Moths comprise a group of insects related to butterflies, belonging to the order Lepidoptera. Most lepidopterans are moths; and there are thought to be approximately 160,000 species of moth, many of which are yet to be described. Most species of moth are nocturnal, but there are also crepuscular and diurnal species. Although the rules for distinguishing moths from butterflies are not hard and fast, 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, Moths evolved long before butterflies, fossils having been found that may be 190 million years old.