Why is a specific species a micro butterfly and not a macro butterfly? It is more of a practice that has been developed over the years without being specifically laid down in any rules of taxonomy. Families consisting mainly of small species fall under the micro butterflies. In most cases, these are species where the fore wing length is less than 10mm. Furthermore, micro butterflies are generally considered to be more primitive and are therefore placed at the beginning of the order of the Lepidoptera. Questionable families are the Pyralidae (Snout moths) and the Crambidae (Grass moths), whose species are often as large as many Geometridae (Geometer moths)