Contrary to the popular "folk" etymology, the word butterfly comes from the Old English
buttorfleoge which is "butter" plus "fly". This was originally the name of a midsize species,
in England during the middle ages, that had yellow wings. As is often the case with moths, the larvae,
caterpillers, of some species are actually better known to most people than the adults. Though the
extreme examples of butterflies and moths can easily be distinguished (monarchs vs isabella moths, for
example), there are many "in between" species which can't readily be classified as one or the other. Most
professional and serious amateur entymologists don't bother making the distinction.