Size: 10.7-12.8mm

Seasonality: April-October

Distribution; WA, OR, CA, ID, w. MT, NV, UT

Information: may be found along muddy riverbanks, ditches, rivers, and streams, as well as in marshes and wetlands. Top photo shows a male specimen of A.alternans, the second shows a female, both are black morph specimens. The description provided below is for the black morph, the bicoloured morph is simply the same overall description, though the elytra have a triangular black marking at the base of the elytra which points towards the elytral apices. Below this is a hazier black marking which covers most of the elytra, leaving the perimeter along the margins and along the triangular mark pale yellow. Scapus orange.

Description: body: elongate-oblong, slender. Dorsal colouration; glossy or matte black, ventral colouration reflects this. Head triangular, face short, smooth, no ridges. One setae near either dorsal lateral margin of the clypeus (with the bicoloured morph, this can be used to tell apart from A.sanctaecrucis). Eyes small, hardly bulging, antennae dark reddish brown, scapus tends to be paler than the rest of the antennae. Antennal pubescence begins on the second half of the first antennomere after the pedicel. Pronotum glossy or matte black, rough, centre typically mostly glossy. Disk covered in numerous tiny punctures near base and around margins. Lateral margins weakly sinuous; front angles obtuse; front margin curved in toward the pronotum weakly; hind angles squared or weakly obtuse; basal margin transverse. All pronotal margins thinly beaded.  Pronotum weakly quadrate-cordate; at widest point, the pronotal disk should be as wide as the elytra. 8 deep striae per elytron, intervals weakly elevated. Humeral angles obtuse; lateral margins of the elytra should be more or less straight; elytral apex pointed, very weakly sinuate. Males tend to be slightly more narrow than females.