Beetles are the most diverse group of plants or animals on earth, with over 300,000 species. More than 16,000 of these species live in water for at least part of their lives. Beetles have egg, larval, pupal, and adult life stages. For different species, one or all of egg, larval, or adult life stages live in the water, but usually the pupae are terrestrial. Some larvae have slender gills for breathing underwater, others simply take oxygen through the soft cuticle or pierce stems or roots of aquatic plants. Adults do not have gills, but have various other strategies--Some hold an air bubble against breathing holes on the sides of their bodies and replenish the bubble periodically at the water surface; others maintain a thin layer of permanently trapped air over those breathing holes. Aquatic beetles can be found in water that is either flowing or still, and some even thrive in brackish environments.