The effect of temperature on embryonic development and local adaptation in the life cycle of Eobiana engelhardti subtropica Bey-Bienko (Orthoptera: Tettigoniidae)

Abstract

Embryonic development in relation to life cycle adaptation was studied in the northern (Aomori, 40.6°N) and southern (Osaka, 34.9°N) strains of Eobiana engelhardti subtropica (formerly Metrioptera hime) (Orthoptera: Tettigoniidae). Eggs of the 2 strains showed similar thermal responses. At 20, 22.5, and 25°C, they developed to the mature embryonic stage without delay and entered an obligatory embryonic diapause (final diapause). At 27.5 and 30°C, on the other hand, embryonic development was markedly delayed. This retardation was more striking in the southern strain. Especially, summer diapause occurred in half-grown embryos at 30°C. A shift of temperature from 30°C to 20 or 25°C terminated summer diapause and the eggs started to develop. The final diapause is never terminated if kept at high temperatures. Final diapause requires an exposure to a cold temperature of 15°C or lower and the intensity is higher in the southern strain. Adults of the southern strain emerged in mid June or earlier and those of the northern in early-mid July. In outdoor experiments, the developmental rate decreased more conspicuously in the eggs laid earlier in the southern population. As a result, the eggs reached the final-diapause stage in autumn more synchronously than expected from the long egg-laying period. The decreased developmental rate and summer diapause appear to secure survival in the hot season and ensure reaching the embryonic diapause stage synchronously in autumn. The local variation in the final-diapause intensity seems to be related to the heat units in autumn which might otherwise cause untimely hatching.