Acilius sylvanus is a species of water beetle commonly found in freshwater habitats such as ponds, lakes, and slow-moving streams. It has a streamlined body shape and is typically around 1-2 centimeters in length. This beetle is known for its excellent swimming abilities, using its long hind legs and paddle-like feet to propel itself through the water. Its coloration varies, but it often has a dark brown or black body with lighter markings.