The wingspan is 92–105 mm. The upperside of the forewing is dark brown with gray streaks along the inner margin and a gray patch at the tip of the wing. The upperside of the hindwing is dark brown with varying amounts of dark orange at the base.
Adults are on wing year round in the tropics and have been recorded from July to October in Arizona, in December in Texas and in September in southern Florida. Adults feed on the nectar of flowers, including Saponaria officinalis and Cynanchum racemosum.
- Lotts, Kelly & Naberhaus, Thomas (2017). "Lassaux's sphinx Erinnyis lassauxii (Boisduval, 1859)". Butterflies and Moths of North America. Retrieved November 19, 2018.
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