The Modest Sphinx has a wingspan of approximately 3.75 to 4.25 inches. The forewing is deeply scalloped. The basal third of the forewing is tan or gray. There is a dark median band and the outer third of the wing is lighter in color. The hindwing is tan with a crimson patch and an inward-pointing bluish-black triangle near the outer margin. The large caterpillars grow to approximately 4.50 inches when mature. They are pale green to blue green with 7 white slanted lines on the abdomen. A thicker white slanted line runs from the proleg to the tip of a short white horn.
The frequency of occurrence is not known. The adults fly in June and July. The larvae are out from July through fall.
This sphinx moth is also called Big Poplar Sphinx. The adults do not eat. The larvae eat the leaves of poplar, cottonwood and willow. It overwinters as a pupa in a shallow burrow in the ground.
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