Where the River Narog flowed into Sirion, there was a lush valley filled with willow trees, a place understandably known as the Land of Willows, or Nan-tathren in Elvish. According to the legends of the Lost Tales, this was the place where butterflies of all kinds came into the world, and they remained common there during the First Age. In Tolkien's better known works, though butterflies are mentioned several times, they are only actually seen on one occasion: Bilbo finds them fluttering above the treetops of Mirkwood in The Hobbit. Like other creatures that lived in that dark wood, its butterflies had turned completely black in colour, and so that variety is referred to as the 'black emperor'.

The Quenya word for 'butterfly' was wilwarin, a name the Elves also gave to a constellation of stars. The identity of this constellation is not completely certain, but Christopher Tolkien suggests that the northern 'W'-shaped group of stars that we know as Cassiopeia was the Elvish constellation of the Butterfly.