Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. 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.