Bug of the Week

Water Penny (Family Psephenidae)


Howdy, BugFans,

Today's BOTW is one that, like caddisflies, tent caterpillars, and wooly bear caterpillars, has an immature/larval/nymphal stage that is much better known than its adult stage.

Whether a tadpole, a leech, a snail, or one of the myriad immature insects for whom the water is a nursery, aquatic organisms face some common challenges. They need a way to breathe, eat and locomote under water. They need shelter from organisms that would eat them, the right habitat, and a plan for overwintering. If an animal lives in swift currents, where today's featured bug hangs out, it has one more problem. Staying put. Some are streamlined, some use glue or silk, others have ways of grabbing their surroundings so they don't end up a mile downstream.

Water pennies are the larvae of riffle beetles in the Family Psephenidae (the first "P" is kind of silent, and "ph" = "f"). Water pennies live underwater on rocks in rapid currents – an unusual habitat for a beetle, but one that offers some protection from predators. Adult riffle beetles can be found in the water or basking on rocks and logs just above the water line, and according to Voshell, in the outstanding A Guide to Common Freshwater Invertebrates of North America, adults may also be found on rocks on wave-washed lakeshores. The adults are hairy, ¼ inch beetles; the larvae, called water pennies for their shape and color, look like well-camouflaged, tiny, suction cups (the official name for the shape is "platyform"). Water pennies occur world-wide (the greatest diversity is found in the Orient) and there are 16 species in North America.

water-penny1
A water penny looks and acts a bit like a limpet.

A female riffle beetle crawls "below-decks" into the swift currents to lay her eggs on the lower surfaces of algae-covered rocks, though eggs may also be deposited just above the water's surface. The hairs on her body hold a film of air. Her eggs hatch into the characteristic limpet-like larvae that cling tightly to rock surfaces, allowing the water to flow over them. The claws on a water penny's tarsi (BugSpeak for feet) help it latch onto rock surfaces, and the "plates" that make up the tops of its body are flexible, allowing it to mold to the shape of a rock. The edges of the plates are fringed with hairs that enhance its "grip."

Adult riffle beetles' basking days are brief, and they probably don't eat (not much is known about them). Water pennies are classed, diet-wise, as "scrapers" that ingest the algae and diatoms that live on rocks (a moderate algal film is good – a thick algal mat is not water penny-friendly). Voshell says that to this end, they are well-adapted. Their cup-shaped jaws have a sharp inner edge, similar to a paint scraper, to dislodge food, and hairs at the base of the jaws help push the dislodged material into their mouths. Another source said that they have scrapers on their legs. They are light-sensitive, clinging by day to the lower surfaces of rocks. But at night, water pennies migrate to the upper surfaces of the rock where the more nutritious algae grow. The larvae of some species are marginally social. In colder climates, water pennies may overwinter, metamorphosing into adults the next year. They pupate in chambers on damp land.

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An upside-down water penny is reminiscent of a tiny, cartoon turtle.

Head and mouth are located at one end, and its nether end, five pair of gills may be seen. Swift currents tend to be oxygen-rich, and the gills of water pennies grab dissolved oxygen from the active waters they live in. The larvae can also absorb oxygen through the general surface of their body. Water pennies are indicators of waterways that are high in oxygen and low in pollution. Hard for us to find, they are said to be eaten by trout living in those same oxygen-rich waters.

So, in summary, these 6mm critters live in strong currents. Under the cover of their "shell, they scrape/loosen tiny plants to eat, food that doesn't get whipped away by the current because it's trapped under them. In the process, they don't get whipped away either. What happens under a water penny, stays under a water penny. Wowsers!.

The BugLady