Dissections of the reproductive system of over-wintering adults of Chrysopa carnea disclose the classical symptoms of gonotrophic dissociation and show these adults to be in a state of reproductive diapause. Experiments testing the efficacy of day length and warm temperatures in promoting the resumption of reproductive activity show that the diapause state is eliminated within 20 days with exposure to daily photophases of 16 hr at 25°C, whereas with daily photophases of 10 hr no indications of the termination of diapause are visible after as long as 55 days at this temperature. Parallel experiments at 3°C give comparable results and provide no evidence that exposure to low temperatures potentiates the elimination of diapause to any significant degree. Rearing experiments show that the induction of diapause is also under primary photoperiodic control which, at least in the male, acts upon some pre-imaginal instar. A reversible colour change undergone by the over-wintering generation is likewise shown to be largely under the control of the adult photoperiod, although certain components of this change are influenced by non-photoperiodic factors to a small extent. The adaptive rôle of certain of these changes in the seasonal cycle of C. carnea is discussed, and the significance of developmental systems which are dependent upon environmental photoperiods for both the induction and termination of diapause is noted.