Populations of Daphnia usually consist of parthenogenetic females that produce only other females. However, during periods of stress, such as cold weather, drought, or overpopulation, males are produced along with haploid eggs. After sexual fertilization, the eggs remain in the brood chamber of female Daphnia until the next molting period. Encased in modified and thickened cases known as ephippia, the eggs can survive many years in severe conditions after they are released. Resistant to freezing, drying, and digestion, the eggs do not hatch until favorable circumstances develop.