lightning bug

Although firefly remains the literary and formal word, lightning bug is the term used by the majority of Americans for the slow-moving flying insect that flashes in the dark. Nearly 80 percent of those interviewed for the Dictionary of American Regional English volunteered lightning bug, while not quite 30 percent said firefly (including those who said both). Only in the northernmost states, especially New England, and along the Pacific coast, does firefly hold its own with lightning bug. Bug itself is nowadays an American term; since the 18th century, the British have preferred insect.