Five groups of final-instar nymphs ofEpitheca cynosuraSay, collected from October 1978 until March 1979, were maintained until emergence at experimental temperatures of 15, 20, 25, and 30°C and at natural photoperiods plus 15-min increments every 5 days. Pre-emergent development proceeded most rapidly at experimental temperatures of 20 and 25°C. The data, demonstrating that the most rapid physiological development of final-instar nymphs in nature occurred during the coldest winter months, strongly suggest a synchronizing effect on development caused by the changing sign of the photoperiod at the winter solstice.