EPHEMERA, the DAY-FLY, in zoology, a genus belonging to the order of neuroptera. It has no teeth or palpi; there are two large protuberances above the eyes; the wings are erect, the two hind ones being largest; and the tail is bristly. There are eleven species, distinguished by their colour and the number of bristles in their tail. This fly derives its name from the circumstance of its living but one day.