7/8-1" (22-25 mm). Above, male blue-violet or silvery-lilac blue; dark margins become marginal dot row on HW. Female brown or blue, has wider margins, especially on FW; HW margin has dot row often capped inwardly with some orange crescents. Both sexes often have black bar at end of FW cell and/or HW cell also (more common on female). Below, both sexes are gray with much white overscaling, dots on HW browner than on FW; HW below has prominent row of metallic bluish-green encircled marginal dots, inwardly bordered by distinct row of orange crescents, in turn bordered inwardly by row of white triangles pointing toward brownish dots.
Northern Blue larger, less dusky. High Mountain Blue lacks orange row. Acmon Blue has bright orange, more distinctly marked below.
Undescribed. Host plants include lupine (Lupinus lyalli), locoweed (Astragalus spatulatus and A. calycosus), and clover (Trifolium dasphyllum).
1 brood; mainly July, varying with altitude.
Chiefly high mountains, often near or above treeline; as low as 7,000' (2135 m) in the Sierra Nevada, and 3,000' (915 m) in Oregon. Alpine fell-fields, arctic-alpine ridges, meadows near streams, and forest clearings; also sagelands, prairie and pinyon-juniper woodlands.
Eastern slopes of central Oregon Cascades and California Sierra Nevada; east to extreme S. Saskatchewan and south through Dakotas, W. Nebraska, central Colorado, Great Basin, and Rockies.
The Shasta Blue is at home as high as 13,000' (3965 m), fluttering weakly over rock and cushion plants. The lower prairie forms are more rapid and erratic fliers. Both tend to live in colonies. There is some geographic variation and individual populations can be variable as well. Males from the Sierra Nevada have broader borders, while those from above treeline in Colorado vary in color from clear to gray-blue. Females from the prairies can be almost as blue as males, and those of Mt. Charleston near Las Vegas, Nevada, are very blue with prominent bright orange crescents above. The common name for this species comes from Mt. Shasta, California.