The younger instars of the caterpillar are decidedly bluish green, but in its final instar, when it is one of the largest prominents (notodontids) in the area, it is bright green and covered with a fine powdery whitish bloom. The head capsule is large and the mandibles are yellow. It is to be found in August and September on Garry Oak or Red Alder, though it is more particularly associated with the former. The moth emerges in mid-June. The prominent hump on its back is doubtless responsible for its name gibbosa, which means hump-backed.