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Size varied; male 10.9-13.6mm, female 11.1-15.0mm; body broad and black; antennae, maxillary and labial palpi, and tarsi black; head finely punctate; pronotum more coarsely punctate (pitted); porsternal intercoxal process slightly elongate; elytral silhouette subovoid, slightly narrowed basally, less narrowed subapically; elytral surface moderately or coarsely rugose in lateral half; pronotum at least as broad at middle as at base, with lateral margins markedly crenulate; elytra evenly convex; elytra finely punctate with striae faint but visible. (See also identification of genus).

Holometabolous. Amphizoids lack structural adaptations for swimming, and are actually more efficient locomotors on land than in water. The beetles are unable to swim, and crawl on substrate at the bottom of streams. Larvae pupate out of the water on adjacent streambanks. Both adults and larvae are strict predators. Adult Amphizoids are able to carry out most life functions- feeding, locomotion, oviposition- as easily on land as in water. Under laboratory conditions, Amphizoid eggs and larvae thrive out of water, and even pupate normally. Larvae are of the caraboid type, and are elongated with flattened lateral lamellate projections, have two tarsal claws, and two short, spine-like cerci. Larvae breathe through the 8th abdominal segment. When disturbed, adults exude a yellowish fluid from the anus which has an odour described as that of cantaloupe melon or rotting wood. The function of this fluid is debated, but it is likely for predator-defense. Both adults and larve are frequently found attached to the roots of undercut vegetation at the stream edge, crawling along rocks and pebbles on the bottom of the stream, or found clinging to driftwood and other debris. When disturbed they can be seen floating on or near the water surface.

Extends southward from Yukon Territory and southeastern Alaska to mountain ranges of southern California; eastward from the Pacific Coast (includes Queen Charlotte Islands and Vancouver Island) to western Alberta, central Montana, western Wyoming, central Idaho, and eastern Nevada.