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A large (5.6-8.0 cm. wingspan) narrow-winged, heavy-bodied moth. The male has pale brown forewings shading to dark brown along the costa and blackish on the outer edge and along the lower margin. The hindwing is pale yellow with a broad black terminal band and a white fringe. The light brown abdomen contrasts sharply with the black thorax. The female is darker and less contrasting, with the forewing dark grey-brown shading to even darker brown on the margins. The hindwing is a light brown with a broad black terminal band and white fringe. The reniform spot in both sexes may be marked by a small white dot.

This large sphinx is unusual for several reasons. It is one of the few species of the genus Sphinx that shows significant sexual dimorphism. Also, the males in particular are apparently mainly diurnal and have been collected while nectaring at flowers, including lilacs and dandelion. Females appear to be active in the evening and at night, and have been collected only at lights. Clemens' Sphinx have also been observed visiting and apparently obtaining nourishment from dead, decaying fish, and this behavior has been observed along the North Saskatchewan River here in Alberta.

No Alberta ata. Elsewhere reported to use a variety of trees and shrubs as larval hosts, including apple, ash, birch, northern bayberry, poplar, wax-myrtle, willow, and others (McGuggan, 1958; Covell, 1984; Handfield, 1999).