It is believed that the name `` butterfly '' originated from a member of this family , the Brimstone Gonepteryx rhamni . It was called the `` butter-coloured fly '' by early British naturalists . It is believed that the name `` butterfly '' originated from a member of this family â '' the Brimstone Gonepteryx rhamni â '' which was called the `` butter-coloured fly '' by early British naturalists .