In crustaceans, typical aquatic beings, there are richly vascularized gills that make contact with water and permit gas exchange. In terrestrial insects the respiration is tracheal and gases flow inside small tubes that connect the animal external surface and ramify to tissues and cells without the participation of blood. In arachnids, besides the tracheal respiration, book lungs (thin folds resembling leaves in a book) may also exist.