The larvae are entirely aquatic, with large gills on the outside and a big tail fin that begins just behind the head. Arms are grown within a short time of coming out of their egg. Some larvae metamorphose quickly. These are known as 'small morph adults'. Other larvae, especially in ancestral pools and warmer climates, may not metamorphose until fully adult size. These large larvae are usually known as waterdogs, and are used many times in the fishing bait and pet trade. Some populations may not metamorphose at all, and become sexually mature while in their larval form. These are the neotenes, and are particularly common where terrestrial conditions are bad.