They are named for the fact that the female has two distinct sexual openings: one for mating, and the other for laying eggs.

They function as herbivores, pollinators, and prey in terrestrial ecosystems, and can also be extremely damaging to crops.

[1] The Lepidoptera group can be divided into the primitive but paraphyletic "micromoths" and the derived monophyletic Apoditrysia, which include mostly larger moths, as well as the butterflies.

[3] While it is difficult to pinpoint the origin of affinities between clades, Tineoidea are found to be useful in understanding the vast diversity in Ditrysia.

[further explanation needed] Apoditrysia, Obtectomera, and Macrolepidoptera will be considered monophyletic if one or more organisms are either included or excluded from the clade.