Water-penny beetle
（重定向自Psephenidae）
Water-penny beetles are a family (Psephenidae) of aquatic beetles. The young, which live in water, resemble pennies. The larvae feed off of algae, larvae, and feces. The presence of water penny larvae in a stream can be used as a test for the quality of the water. Among the pollution sensitivity categories sensitive, somewhat-sensitive, and tolerant; water pennies belong to the sensitive category. They cannot live in habitats where rocks acquire a thick layer of algae, fungi, or inorganic sediment. Therefore, their presence along with other diverse phyla signifies good quality water. They are around 6 to 10 millimeters in length.