Young newt. Scanning electron micrograph (SEM) of a juvenile (immature) newt. Although this newt is in the last stages of development, as it has four legs but still has gills (frilly structures at upper left) on its neck. Gills are specialised organs used for breathing underwater. When tadpoles undergo metamorphosis (developmental changes) into adult newts, their gills change into air-breathing lungs for living on land. Adult newts are mainly terrestrial, only visiting the water to breed.