Bug o’the Week – Water Penny Redux

Howdy, BugFans,

In the spirit of New Year’s Eve, here’s a rerun about a really spiffy little aquatic insect, revised from 10 years ago with some new words and pictures and also a correction.  In the original episode, the BugLady erroneously referred to adult water penny beetles as riffle beetles, but they aren’t riffle beetles (despite the fact that they hang out near riffles); true riffle beetles are members of the nearby family Elmidae.

Scooping for aquatic invertebrates is a great “gateway” to nature studies for many people – who knew that the world below the water’s surface was peopled by such an amazing bunch of critters!

Whether tadpole, fairy shrimp, leech, snail, planarian, or one of the myriad insects for whom the water is either a temporary nursery or a permanent home, aquatic animals face some common challenges.  They need a way to breathe, eat and locomote under water.  They need shelter from predators, a habitat that fills their needs, and a plan for overwintering.  And if an animal lives in swift currents, it has one more problem – staying in one place.  Some inhabitants of moving waters are streamlined, some attach with glue or silk, and others (like the sculpin that faces upstream with its fins braced against adjacent rocks) have structural adaptations to grab their surroundings so they don’t end up downstream.

Water penny beetles are in the family Psephenidae, a family whose immature or larval stage is much better known than its adult stage.  Adults – hairy, quarter-inch beetles https://bugguide.net/node/view/131447/bgimage – can be found in the water or basking on rocks and logs just above the water line, and also, according to Voshell, in his outstanding A Guide to Common Freshwater Invertebrates of North America, on rocks on wave-washed lakeshores.  There are 16 species of water pennies in North America; fewer than three hundred species occur world-wide, and the greatest diversity is found in the Orient.

Called water pennies for their shape and color, the larvae look like tiny, oblong suction cups https://bugguide.net/node/view/477142/bgimage dressed in camouflage.  They live underwater on rocks in rapid currents – an unusual habitat for a beetle larva, but one that offers some protection from predators (we have trouble finding them, but it’s said that trout don’t).

[Etymology alert: the official name for the larva’s shape is “platyform” – “platy” means “flat.”  Platyform is somewhat synonymous with “onisciform,” a word that refers to the flattened body shape of a wood louse (or sow bug or pill bug or roly-poly or potato bug or whatever regional name you learned as a kid).  Interested in the etymology of entomology? See https://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1001&context=onlinedictinvertzoology].

Head and mouth are located at fore end, and filamentous gills are found at the aft end https://bugguide.net/node/view/207646/bgimage (great series of pictures).  Swift currents tend to be oxygen-rich, and the gills of water pennies grab dissolved oxygen from the active waters they live in (they also have spiracles for taking air in ala terrestrial insects).  Water pennies are indicators of waterways that are high in oxygen and low in pollution.

The claws on a water penny’s tarsi (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 so that water flows over it.  The edges of the plates are fringed with hairs that enhance its grip.  It’s not physically suctioned to the rock, just very streamlined.

A female water penny beetle crawls “below-decks” into the swift currents to lay her eggs on the lower surfaces of algae-covered rocks, though she may deposit eggs just above the water’s surface, too.  The hairs on her body hold a film of air for her to breathe.

The larvae of some species are marginally social, tolerating nearby larvae, but they’re mostly solitary.  In colder climates, water pennies may take two years to mature, but they usually metamorphose into adults the next year.  They pupate in damp spots on land near the water’s edge or in air-filled chambers underwater, beneath that wonderful larval skin.

Adult water penny beetles’ basking days are brief, and they probably don’t eat (not much is known about them).  Water penny larvae are classed, diet-wise, as “scrapers” that ingest the algae and diatoms that live on rocks (a moderate algal film is desirable, but 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 to dislodge food, similar to a paint scraper, and hairs at the base of the jaws to help push the dislodged material into their mouths.  Other sources say that they have scrapers on their legs.  They are light-sensitive, clinging by day to the lower surfaces of rocks, migrating at night to the upper surfaces of rocks where the more nutritious algae grow.

So, in summary, these 6 mm critters live in strong currents and are the epitome of “streamlined.”  They don’t get swept away, and under the cover of their “shell,” the material that they scrape and loosen to eat doesn’t get swept away either, because it’s trapped.  What happens under a water penny, stays under a water penny.

Kate Redmond, The BugLady

Bug of the Week archives:

Become a Member

Take advantage of all the benefits of a Riveredge membership year round!

Learn More