By the time most female butterflies emerge from the chrysalis, their eggs are fully mature, permitting immediate fertilization. Minute pores in the egg allow entry of the male sperm for fertilization. Eggs may be laid singly, in rows, in clusters, or in rings around a plant stem. The eggs are highly vulnerable to predators and parasites. Females lay up to 600 eggs at a time. A sticky substance produced during egg laying glues the eggs to the host plant. Most eggs hatch within a few days, although those of some species remain dormant over winter and hatch in the spring.