Robustness of latitudinal life-cycle variations in a cricket Dianemobius nigrofasciatus (Orthoptera: Trigonidiidae) in Japan against climate warming over the last five decades

Recent climate warming is expected to have changed the phenology of organisms. The cricket Dianemobius nigrofasciatus (Matsumura) (Orthoptera: Trigonidiidae) has univoltine and bivoltine life cycles in northern and southern regions of Japan, respectively, because of latitudinal variation in the growing season length. Its adult body size increases with decreasing latitude, and decreases at a latitude where the number of annual generations increases. The present study aims to examine whether the range of the bivoltine life cycle has expanded northward due to climate warming. We compared a latitudinal saw-tooth body size cline between the 1960s and the 2010s. The body size showed a latitudinal saw-tooth cline in adults collected in recent years, as it did in adults collected five decades ago. However, no significant difference was observed between these two clines, suggesting that the bivoltine life cycle has not expanded in the last five decades. These results contrast to those recently reported in a closely related species, Polionemobius mikado (Shiraki) (Orthoptera: Trigonidiidae), in which the turning points of the saw-tooth cline had shifted northward in the last four decades. The stable latitudinal distribution of life cycles in D. nigrofasciatus might result from the different photoperiodic regulation of growth rate involved in the different responses to climate warming.