At night and during rain slugs crawl up trees and bushes. In the morning, as the air humidity decreases, they do not retrace their track but just crawl downwards. If the branch is bent down, the slug will crawl to its end and then continue the downward movement on a thread of mucus. The mucus thread of Deroceras reticulatum (O. F. Müll.) gets up to 60 cm long with the slug descending at 10 cm per minute; that of Arion fasciatus (Nilsson) is up to 27 cm. Going down on mucus threads can be considered a behavioural feature of slugs as an ecological form. This method of descending has appeared in different families as a result of convergence.