Psephenus herricki

Larvae can be distinguished by their strongly flattened, disc-shaped body and association with stream riffles. The combination of extreme dorsoventral compression, oval carapace, and presence in fast-flowing water habitats separates this species from other aquatic beetle larvae. Adults are found on wave-splashed rocks in riffle areas.

Larvae are highly flattened with a disc-like, oval carapace. The body shape is dorsoventrally compressed, an adaptation to life in fast-flowing water. Adults have the typical beetle form with hardened forewings (elytra).

Stream riffles with turbulent, fast-flowing water. Larvae occupy interstitial spaces within the substrate, crevices, and undersides of stones. Adults are found on moist, wave-splashed rocks that protrude above the water surface.

Eastern North America. Documented in Canada from New Brunswick, Nova Scotia, and Ontario. Presence confirmed in North America generally, with eastern distribution emphasized in sources.

Larval stage lasts approximately 2 years. Eggs are deposited in subsurface materials associated with wave-splashed rocks in riffles. Larvae have extended residence time in riffles due to limited dispersal behavior.

Larvae exhibit thigmokinesis—movement inhibition when in contact with substrate surfaces—reducing vagility when wedged in interstitial spaces. They withstand water flow through leg purchase on the substrate; loss of grip leads to dislodgement. Active pumping of water through lateral carapace slots and posterior expulsion reduces turbulence and may decrease drag at high Reynolds numbers. Adults aggregate on wave-splashed rocks for mating and oviposition.

• Other PsephenidaeOther water penny beetles share the flattened larval form, but P. herricki is distinguished by its specific association with turbulent riffle habitats and eastern North American distribution.

The flattened body shape was historically interpreted as an adaptation to fast currents or predator avoidance, but this interpretation is questionable because larvae are typically found in sheltered microhabitats (crevices, undersides of stones) rather than exposed to direct current or predation pressure.

• aquatic
• lotic
• rheophilic
• flattened larvae
• thigmokinesis

• BugGuide
• Wikipedia
• GBIF taxonomy match
• iNaturalist taxon
• NCBI Taxonomy
• Ecology of the Water Penny Beetle Psephenus herricki (DeKay)
• The behaviour and form of Psephenus herricki (DeKay) (Coieoptera: Psephenidae) in relation to water flow*