About 100 species in 27 genera have been found in North America.
There are about 1400 species known world-wide, but probably many more have not yet been discovered. What happens under a water penny, stays under a water penny.The Elmidae, commonly known as riffle beetles, are found in freshwater streams all around the world.
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. 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. Other sources say that they have scrapers on their legs. 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.
Voshell says that to this end, they are well-adapted. 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). They pupate in damp spots on land near the water’s edge or in air-filled chambers underwater, beneath that wonderful larval skin.Īdult water penny beetles’ basking days are brief, and they probably don’t eat (not much is known about them). In colder climates, water pennies may take two years to mature, but they usually metamorphose into adults the next year. The larvae of some species are marginally social, tolerating nearby larvae, but they’re mostly solitary. The hairs on her body hold a film of air for her to breathe. It’s not physically suctioned to the rock, just very streamlined.Ī 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 edges of the plates are fringed with hairs that enhance its grip. 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. Water pennies are indicators of waterways that are high in oxygen and low in pollution. 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).
#RIFFLE BEETLE LARVAE SERIES#
Head and mouth are located at fore end, and filamentous gills are found at the aft end (great series of pictures). 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. And if an animal lives in swift currents, it has one more problem – staying in one place. They need shelter from predators, a habitat that fills their needs, and a plan for overwintering.
They need a way to breathe, eat and locomote under water. 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. 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! 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. 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.
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