Reproduction of Dugesia tigrina under short-day and long-day conditions at different temperatures. I. Sexually derived individuals.

Five successive generations of asexually derived populations of D. tigrina and their sexually derived parental population were reared under each of six combinations of 20, 23 and 26 °C with 12 and 16 h of daylight. Asexual and sexual reproduction were recorded for each population. Peaks of asexual reproduction occurred increasingly earlier in consecutive generations to finally disappear or occur later again. Peaks of sexual reproduction likewise occurred increasingly earlier in consecutive generations to be retarded again or disappear in later generations. Retardation occurs in earlier generations under short-day conditions than under long-day conditions. All generations later than the third, asexually derived, were sterile. The implications of these results are discussed and it is suggested that the time of cocoon deposition of all asexually derived offsprings is determined regardless of intervening generations during early development of the sexually derived parent. Attention is drawn to the similarity between the suggested mechanism and environmental control of aphid reproduction.