Butterflies choose a stage in their life cycle to hibernate according to their species. Some butterfly species, such as the Banded Hairstreak variety, lie dormant in the egg waiting for winter to move on. The Viceroy butterfly hibernates during the larval stage under the soil or inside curled leaves. Mourning Cloaks spend the winter as adults under loose tree bark and in log piles that people store outside their homes. The chrysalis stage is another cycle that some species of butterflies choose for hibernation, despite their vulnerability in this state. Another aspect of butterfly biology that helps them survive is their ability to create chemical substances in their blood, such as glycerol, that acts as a sort of anti-freeze to protect them from extreme cold.