1911 Encyclopædia Britannica/Butterfly and Moth

BUTTERFLY and MOTH (the former from “butter” and “fly,” an old term of uncertain origin, possibly from the nature of the excrement, or the yellow colour of some particular species; the latter akin to O. Eng. mod, an earth-worm), the common English names applied respectively to the two groups of insects forming the scientific order Lepidoptera (q.v.).