Very like a small S. ocellatus , but with incomplete small black ocelli containing two small blue streaks. The foretibia lacks an apical thorn. Variation even less than in S. ocellatus , mainly consisting of differences in the amount of blue in the ocelli, and in the intensity of forewing markings. Like M. quercus , it occurs at low population densities and may remain undetected for long periods.
Males come freely to light.
Oval, greenish white. Resembles that of S. ocellatus . Both colour forms of S. caecus have distinct black markings on the dorsal surface of the horn, base of the claspers, prolegs, and ventral surface of the head. The white body tubercles are round and blunt, unlike in S. ocellatus , where they are sharp and pointed. Found throughout July and August.