Acneus

Larvae are identified by their characteristic disc-like, flattened shape and association with fast-flowing, rocky streams. They can be distinguished from other psephenid genera by subtle differences in thoracic and abdominal plate structure, though precise diagnostic features require specialist examination. Adults are difficult to identify without dissection and comparison with type specimens.

Aquatic larvae inhabit clear, cool, fast-flowing streams and rivers with rocky substrates. They attach to the undersides of stones and cobbles in well-oxygenated water. Adults are terrestrial and found in riparian vegetation near larval habitats.

Western North America, including the Pacific Northwest and Rocky Mountain regions. Documented from Oregon, Washington, and adjacent areas. The genus appears restricted to this region based on species descriptions.

Larval activity occurs year-round in suitable aquatic habitats. Adult emergence timing is poorly documented but likely coincides with warmer months.

Larvae are fully aquatic and develop on submerged rocks. Pupation occurs in moist terrestrial environments near the water margin. Adults are short-lived and terrestrial, returning to water only to oviposit. Specific duration of developmental stages is not documented.

Larvae cling tightly to rock surfaces using their legs, resisting displacement by current. They are slow-moving and remain stationary during daylight, feeding at night. When disturbed, larvae may release their grip and drift downstream before resettling.

Larvae function as scrapers or grazers in stream ecosystems, processing periphyton and biofilm from rock surfaces. They serve as prey for fish and other aquatic predators. The genus contributes to nutrient cycling in headwater stream systems.

No direct economic importance. Occasionally used as bioindicators of stream water quality due to sensitivity to pollution and habitat degradation. Rarely encountered by the general public due to cryptic larval habits and brief adult life.

• PsephenusAnother genus of water-penny beetles with similar larval morphology; Acneus species generally smaller with subtle differences in thoracic plate structure and more restricted western distribution.
• EubrianaxAsian and North American water-penny genus; Acneus distinguished by geographic range and details of larval plate sclerotization.

• aquatic
• Coleoptera
• Psephenidae
• western North America
• stream insects
• bioindicator

• BugGuide
• Wikipedia
• GBIF taxonomy match
• iNaturalist taxon