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Water Penny (Psephenus spp.)

Detailing the physical features, habits, territorial reach and other identifying qualities of the Water Penny

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Image Credit: Alex -icycatelf- Bowen
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Water Pennies have larvae that look more like crustaceans or fossils than beetles.

Updated: 07/15/2021; Authored By Staff Writer; Content ©www.InsectIdentification.org
An adult Water Penny looks like many types of beetle: six legs, black body, shiny, and small. Its larval stage looks quite unlike most beetle larvae. Instead of a worm-like, white grub with a brown head curling in the soil, this larva looks like a legless oval and is aquatic. Its copper color, round shape, and water habitat helped coin its common name. It is often compared to a trilobite, whose fossil is a similar shape with armor plating. Young Water Pennies are often found clinging to underwater stones and rocks in well-circulated streams, rivers, and lakeshores. They may even hold fast to sunken wood. Feathery gills allow them to breathe. Legs are hidden under the ?shell? and have scrapers used to nibble algae on the substrate they adhere to. They also eat decaying organic matter in the water. Once in adult form, they live on land, but near the water they came from.

The presence of Water Pennies signifies good water quality. In unhealthy, polluted streams that have overgrown algae or deep sediment, their ability to cling to stones and rocks is compromised, so they cannot establish a population. Looking for Water Pennies in shallow water can be fun for those who don?t mind getting their feet wet.

General Characteristics
Capability, Shape, Texture/Pattern, Benefits, Dangers
Rounded insect body icon


Taxonomic Hierarchy
Species Breakdown
Kingdom: Animalia
  Phylum: Arthropoda
    Class: Insecta
      Order: Coleoptera
        Family: Psephenidae
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          Genus: Psephenus
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            Species: spp.
Identifying Information
Size, Colors, Features
Scientific Name: Psephenus spp.
Category: Beetle
Size (Adult; Length): 4mm to 6mm (0.15" to 0.23")
Colorwheel Graphic Colors: brown; black
Descriptors: round; shell; stuck; horseshoe crab-like; underwater; fossil; trilobite

Relative Size Comparison
Typical Size Between 4mm (0.2in) and 6mm (0.2in)
Lo: 4mm
Md: 5mm
Hi: 6mm
Territorial Map*
U.S., Canada, and Mexico
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Territory map graphic of the country of Mexico
Contiguous United States shape map layer graphic
Prince Edward Is.  
* MAP NOTES: The territorial heat map above showcases (in red) the states and territories of North America where the Water Penny may be found (but is not limited to). This sort of data is useful when attempting to see concentrations of particular species across the continent as well as revealing possible migratory patterns over a species' given lifespan. Some insects are naturally confined by environment, weather, mating habits, food resources and the like while others see widespread expansion across most, or all, of North America. States/Territories shown above are a general indicator of areas inhabited by the Water Penny. Insects generally go where they please, typically driven by diet, environmental changes, and / or mating habits.


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