1 3/4-2" (44-51 mm). Above and below 2-toned; inner dark and outer light, separated by a sharp border. Above, inner dark area chocolate-brown with 2 red-orange patches along FW costa, outer part has yellow band blending into bright orange band. Below, inner area purplish-brown; outer band tan. Dark margin above and below punctuated by faint blue bars, has irregular but not really ragged outline.
Egg pale green; deposited in clusters, often several hundred together. Caterpillars at first live in colonies in silken nests, later become solitary leaf-folders. Mature caterpillar black with narrow yellow band above and green side stripes, white speckled with 7 rows of short spines. Host plants
are nettles (Urtica). Chrysalis grayish or greenish-tan, thorny. Adults overwinter.
2 or 3 broods where season permits; spring, summer, and fall.
Dry stream beds and canals, riversides, beaches, meadows, alpine rockslides, roads, and trails.
Far North except Alaska, south to S. California, Oklahoma, and West Virginia.
This unmistakable butterfly prefers northern latitudes and higher altitudes, although it occupies lowlands if they are cool enough. Extremely versatile, it inhabits every kind of place within its range, from cold desert to rain forest and city lot to alpine summit. Milbert's Tortoiseshell may sometimes be seen even in midwinter on a warmish day in many temperate areas. As with the Compton Tortoiseshell, the abundance of Milbert's fluctuates radically, but unlike the California Tortoiseshell, Milbert's never emigrates in impressive masses. Some specialists consider this butterfly to be the same species as the European Small Tortoiseshell (A. urticae). These 2 small, bright nettle-feeders may have evolved from common stock and, having been successful in their respective hemispheres, now differ significantly in appearance.