is a genus of beetles
, placed in its own family
. It comprises six species, three from western North America
and three from China
. The vernacular name "trout-stream beetle" comes from the original finding of A. insolens
and A. lecontei
in high mountain streams, although other species occur at lower elevation. They are notable as a possible intermediate stage between terrestrial and aquatic beetles; while living in the water, they are not good swimmers and physically resemble ground beetles
more than other types of water beetle.
Ranging in length from 11 to 16 mm, they are generally a dull black, with a squarish head and a pronotum significantly narrower than the elytra.
The larvae also live underwater, but breathe through the eighth abdominal segment and must therefore stay close to the surface. They cling to rocks or floating debris. Both adults and larvae are predators, but will also scavenge dead insects.
When disturbed, adults exude a yellowish fluid from the anus, with an odor described as that of canteloupe or decaying wood, probably as a defensive mechanism against hungry frogs and toads.