If you can get a good look at them, the shape of the antennae is the best way to distinguish adult butterflies from moths. Except for one group of tropical butterflies, all butterflies have simple antennae that end in a swelling or "club." The clubs may be very pronounced, as in the Nokomis Fritillary or more subtle, as in the White-dotted Cattleheart. Moth antennae range in shape from simple, as in the Rustic sphinx, to feather-like, as in the male White-streaked saturnia moth, but all of them lack the clubbed tip.
Most adult butterflies are diurnal and brightly colored. Their bodies are generally slender and not especially pubescent (hairy). In contrast, adult moths are generally nocturnal or crepuscular. Although some, such as the Io moth are brightly colored or have colorful "eyespots," most moths are drab, with cryptic wing patterns. Bodies tend to be bulky and are often quite pubescent.
Caterpillars and Pupae
Unfortunately, there are no simple characteristics that universally distinguish moth caterpillars or pupae from those of butterflies. If you have a caterpillar or pupa to identify, try filtering the Image Gallery by stage, and browse all of our images of these life stages. Two resources that may help if you are trying to identify a caterpillar are Caterpillars of Eastern Forests and Caterpillars of Pacific Northwest Forests and Woodlands. These cover a broad range of butterfly and moth caterpillars, and together with larval photos on this site may allow you to identify your specimen,or at least narrow it to family. Pupae are much more difficult to identify. If you find a good resource, let us know.