Fireflies are not flies, but beetles in the Family Lampyridae. Typically you do not see their heads which are usually hidden by the very broad pronotum.

This is a large firefly (about 10mm). You can see the big eyes which characterize insects that are most active at night. You can also see the yellow tip to the abdomen which produces the light (bio-luminescence)we see flickering on and off at night. While it might seem that this is done to enchant us (fairy lights as they are sometimes called), the more prosaic reason is to attract a mate. Both males and females flash!