Japan. These crickets display significantly reduced body sizes compared to those in the surrounding grasslands. To investigate the underlying causes of this body size reduction, we examined the influence of environmental and genetic factors. Laboratory-reared mid-instar nymphs from the RS grew into larger adults than their field-collected counterparts, suggesting an environmental basis for reduced body size. High temperatures led to smaller adults, while high rearing densities (100 individuals per container) significantly reduced adult size compared to low densities (1–2 individuals). Photoperiod (11–16 hours) also influenced growth: longer daylengths extended development and produced larger adults. Small but significant size differences between the RS and surrounding populations suggest possible genetic differentiation, although further genetic analysis is needed to confirm evolutionary divergence. Delayed hatching at the RS exposes the crickets to shorter daylengths, likely accelerating growth and leading to smaller adults. Year-to-year variation in adult head width in the RS population primarily correlated with nymphal densities rather than temperature. Interestingly, smaller adults laid smaller eggs, yet nymphs from these eggs exhibited compensatory growth, often reaching sizes comparable to those from larger eggs, resulting in no significant correlation between adult body sizes of females and offspring. Overall, body size variation in<br> T. emma<br> at the RS appears to be shaped by both phenotypic plasticity in response to environmental conditions and genetic differentiation. Despite these fluctuations, body size in some populations has remained stable for over half a century, suggesting possible stabilizing selection or genetic constraints.<br>
