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The forewing is distinctly marked with a white costal triangle before the postmedian area. The inner half of the forewing is pale yellowish brown, while the remainder is darker brown with some black shading towards the anal angle. The outer margin is sometimes pale yellow and reticulated with darker brown. In some specimens there are small irregular purple blotches through much of the wing. In the subspecies C. p. forbesi which occurs in western areas and has some intergrades in SW Alberta the forewing is overall more orange-brown and the triangle is more slender. The larva is green with darker green mid-dorsal and sub-dorsal stripes. The head is brownish yellow with some black in the ocellar area and a dark lateral stripe. (Duncan 2006)

Eggs are laid in masses on foliage of the host and hatch in 10 to 12 days. First instar larvae are unusually active and will either descend from the tree on a silken line to the ground or climb higher in the tree. The solitary larva hibernates on the ground and resume feeding in the spring until June. They start by feeding on low herbaceous plants and then later ascend up trees. Unlike most related species the later instars do not often descend on silken lines and will leap off of their substrate if disturbed. They pupate under loose bark or on the ground. (Duncan 2006; Chapman & Lienk 1971).