Water pennies have a jelly-like appearance and are typically found adhering to the underside of rocks in rivers and streams. They have six fuctioning legs on their underside, though not evident from most photographs.
Water Penny species are sensitive to pollutants and sedimentation, and their presence is a sign of good water. They are often found in riffles and whitewater, where the oxygen content of the water is greater.
The adults take their name from the larvae, and are called Water Penny Beetles.
American Insects site