The red-fringed emerald has two color forms depending on the season. Adults that emerge in summer are green, but those that emerge in spring are tan. On both, two thin white stripes on the wings parallel the outer edges; these markings flow uninterruptedly from forewings to hindwings. In the summer form, the edges of the wings can have a pink or pink-checkered “fringe” (thus the name).
Larvae are grayish, brownish, or tan, with each segment bearing a pair of finlike projections. Their slight fuzziness makes them look like the remnants of a dead leaf. When they rest motionless on the edge of a dying leaf, they are perfectly camouflaged.
Similar species: An entire subfamily of moths in the family Geometridae are called "emeralds," and they all look very similar — in North America north of Mexico, the emerald subfamily comprises some 62 species in 12 genera. Here are two examples:
- The wavy-lined emerald (Synchlora aerata) is very similar but lacks the reddish fringe on the wing margins; instead, the fringe is pale green. Considering that the fringe can wear away, note also that the wavy-lined emerald has an unbroken white stripe running all along the top of the abdomen. On red-fringed emeralds, the abdomen has some white spots that are usually outlined with reddish color.
- The southern emerald (S. frondaria) is similar to the wavy-lined emerald, but its wing lines are scalloped instead of wavy.