Seasonal changes in the photoperiodic response regulating diapause in a tropical beetle, Stenotarsus rotundus

Abstract

Adults of Stenotarsus rotundus aggregate at the base of a palm tree and remain in a photoperiodically regulated diapause for up to 10 months each year in a tropical lowland forest, at 9°N. To examine seasonal changes in the photoperiodic response, beetles collected from the field at different times of the year were exposed to different photoperiods, and their gonad and flight muscle development were examined. The physiological responses of the adults suggest that diapause may be divided into three phases. During the initial phase (June–September), some beetles developed gonads in response to long photoperiods (12 h light-11 h dark), but the number of such individuals decreased towards the end of this phase. At short photoperiods (12 h light-12 h dark), no development occurred. During the intermediate phase (October–December), beetles showed no rapid development at either photoperiod. However, diapause development before the winter solstice proceeded more rapidly at short photoperiods than at long photoperiods. During the final phase (January–April), gonad and flight muscle development occurred earlier and more rapidly at long photoperiods. Thus, the beetle's response to photoperiodic changes throughout the season serves to coordinate the seasonal cycle of development in this tropical species.