I turned this rather large firefly upside down to take a photo of the tip of the abdomen, the yellow part. This is the section which lights up (bio-luminescence)and flashes. Not all firefly species have this ability, but most do. There is nothing more magical than to watch the little 'fairy lights' floating across a field on a warm summer night. This is done to attract a mate, and both sexes "flash"! Different species have different light intervals too, so it is more complicated than we think. And by the way, fireflies are not flies, they are beetles!