 In newts and salamanders , metamorphosis is less dramatic than in frogs . This is because the larvae are already carnivorous and continue to feed as predators when they are adults so few changes are needed to their digestive systems . Their lungs are functional early , but the larvae do not make as much use of them as do tadpoles . Their gills are never covered by gill sacs and are reabsorbed just before the animals leave the water . Other changes include the reduction in size or loss of tail fins , the closure of gill slits , thickening of the skin , the development of eyelids , and certain changes in dentition and tongue structure . Salamanders are at their most vulnerable at metamorphosis as swimming speeds are reduced and transforming tails are encumbrances on land . Adult salamanders often have an aquatic phase in spring and summer , and a land phase in winter . For adaptation to a water phase , prolactin is the required hormone , and for adaptation to the land phase , thyroxine . External gills do not return in subsequent aquatic phases because these are completely absorbed upon leaving the water for the first time . 
