The name water penny beetle is a colloquial term for any beetle belonging to the family Psephenidae. Roughly 260 known species exist, but it's likely there are many more that have not yet been discovered. Their name comes from the flat, round appearance of their larvae.
Water penny beetles look very different from their larvae. Adults are dark-colored, generally black or dark brown, and measure 4 to 6 millimeters (.15 to .23 inches). Their bodies are slightly flattened and their antennae are short and threadlike. The larvae vary slightly in appearance according to their species. All of them have a flat, round shape, with their legs and heads covered by their dorsal plates. However, they can range in length from 3 to 10 millimeters (.12 to .39 inches) and vary in color from pale amber to almost black.
Water penny beetles are found all across the globe, on every continent except Antarctica. The larvae -- beetles before they pupate into their adult form -- are aquatic and are generally seen in fast-moving rivers and streams, or occasionally lake shores, clinging to rocks or other hard substrates. Adult beetles from this taxonomic family are land-dwelling, but still tend to live close to water and often hide under rocks or logs.
Feeding and Predation
Since adult water penny beetles only live for a short period of time, experts think they may not feed at all. However, little is known about members of family Psephenidae, and more research is required. Water penny larvae are known to be strict herbivores who feed on algae and other microorganisms that grow on freshwater rocks. When it comes to predation, both the beetles and their larvae are seen as prey by a range or birds, fish and rodents. They're more active at night than during the day to avoid potential predators.
Adult water penny beetles must use their short life -- probably only a couple of weeks -- to mate and lay eggs so the next generation can come into being. Exact details of reproduction depend on the species, but all members of this family are thought to be sexually dioecious and utilize internal fertilization. Females lay their eggs near the edge of the water so the aquatic larvae can drop into it once hatched. Over the next one to two years, the larvae grow and then emerge from the water to pupate into their adult forms.