3/4-1" (19-25 mm). Above, male bright lilac-blue with narrow black margins; HW has pinkish-orange submarginal band surrounding small marginal black dots. Female dark brownish; blue, if present, limited to wing bases (mostly FW); red-orange submarginal band more prominent. Below, both sexes white to gray with many small black dots; orange submarginal band prominent on HW (never on FW); outermost row of HW black dots have metallic green caps. Fringes are white; FW below has black dot in middle of cell.
Euphilotes blues lack metallic scaling on HW below and are confined to buckwheat host plants. Lupine Blue almost indistinguishable, generally larger with more and brighter orange markings, female often blue. Orange-bordered Blue male lacks orange on HW above; female has orange on both HW and FW above. Shasta Blue duskier, with less orange.
Egg pale green; laid on wild buckwheat (Eriogonum), locoweed (Astragalus), bird's-foot trefoil and deer weed (Lotus), lupine (Lupinus), other legumes, and knotweed (Polygonum aviculare). Young caterpillar overwinters; mature caterpillar dirty yellow, covered with fine white hair, green back stripe, and various side markings. Chrysalis brown with green abdomen.
Multiple broods; February-October, according to locale.
Virtually any habitat in West except driest deserts, dense forests, and urban areas, sea level to 10,000' (3050 m); rarer at higher elevations.
Canada to Mexico; Pacific Coast east to Saskatchewan and Dakotas, western edge of Great Plains, and W. Texas. Isolated populations in Minnesota, Nebraska, and Kansas.
Nearly the most ubiquitous western blue, this species flies close to the ground between flower visits. Spring brood females may be quite blue. Almost any crowd of blues at a mountain mud puddle will include some Acmon Blues. As they drink they twitch their hind wings, and in so doing, flash their emerald scales in the sunshine. This immediately distinguishes them from the similar, smaller buckwheat blues in the genus Euphilotes. The Acmon Blue was one of the few species of butterflies that lived on the slopes of Mt. Saint Helens before the volcanic eruption of 1980 destroyed this habitat. Versatile and quick to colonize, this species will probably return to the mountain soon.