Water Penny Beetle Larvae

Beetles in the family Psephenidae

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Photo of two water penny beetles clinging to a wet rock.
Water penny larvae cling to stones and other submerged objects in flowing, unpolluted water.
Noppadol Paothong
Family

Psephenidae (water penny beetles) in the order Coleoptera (beetles)

Description

The flat, brown, nearly circular aquatic larvae of water penny beetles are more often noticed than the adults. Upon close inspection, you can see the segmented abdomen, thorax, and head. If you inspect the underside of a water penny, you will see 6 legs in the thorax region and tiny, feathery gills under the abdomen. Some species are more circular than others; others have slightly elongated bodies.

Adults are black or brown, oval, flattened beetles, usually wider at the hind end than in the front.

Size

Length: larvae to nearly ½ inch; adults to ¼ inch.

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Photo of a water penny beetle on a rock in an aquarium.
Water Penny Beetle on Rock
The presence or absence of water pennies in a stream can indicate the water quality.

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Photo of water penny beetle showing underside.
Water Penny Beetle (Underside)
Inspecting the underside of a water penny reveals feathery abdominal gills and six small legs.

water_penny_2-2-15.jpg

Photo of a water penny beetle clinging to a wet rock.
Water Penny Beetle
The flat, round aquatic larvae of water penny beetles are more often noticed than the adults.

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Photo of a leech and a water penny both clinging to the same rock.
Leech and Water Penny
Leeches, which are worms, and water pennies, which are beetles, both have segmented bodies.
Habitat and conservation

Water penny larvae cling to the undersides or sides of stones and other submerged objects in flowing, unpolluted water. Sometimes they cling to stones at the waterline, where splashing water keeps them moist. They don’t tolerate streams with high sediments or silt or with lots of algae. The flat shape and ability to cling to rocks helps keep them from being swept away in the current. It also helps them avoid predation. Adults are terrestrial and are often found on vegetation near streams.

Foods

The larvae eat algae by scraping it off of rocks using rasps on their legs. Depending on species, they may also scavenge from a variety of other materials, including other aquatic invertebrates. We’re not sure, but it’s possible the short-lived adults might not eat at all.

image of Water Penny Beetles Distribution Map
Distribution in Missouri

Statewide. Probably most abundant in the Ozarks, where there are so many clear, fast-flowing, rocky streams.

Life cycle

Water pennies go through the same life stages as other beetles, except that the larva is flat and looks like a penny instead of a grub! The larvae may take more than a year to mature. When they are large enough to pupate, they do so either underwater, still attached to stones, or in the soil near the stream. When they emerge as adult beetles, their mission is to locate the opposite sex, mate, and lay eggs; they don’t live long as adults.

Human connections

Many people are fascinated by the natural world. Water pennies are one of the truly nifty aquatic invertebrates that bring out the child in all of us. Some of them really do look like pennies!

Ecosystem connections

Biologists can gauge the health of a stream by taking a census of the aquatic insects that live in it. Because water pennies cannot tolerate pollution, high sedimentation, and high amounts of algae and fungi, their presence or absence in a stream is an indicator of the stream’s water quality.