Rhopalocera and Heterocera are non-standard divisions in the taxonomy of Lepidopterans, used in an attempt to formalize the popular schoolyard distinction between butterflies and moths.
Although the rules for distinguishing these groups are not completely hard and fast (see Difference between a butterfly and a moth), one very good guiding principle is that butterflies have thin antennae and (with one exception) have small balls or clubs at the end of their antennae. Moth antennae can be quite varied in appearance, but in particular lack the club end. The divisions are named by this principle: "club-antennae" (Rhopalocera) or "varied-antennae" (Heterocera).
