Forewings of adults are white with black spots. The amount and size of the black spots varies, but the spots are configured in a way that, if you connected them, they would form lines and bands. There are usually 5 fairly prominent spots evenly spaced along the costa (leading edge) of the forewing. Some individuals lack spots completely. Hindwings are different in the two sexes: on males, they are yellowish-orange, and on females, they are white. Both sexes usually have about 3 or 4 black spots on the hindwings.
The bodies of adults are white on the head and thorax, with the abdomen orangish-yellow. On females, the tip of the abdomen is white.
Caterpillars are mottled with yellow and brown and are hairy like a woolly worm. The long, bristly hairs arise from orange or black, raised tubercles. The colors of the hairs can vary quite a bit. The hairs can be all grayish-white, yellowish, reddish brown, or blackish. One common coloration has grayish brown hairs along the top side and reddish brown hairs on the sides. The caterpillars typically walk very quickly.
Similar species: A number of moths look confusingly similar to this species. The Virginian tiger moth (Spilosoma virginica) adult is very similar, but its body lacks the different white versus orange regions; instead, the abdomen is mainly white, with a black dot on the top of each abdominal segments, and some orangish bandlike markings separating the different segments.
The Virginian tiger moth’s caterpillar looks similar, too. The salt marsh caterpillar, however, has noticeably longer hairs at the front and end of the body, especially at the rear, while the Virginian’s hairs are long, soft, and vary considerably in length over the whole body. In both species, the color of caterpillar hairs can vary tremendously, so hair color is not a good clue.
Other similar moths in our region include the agreeable tiger moth (Spilosoma congrua), which has a pure white abdomen, and the fall webworm moth (Hyphantria cunea). The color patterns on the abdomen should help separate these from salt marsh moths. After that, however, you need to look at color patterns of the legs and other arcane characteristics. Fortunately, many Internet resources exist to help you with these fine points.